Choosing the Right Fastener - Part ll

By Carl Tamm

In the first part of this article, a brief mention was made regarding “conductivity” of fasteners.  This second section will address that issue.

Peculiarities of electrical connectors give rise to further thought of fasteners.  It is not exceptionally difficult, using reasonable workmanship and materials, to make an electrical connection of reasonable conductivity initially – however, to make one which will provide exceptional duration for many decades is another story.

While there are many types of “bolted connectors” used in the utility industry, for simplicity references this article will relate to flat “pad-to-pad” type connections.  The physics involved in the atomic structure of the electrical interface are too involved to address in this article, however sufficient evidence is readily available to show that a typical 4-bolt pad-to-pad connection will achieve less than 2% of the “available” surface area that will actually make an electrical connection.  Some of the earlier references to this phenomenon of area immediately surrounding the bolt holes being the only actual contact area date back over 55 years.

Kaiser Aluminum – Electrical Bus Conductors, 1957

Accepting this long stated peculiarity to be true, one might ask, “What could be done to improve that value?”  The industry has been striving to improve the area of contact “between” the pads – but the obvious option of allowing the bolts to serve as conductive paths is often overlooked.

Use of conductive washers and fasteners can provide additional current paths from the backside of the respective pads, through the fasteners!  How obvious is that?  Why do so many disregard this simplistic improvement?  Some other misunderstandings, such as reusing fasteners, will be addressed in a future part of this article.

Perhaps a simplistic understanding of fastener conductivity will help.  One may look at either conductivity or resistivity.  I prefer the former, as the numbers are more simplistic and easy to understand.  The common basis used for conductivity values is the International Annealed Copper Standard, abbreviated IACS.  The following chart indicates values for a number of materials commonly used in our industry, and a few more for reference.

Most are surprised to learn that Gold is not the most conductive material, but is significantly superseded by Silver! The attributes of gold, and its place in Contact Physics is not the subject of this article. Of equal surprise to most, is that while we all recognize iron and steel are conductive, its value of conductivity is only 2.5% IACS. This value is close enough for argument, not to get into the specific values of different alloys, and represents all common ferrous alloys, and includes “stainless” alloys.

Alloys used for connectors commonly range from 16% to about 44%. While Silicon Bronze bolts, or the common aluminum alloys used for bolts are not nearly as conductive as the materials used for conductors, they commonly do approach the conductivity of many connectors, of approximately 16% to 24%. The lesser degree of conductivity of these fasteners is due to the alloying elements used to provide the strength needed for fasteners. Still, these conductive fasteners provide 8 to 12 times the conductivity of steel.

What is the result of using steel or stainless steel bolts in connectors? While very low in conductivity, they are yet conductive, and the low value of conductivity is the reason for the “high resistance” associated with steel. The very definition of resistance is associated with the thermal rise of the material as the result of passing electrical energy through it. The physical effect is the expansion of the material due to the thermal rise. With bolts, the higher the temperature, the less clamping force provided!

To counteract this, properly designed and applied Belleville washers are used to maintain the clamping force, as Mr. Goch covered in Part 1 of this series. As Part 2 began, our purpose is to provide a bolted joint that will maintain the electrical integrity of the connection over many decades. Maintaining the clamping force is paramount to achieving this goal. The following illustration depicts the effects of the difference in the coefficient of thermal expansion between differing materials used in electrical connections.

As the illustration depicts, differing materials expand at differing rates over a given temperature rise.  This property is given stated values for different materials, known as the coefficient of thermal expansion related to the respective materials.  Both aluminum and copper alloys expand at a greater rate than steel.  The use of steel bolts, without the benefit of properly sized and applied Belleville washers will result in rapid creep of softer material of the connectors, and the joint will loosen over time.  Of course, as it loosens, the resistance will rise, and with that given rise in resistance, the thermal rise for a given current will increase.

There exist several design features incorporated in ClampStar, the result of which provide a superior connection to other types of connections, including compression connectors.  Thus another reason ClampStar provides the properly engineered Belleville washers in its fastening system!  As Mr. Goch stated in the previous part 1, the torque nuts, designed by Classic Connectors, Inc., prevent over-tightening of the fasteners, and thereby prevent over compression of the Belleville washers.

Additional information on Belleville washers will be incorporated in another series!

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Choosing the Right Fastener

By Waymon P. Goch

With proper attention to the fastener assembly, the best choice is almost always fasteners of the same material as the components being joined. The primary reason is it eliminates differential thermal expansion and contraction, and just as important, since we are talking about electrical connectors, is that it will be conductive, at least more conductive than steel fasteners.  More discussion on this subject in a following article.

A common misconception is that stainless steel fasteners do not rust, but they do rust under certain conditions. Stainless steel remains stainless in unpolluted atmospheres and when immersed in moving or flowing fresh and seawater. In humid marine environments or stagnant moisture conditions, type 304 stainless, for example, may rust locally in pitting, crevice, stress, concentration cell, knife edge, and galvanic corrosion. The exposure environment and chemical composition of the stainless steel will dictate whether or not rusting will occur.

Stainless steel owes its corrosion resistance to a very thin, compact passive surface layer that forms upon exposure to oxygen. If the surface is scratched and exposed to oxygen, the scratches will again become passive. It is primarily this passive layer that is also responsible for galling and seizure of stainless threaded fasteners. The layer may flake with small particles becoming trapped in the thread bearing surfaces where the pressure is sufficient to cold weld the internal and external threads together. The use of anti-seize compounds and different grades of stainless are often recommended to prevent or minimize galling and seizure but these options may only be partly successful. By far, the best solution I have found over the years is to mechanically zinc plate either the stainless bolt or nut (normally the nut).

Particular attention should be given to this “galling” or seizing tendency.  The purpose of the bolt is to provide a clamping force between two or more components.  During assembly, the tensile stress which provides this clamping force is obtained by torque applied, less the friction between the threads of the bolt and nut, and the friction of the face of the bolt against the mating surface.  The most common means of determining that the appropriate tensile stress and elongation of the bolt is achieved during assembly is calculated against torque.  “Galling” or seizing of these threads consumes the force provided by the torque applied, giving a false indication of clamping force achieved.

There are many applications in which stainless steel fasteners are appropriate but they should not be considered universal.

How does a bolted joint work?

A properly tightened bolt is essentially a spring; achieved through elastic elongation of the bolt. Under constant static loading at a constant temperature, that alone would be entirely satisfactory without the need for lock washers, Belleville spring washers, or other devices to maintain clamping force. However, most applications are dynamic and require additional consideration.

The well-accepted equation for computing torque-tension relationships in bolted joints is:

T = kDW/12

Where T = torque in lb-ft, k = friction factor or torque coefficient, D = bolt diameter in inches, and W = bolt tension in lbf.

The static friction constant k varies with the fastener material and material condition (lubricated or dry) and ranges from 0.11 for lubricated steel to around 0.30 for lubricated aluminum and stainless steel. The overall torque coefficient depends upon the materials being joined, thread clearance, and the torque coefficient of the bearing surface against which the bolt head or nut is being turned. Friction is an important consideration because it represents torque that is lost in overcoming friction and not applied to create bolt tension.

Recommended fastener torques for common sizes, grades, and materials are provided by manufacturers as well as industry standards such as the Industrial Fasteners Institute, ANSI C119.4, and others. Recommended torque is usually based on final fastener tension within the elastic limit and is typically 60 – 70% of the proportional limit, yield point, or proof load. The bolt will continue to stretch if loaded beyond yield but will be unable to return to its original length, thereby reducing the clamping force.

The absence of torque wrenches in tool belts and bags of line personnel and installers frequently results in a policy of tightening “until tight” then applying another half or full turn.

ClampStar® avoids this potential problem, assures proper installation torque, and eases installation by providing torque-limiting nuts with an outer section that shears off at the proper torque, leaving a permanent hex nut in place, regardless of the type of wrench employed. This facilitates the use of pneumatic, electric, hydraulic, or battery operated nut runners, wrenches, and rattle guns.

Conventional spring or split lock washers are frequently recommended for maintaining fastener tension under dynamic loading. However, a good definition of a spring lock washer is a flat washer with a split. They are ineffective in maintaining live spring follow up because they completely flatten under relatively low compressive loading; typically around 350 lbf for ½” lock washers.

The most effective means of assuring live spring follow up is the use of Belleville type spring washers, properly sized, that will remain within its working range and not flatten or reverse, under any anticipated thermal or mechanical load excursions.

So, how do we choose the right fasteners?

The majority of electrical connectors are aluminum-to-aluminum, copper-to-copper, aluminum-to-copper, galvanized steel-to-galvanized steel and galvanized steel-to-copper or aluminum.

Proper surface preparation and the use of the correct inhibitors and joint compounds are critical steps in the creation of a low resistance electrical connection to assure long service life, but those are subjects for another time.

The preferred and recommended fastener materials for joining like and unlike metals are shown in the following chart. Although the chart references flat bar connections, the same applies to pad-to-pad as well as connections of other shapes.

Aluminum bolts are typically 2024-T4 with a #205 Alumilite finish, washers are 7075-T6 and nuts are 6061-T6 with a wax finish.

Silicon Bronze alloy bolts, washers and nuts are preferred for copper to copper connections.

Stainless steel bolts, washers and nuts are primarily type 304, 304L or 316 austenitic stainless. Type 316 has better corrosion resistance and greater creep strength than 304 or 304L due to its slightly higher nickel content.

Hot dip galvanized steel bolts are normally ASTM A307 grade 2 low carbon steel or ASTM A325 grade 5 medium carbon or low alloy steel. Grade 2 bolts do not have a grade marking on the head whereas grade 5 is marked with 3 radial lines. Both may contain the bolt manufacturer’s identification and both are galvanized according to ASTM A153. Galvanized or stainless steel flat and Belleville spring washers may be supplied and used with galvanized steel bolts. Galvanized steel nuts are tapped oversize for a class 2 fit on galvanized bolts.

We’re sure that readers will find this brief discussion worthwhile.

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Fault Current and the Effects on ClampStar®

By Waymon Goch

The terms fault and short circuit in electric power systems are frequently used interchangeably but there are slight differences. A fault can be defined as any abnormal flow of current whereas a short circuit is a current that completely bypasses the load by flowing directly to ground (earth) or by returning to the source. Thus, a very low or zero impedance short circuit is a fault but a high impedance fault may not be a short circuit since current may continue to be supplied to the load.

Single phase faults can be line to ground, line to neutral, arcing, or open and can be momentary (transient) or persistent. A transient fault is cleared after power is disconnected for a short time whereas a persistent fault must be corrected. The effect on insulation depends upon whether the insulation is self-restoring or not.

In polyphase systems a fault that effects all phases equally is a symmetrical fault. If only some phases are involved, an asymmetrical fault occurs. Asymmetrical faults are more difficult to analyze because equal current magnitudes in all phases no longer applies and methods such as symmetrical components are normally used for analysis.

The suggestion for this particular article came from recent laboratory fault current tests during which the participants were surprised by melting (fusing) of the test conductors while the ClampStar® units remained near room ambient temperature. From our viewpoint, those results were as expected. (See Video Gallery, High Current Test at www.classicconnectors.com).

Conductor fault current ratings are used to determine the suitability of a given conductor material and construction to withstand the anticipated fault current magnitude and duration without jeopardizing the mechanical integrity of the conductor.

Fault currents can result in very high currents in conductors from initiation until the fault is cleared by protective devices such as circuit breakers, fuses, reclosers, etc. Modern relaying typically limits the fault duration to fewer than 20 cycles on transmission circuits but may be longer for distribution. When multiple attempts at reclose occur, the total fault current exposure time is the sum of the interruption times.

Figures 1 and 2 are fault current versus time curves for a range of AAC and ACSR conductors. These particular curves are taken from the Southwire Overhead Conductor Manual. They are also available in the Aluminum Association, Aluminum Electrical Conductor Handbook as well as other handbooks and publications (and if such tables are not available, they can be generated by the following equations).

ACSR:     I = 0.0862 A / √ t

AAC:       I = 0.0671 A / √ t

Where: I = Current in Amperes, A = Cross-sectional area, cmil, and t = Time, seconds

The following metal characteristics are needed to calculate fusing (melting) time;

  • melting point, ⁰C
  • density, grams/cm3
  • specific heat, calories/gram/⁰C
  • resistivity (ρ), ohm-cm
  • sp ht x melting point x density = cal/cm3 to melt wire or section
  • multiply cal/cm3 x volume (cm3 ) = calories to melt
  • multiply by 4.61 joules/cal = joules required to melt
  • resistance (R) = ρl/A (resistivity x length / area = ohm-cm x cm/cm2 = ohms
  • time to melt = I2 Rt = joules or watt-seconds to melt
  • t = joules / I2 R (time to melt is inversely proportional to I2 )

The difference in acceptable fault current limits for ACSR and AAC results primarily from the difference in established allowable temperature. For AAC and other all aluminum construction a limit of 340⁰C has been established since momentary exposure to this temperature does not result in significant loss of strength.

For ACSR, and other conductors with high steel core content, an upper limit of 645⁰C represents the threshold of melting of the aluminum and the unaffected steel core is expected to maintain sufficient mechanical strength, although the contribution to overall conductor strength (RBS) of the aluminum strands may be substantially reduced.

Of significant interest to most people, and the reason the referenced fault current tests were conducted, is that the electrical interface of a connection is commonly known to be significantly degraded by severe fault currents.  This is particularly true of aged connectors where some portion of the interface has already been degraded due to natural aging, and while the remaining available interface may be sufficient to carry the normal current load of the system, the mechanical and electrical stress induced in a connector during a fault event will often destroy the remaining interface resulting in a catastrophic failure, or degrade it such that a mechanical failure is imminent in the near future under moderate to heavy loads. It is also important to note that a partially degraded interface resulting from natural aging is almost impossible to detect with either resistance readings or infrared under normal loading conditions.

Heating of the conductor occurs more rapidly than cooling and although fault currents can also result in mechanical forces (which can be calculated from readily available information in handbooks and other sources) the primary consideration for conductors is thermal.

Fault current limits for copper conductors and accessories can be calculated in the same manner and three common formulae have been developed over the years.

A formula developed by W.H. Preece (I = αd3/2) in which I = fusing current, d = wire diameter in inches, and α = a constant depending upon the material which, for copper is 10,244. Although this formula was widely used it proved to be inaccurate in many cases because it assumed that all heat loss was due to radiation. The formula, I = kdn can be used with accuracy if k and n are known for a particular case.

Figure 3 is a fusing current curve for copper conductors from 30 AWG to 500 kcmil prepared by E.R. Stauffacher that assumes no radiant heat loss due to the short times involved, a copper melting temperature of 1083⁰C and ambient temperature of 40⁰C.

I.M. Onderdonk also developed a simpler equation for calculating the fusing time for copper conductors and copper connectors:

33 (I/A)2 S = log [(Tm – Ta / 234 + Ta) + 1]

or:

I = A √ log (Tm – Ta / 234 + Ta + 1) /33S

Where: I = current in amperes, A = conductor area, circular mils, T = time current applied, seconds, Tm = melting point of copper, ⁰C, and Ta = ambient temperature, ⁰C

Not surprising, this is called the Onderdonk equation which can also be used to estimate the performance of soldered, brazed, and bolted copper connections, using appropriate melting temperatures for the solder and brazing alloys of interest. For bolted connections, a generally accepted value of Tm is 250⁰C.

It is important to determine the available fault current at the location of interest. Such studies normally begin with a line diagram showing all loads and potential sources of fault current. (During a symmetrical fault, induction motors will contribute only during the asymmetric portion of fault current but synchronous motors may contribute 4 – 6 times their full load current to all fault locations). Capacitors may also be a factor under some conditions. Protective devices are not normally included in the line diagram.

Worst case short circuits are normally based on bolted 3 phase fault conditions in which all three phases are “bolted” together to obtain a zero impedance fault. This results in maximum thermal and mechanical stress in the system and typically assumes infinitely available fault current from the primary source.

Figure 3 Copper current - time

Recommended reading: “Hard to Find Information About Distribution Systems, Volume 1”, Jim Burke, September 18, 2006 (www.quanta-technology.com).

References: “Standard Handbook for Electrical Engineers“, Fink and Beaty, McGraw-Hill

IEEE Std 738, “IEEE Standard for Calculating the Current-Temperature Relationship of Bare Overhead Conductors” may also be useful. It can be used to calculate both steady-state and transient thermal ratings and conductor temperatures.

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Corona, RIV/TVI Testing Complete

By Carl R. Tamm

If your concept of corona is that it is commonly consumed with a slice of lime, you might wish to read a bit more about issues in our industry that differ slightly from that concept!

Fair weather radio and television interference (RIV and TVI) generated by overhead lines are most frequently caused by corona. Corona results from voltage overstress on the surfaces of conductors and other energized components. On distribution lines, sparking at lightly loaded connections and associated non-conductive films may also generate RIV/TVI. Corona also results in resistive power loss on transmission and distribution lines. Under foul weather conditions virtually all energized conductors and components are in corona.

With all this concern about Corona and at the request of a few customers we recently conducted RIV and Corona tests on CSF-1108-024-COR345 and CSF-1302-024-COR500 standard production ClampStar® units.

The tests were conducted in the HPS (Ohio Brass Company) high voltage environmental test chamber in Wadsworth, OH on September 13, 2012. The RIV tests were conducted in accordance with NEMA 107, “methods of measurement of radio influence voltage (RIV) of high-voltage apparatus”. Corona observations in complete darkness were made with the aid of a Noctron V night vision system (night image intensifier) S/N 5046. Click here to get the full report.


Important Facts You Need To Know About Surge Arrester Currents

Surge Arrester Currents
By Waymon P. Goch

Discharge current is the surge current that flows through a surge arrester during discharge of an overvoltage surge (and discharge voltage is the voltage that appears across the terminals of an arrester during that time). There are four additional currents that are of significance in the design, application, and performance of a surge arrester. Those currents can be defined as follows:

  • Grading current: Current that flows through the arrester internal grading circuit.
  • Leakage current: Current that flows over the outer surface of the arrester housing that is primarily a function of the application environment.
  • Fault current: Current from the connected power system that flows in a short circuit.
  • Power follow current: Current that continues to flow following discharge of the surge by the arrester.

The difference in fault and power follow current is timing. The low arrester impedance during discharge is, for all practical purposes, a short circuit but the arrester must interrupt and reseal against power follow current.

Fault current is dictated by the power system and the available current at the arrester location. Power follow current is dictated by the power system and the surge arrester design. The others depend upon the arrester age, class, rating, and design [gapped silicon-carbide (SiC), gapped or gapless metal oxide (MOV)].

Surge arresters manufactured before about 1977 are gapped SiC and there are many distribution, riser pole, intermediate, and station class arresters in service today that are of that design. The majority of these arresters were manufactured from 1950 through 1977. They represented state of the art at the time they were installed and for the most part their service history has been satisfactory.

The design of the earliest SiC arresters consisted of a simple multigap structure in series with non-linear SiC valve blocks. In those arresters, all system voltage was applied to the gap structure. The gap structure sparked over in response to an overvoltage surge to prevent damage to line or equipment insulation and the resulting power follow current flowed through the series gap-valve block combination. The non-linear blocks limited the follow current to a level that the gap structure could typically interrupt on the next voltage zero crossing (although restrikes were not uncommon). Following successful reseal the arrester returned to normal operation.

A major improvement in that design, primarily for station and intermediate class arresters, occurred with the introduction of the current-limiting gap in 1957. The current-limiting gap helped limit system follow current by generating a back EMF which, in combination with the non-linear SiC blocks, allowed
current interruption without reliance on a voltage zero crossing.

Most gapped SiC arresters also utilized resistive (R), capacitive (C) or resistive-capacitive (RC) grading circuits to grade the system voltage and obtain uniform voltage distribution across the gap structure. These grading circuits were electrically connected external to the gaps and blocks so that grading current flowed only through the grading circuit. Typical grading circuits resulted in a few milliamps of line to ground current.

A concern with gapped SiC arresters was operation in severely contaminated environments. Severe external contamination and the resulting leakage currents could couple and upset weaker internal grading circuits and alter the voltage distribution over the gap structure.

One method of monitoring grading and leakage currents as well as the number of line to ground discharges primarily through station arresters is a discharge counter with a leakage/grading current meter, Counters are used with gapped SiC and gapped and ungapped MOV arresters to assist in monitoring their duty and condition. Installation of discharge counters requires grounding the arrester through the discharge counter. This is typically done by mounting the arrester on an insulating sub base, as shown in Figure 1. (Many years ago, one manufacturer offered a discharge counter that also contained a mirrored replica gap in the discharge path. By examination of the replica gap and the copper mirror electrodes one could theoretically judge the condition of the arrester internal gaps and determine the duty to which they had been exposed).

The introduction of metal-oxide [primarily zinc-oxide (ZnO)] semiconductors for use in MOV surge arresters in 1977 was the second major advancement in surge arrester design and performance. The metal-oxide varistor is characterized by an extremely non-linear current-voltage relationship resulting in a much higher voltage exponent over the nonlinear portion of the volt-amp curve than SiC. This characteristic is what allows the design of gapless surge arresters. It also requires the introduction of another current called the reference current (Iref), which is an AC current specified by the surge arrester manufacturer in conjunction with a reference AC voltage (Vref) that essentially defines the point at which the arrester elements go into conduction. Below that point (and when energized at normal line to ground operating voltage) MOV elements can be characterized as lossy capacitors with current leading voltage by almost 90⁰. As voltage is increased above Vref the MOV elements become more resistive and at full conduction almost purely resistive with current and voltage in phase.

The significant improvement in operating characteristics and protective levels afforded by gapless MOV surge arresters also renders them virtually immune to the effects of contamination and external leakage currents.

Figure 1 Typical Installation (Cooper)

The IEEE C62 family of standards covers surge arresters and their application. For example, C62.11 is titled, “IEEE Standard for Metal-Oxide Surge Arresters for AC Power Circuits (>1kV)”.

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Problem Connectors on Multiple Sub-Conductors

By Carl R. Tamm

If you have a line with two or more sub-conductors per phase, and during an infrared inspection, a scenario is found where one of the sub-conductors has a fitting that is hotter than the one next to it, the natural tendency is to assume it is a high resistance connection.  Were it a single conductor and we find a “hot fitting” it is a “no brainer” to understand that it is high resistance.

However, when we have two or more sub-conductors, and one fitting is hot and the other is not, it is not such a simple matter, as more often than not, the problem is elsewhere!

While this phenomenon applies to any multiple of sub-conductors, for simplicity, we will use the scenario of only two, or “twin sub-conductors” for the rest of this article.

To gain an appreciation of what happens, it may be easier to think of the old analogy of comparing electrical energy flow through conductors to water flowing though pipes.

If we think about having a specific flow rate of water, say 100 gallons per minute, and instead of having that flow through one pipe, we provide two pipes of equal size and equal levels (for simplicity, we’ll ignore number of fittings and other restrictions) we can expect the flow of water to be divided in half, with 50 gallons per minute flowing through each pipe.

When we design a transmission line with twin sub-conductors, care is taken to assure both conductors are of equal length, with the same number of fittings so that if measured, they would have the same “electrical resistance” measured in ohms.  And, if this is the case, we will achieve an equal voltage division, and therefore an equal current division, with each conductor carrying 50% of the entire electrical load (or flow).

If we go back to the pipes, and we put a restriction in one of them, possibly partially closing a valve, or some other mechanical restriction….

….now we expect more water to flow through the open pipe, such as 40 gallons through the restricted pipe, and 60 gallons through the open pipe!

With electricity, it works the same way.  IF both sub-conductors are exactly the same, they will carry equal current.

However, if one has more resistance than the other, such as having a high resistance connector in the circuit, MORE current flows through the conductor that has less resistance!  Now – MORE CURRENT results in HIGHER OPERATING TEMPERATURES!

A transmission line has many connectors, and while they may be close, there will inevitably be some difference in electrical resistance, but with reasonably good craftsmanship, the normal result is so close it is of no concern.

However, when one or more of those connectors begin to increase in resistance with age, rarely will that degradation be equal, and the result will be an imbalance of current between the two sub-conductors.

Because the current is always trying to find equilibrium, the current will try to find other paths in order to maintain that balance.  Oftentimes, this is through a yoke plate or other mechanical connection, which may be miles from the location where the hot connector was originally found!

While the hot connector may be bad, or going bad, it is very likely not the ONLY bad connection on the line.  When using a ClampStar® to shunt that hot connector, it will behave as one would expect, it WILL cool down, simply because the ClampStar® has increased its ampacity by a factor of 3 or more, so it would take 3 times as much current to cause it to heat.  But the story does not end there.

It becomes necessary to look for that current imbalance.  Begin by doing a much more thorough inspection of the circuit, using a much more narrow range of bandwidths on your IR equipment, and look at all hardware that joins the sub-conductors in any manner, including spacers and yoke plates.  It would be advisable to look for high resistance fittings using an Ohmstik (SensorLink®) which allows the measurement of resistance by measuring micro-voltage drop across a connector.

It may be advisable, while you are out on the line, up close and personal, to consider covering any critical connections with ClampStar®, even if they are not really bad today.  If the line is aged, and you are finding some bad connectors today, you will find more next year, and more the year after that.  A great portion of your expense is mobilization, and while you are there, it only takes minutes to correct a potentially bad situation.

Connections over roadways, waterways, or places frequented by pedestrian traffic should be given serious consideration.  One failed connector and a dropped line will cost more than covering every connector on the line while you are there.

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Surge Arrester Lead Length Revisited

By Waymon P. Goch

Surge arrester manufacturers always recommend that line and ground leads be as short and straight as possible in all surge arrester applications. Why?

The primary reason is inductive surge impedance. The selection of the best surge arrester for a given application can be negated by poor installation practices. The length and configuration of the line and ground leads connecting the arrester to the apparatus being protected is critical in the determination of the arrester protective levels and margins.

Modern surge arresters are designed to protect insulation from impulse over voltages that could cause flashover or damage to the insulation that is in parallel with the arrester. For distribution class arresters the primary impulse overvoltage is lightning whereas for station class arresters the primary impulse overvoltage is switching. As the class name suggests, Intermediate class arresters fall in between. The consequence of failure to protect is a function of the nature of the insulation and whether it is self-restoring (air) or non-self-restoring (solid, liquid, or composite) insulation.

Internally, the critical components of surge arresters are non-linear resistance elements; either in combination with series multi gaps or more modern gapless metal-oxide varistors (MOV). Internal gaps were necessary in older silicon-carbide arresters to limit grading current through the silicon-carbide resistance elements and to interrupt the flow of system follow current through the arrester following discharge. The composition of MOV (primarily zinc-oxide) resistance elements results in a very highly non-linear voltage / current relationship and allows the construction of gapless surge arresters.

When an overvoltage surge is impressed across the arrester terminals, the arrester begins to conduct the resulting discharge current to ground. The flow of discharge current through the arrester causes a discharge voltage to appear across the terminals of the arrester. If the arrester line and ground leads are also installed in parallel with the insulation being protected, the combined lead inductive voltage drop is additive to the arrester discharge voltage.

The inductive voltage drop in the line and ground leads is a function of the lead inductance, current rate of rise and time according to the formula: V = L di/dt .

For a straight lead wire, the inductance (L) can be assumed to be 0.4 µH/foot. If the lead wires are coiled the inductance will be significantly greater.

Arrester manufacturers’ catalogs, drawings, and data will usually provide protective characteristics of their arresters, including maximum discharge voltages for several discharge currents and voltage times to crest from steep wave through switching surge. Those arrester discharge voltages plus lead inductive voltage (if appropriate) are usually plotted and  compared to the corresponding insulation characteristics to determine the protective margins on insulation coordination curves similar to Fig 1. Fig 1 also illustrates the typical volt-time characteristic of most insulation. That is, the shorter the time the greater the insulation or dielectric strength.

Published arrester IR discharge voltages are normally based on a standard 8/20 impulse current wave (8 µs to crest/20µs to half crest on the tail) however, the highest voltages to which the insulation is subjected are rapidly rising steep wave impulses due to lightning. It is now known that rapidly rising impulse currents are far more common that previously thought. More accurate measurements have shown that about half have rates of rise of 13 kA/µs with a maximum value of approximately 60 kA/µs.

Consistent with this rate of rise, a lead wire voltage drop of 6 kV/ft is often used in calculating the protective levels for installations exposed to rapidly rising fast front surges.

Let’s look at an example to illustrate the effect of arrester line and ground leads connected to a surge arrester and connected in parallel with insulation.

Assuming an 8.4 kV MCOV gapless distribution arrester protecting a 95 kV BIL transformer, a typical gapless heavy duty distribution class arrester might have a 0.5 µs 10 kA IR of around 36.5 kV.

Assuming line and ground lead impedance of 0.4 µH/ft, the resulting inductive voltage developed across the leads = 0.4 x 10‾⁶ x (10 x 103 A / 0.5 x 10‾⁶ sec) = 8 kV/ft. Thus, for every foot of line and ground lead, 8 kV must be added to the arrester IR discharge voltage when calculating the overvoltage protective margins provided by the arrester and its connections. This could be particularly important for the protection of underground cable and aged or degraded equipment.

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Avoiding Splice Failures

By Lisa Nelson, EDM International, Inc.

One of the primary root causes for splicing failures is poor cleaning of aluminum strands prior to compression.   The Electric Power Research Institute has developed technology that enables line crews to properly prepare conductors quickly, efficiently and affordably.  Improper cleaning of conductor strands can result in higher resistance terminations and splices that cause fittings to operate at higher temperatures leading to premature failure. To alleviate this concern, EPRI developers have come up with a system for cleaning the ends of overhead conductors prior to installing compression terminations and splices.

Prior to EPRI’s conductor cleaning research, the predominant method to clean conductors before splice assembly was wire brushing.  However, for complete and thorough cleaning, the conductor must be unstranded. This unstranding is an impractical requirement in most field conditions. Thus EPRI has initiated a multi-phase initiative to develop a method or tool for cleaning aluminum conductors.

The technology involves the agitation of a specialized solution to remove oxidation and grime from conductor strands, and can be adjusted for various cleaning cycle time, depending on the condition of the conductor. This methodology allows line crews to thoroughly clean conductors in much less time than traditional hand-cleaning methods. The technology was designed to be compact and portable to allow linemen to operate wherever the splice is most efficiently made – whether on the ground or up in a bucket.

Not long ago, Southern Company ran a beta test with the conductor cleaning tool. The Southern Company team put the tool to use during a restoration effort at Plant Bowen in Cartersville, GA, where three 500 kV feeders comprised of six structures were destroyed due to a tornado. The team completed 80 to 90 conductor cleanings over a two-week period. “With this new tool, Southern Company was able to do a single cleaning in about six minutes,” according to Andrew Phillips, Director of Trans­mission Increased Power Flow at EPRI. “Using manufacturer conductor cleaning recommenda­tions, which involves cleaning each strand, it would have taken 30–45 minutes to clean each one. A conservative, rough estimate of the time savings would be in the neighborhood of 1,920 minutes, or 32 hours saved.”

For the first-time users at Southern Company, the tool proved to be a device you could learn quickly and put to work immediately with basic expert advisement. “The team saw the tool for the first time and adapted quickly to it,” Phillips commented. “This was really a good situation to test the effectiveness of the device. They were experiencing an outage and in a worst case scenario they were prepared to spend 30 or more minutes to clean each conductor using the manufacturer’s suggested cleaning method. If the first few did not go well, they would have gone back to the recommended method and nothing substantial would have been lost. There was only an upside in choosing to use the device.” Alan Holloman from Southern Company shared this perspective, “The crew was amazed at how well and thoroughly the tool cleaned each conductor. It did a superb job. We would have missed an opportunity if we had not used it.”

There were a number of benefits from using the conductor cleaner. The device enabled crews to be timely and efficient in the splice making process and the splice that was made was more efficient than if they had used the traditional method. The cleaning process was also much faster, saving significant man-hours. Southern Company was also able to make its 500 kV lines available much earlier. Since the conductor is cleaned to the core, the finished product is also of better quality and this could help limit sleeve failures in the future.

A total of seven utilities were part of the project to develop the conductor cleaner. The group of utilities included American Transmission Company, Tennessee Valley Authority, Oncor Electric Delivery, Public Service Electric & Gas Company, CenterPoint Energy, East Kentucky Power Cooperative and Southern Company. Southern Company was the first to use it in scale.  Heat-cycle testing of compression connectors by EPRI shows that connectors installed using this technology consistently outperform connectors installed using wire brushing as evidenced by lower operating temperatures and longer life.

For additional information on proper splice methodology, or for information on other topics related to effectively managing electric utility assets, please contact EDM International, Inc. (970-204-4001).

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Corrosion and Splices

By Waymon P. Goch

Worldwide, the annual cost of corrosion is $2.2 trillion (US); and is currently estimated to be $429 billion (US) annually in the United States.(1) Corrosion that results in failure of aircraft, pipelines, bridges and other critical structures receives a lot of publicity and attention but other failures that are primarily due to corrosion do not receive that kind of scrutiny.

Corrosion can be defined as the destruction of metals by chemical and electrochemical processes. The corrosion products are oxides, hydroxides, sulfides, sulfates, and carbonates in most cases. This represents a return to a natural state in which most common metals occur as ores. There are several types of corrosion. Atmospheric corrosion, such as rust on the surface of ferrous metals, is an example of direct chemical attack. Oxidation is a direct chemical attack that requires only oxygen. Moisture is not required for oxidation to occur. Oxides form immediately on copper and aluminum upon exposure to the air. Copper surface oxides tend to be soft and electrically conductive in contrast to the hard, almost impervious surface oxide that forms on aluminum alloys. The aluminum oxide is electrically non-conductive and the oxide thickness increases with time and temperature.

Additional accelerants present in the atmosphere are carbon dioxide, water vapor, sulfur, sodium and chlorine compounds and the severity of the attack is directly related to the amount of those compounds in the service environment.

Although splices and other connectors and hardware on overhead transmission and distribution lines are subject to direct corrosion with pitting and other outside surface changes, it usually does not result in mechanical or electrical failure, with one exception and that is the oxidation of internal aluminum splice components and conductor. That oxidation contributes to increased contact resistance, heating, and subsequent failure of splices and, to a lesser degree, dead ends and other current-carrying compression connectors.

The more significant corrosion type that affects splice life and performance is galvanic. Almost everyone is at least casually familiar with galvanic corrosion but may not be aware of some of its causes and characteristics. Galvanic corrosion is an electrochemical process that involves metal in the presence of an electrolyte. It normally requires two dissimilar metals of different electrical potential in the presence of an electrolyte.

All metals have a specific relative electrical potential as shown in the galvanic series chart of Fig 1. When metals of different electrical potential are in contact in the presence of moisture (electrolyte) a low energy electric current flows from the metal having the higher potential (anode) to the one having the lower (cathode). This galvanic action is shown graphically in Fig 2.

The actual process involves an anode reaction, the conduction of electrons through the metal from anode to cathode, and the conduction of ions through the electrolyte solution. Corrosion occurs in the anode area (less noble) while the cathode area (more noble) is protected. The corrosion deposit is analogous to ash from burning wood.

This process is intentionally employed in cathodic protection systems to prevent corrosion of underground pipelines and other structures and equipment (including hot water heaters in which the anode is normally magnesium).

Concentration cell, crevice, stress, deposit, impingement, intergranular and fatigue corrosion are all forms of galvanic corrosion.

It was mentioned earlier that galvanic corrosion normally involves dissimilar metals but it is possible to experience galvanic corrosion in a single metal under certain conditions. If a deep crack or fissure develops and can contain stagnant moisture, it is possible to create galvanic corrosion in one metal.

It is important to recognize that the one element required for galvanic corrosion to occur in any form is moisture. If there is no moisture, there is no electrolyte and therefore no galvanic corrosion.

I’d like to call attention to an interesting characteristic in Fig 1. That is the difference in potential of active and passive types 304 and 316 stainless steel. Exposure to stagnant or poorly aerated water causes passive stainless steels to become much more active. This same characteristic can be seen in other metals in the galvanic series.

Within a line splice galvanic cells are created by the ingress of moisture, salts, and other surface and airborne contaminants. Evaporation is retarded and an electrolytic cell is created. This is the primary cause of internal corrosion, accelerated aging, and higher contact resistance resulting in shifting of the dynamic current path. This also emphasizes the importance of proper conductor cleaning and preparation to remove the non-conductive aluminum oxides immediately before splice installation and the use of the proper inhibitors to seal the contact area and conductor strand interstices.

Galvanic corrosion is not a concern with ClampStar® Connector Correctors because a galvanic cell cannot be established. There are no pockets in which stagnant moisture can accumulate and the conductor grooves and interfaces are permanently sealed with CC², a proprietary high temperature inhibitor compound that will not wash out under any service condition.

(1)    U.S. Congress, Federal Highway Administration, and NACE.

Figure 1
Figure 2

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Inner Workings of an Automatic Splice and Using ClampStar as a Safety Tool

By, Carl R. Tamm

ClampStar is used as a tool by several utilities as a “safety” when performing line work on a conductor that contains an automatic splice in the span.

As a work method precaution, many utilities require the crew to install a “safety” over an automatic splice if any work is to be done on a live line, including changing insulators or crossarms, or any work during which the subject span containing an automatic might be disturbed.  Properly installed, automatic splices are predominantly reliable.  However, due to the number of incidents that have occurred, many times it is found that the conductor was not properly inserted.  The most prominent “improper insertion” is known as “partial insertion.”  In this case, for various reasons, usually because the installer failed to properly mark the conductor before starting, the conductor is not inserted to a sufficient depth to push the pilot cup completely through the jaws.

The problem then becomes that the jaws cannot close completely on the conductor, and obviously, it has a propensity to slip.

To state our position once again, “Properly installed, automatic splices are predominantly reliable.”  Some people have thought that CCI is pushing for the “ban” of automatic splices – but that is not the case.  We have also stated that automatic splices are “a must – for storm restoration.”  They are, without question, the most economical, and certainly the fastest means of reliably splicing a conductor.  Do other methods, such as compression connectors, offer a higher integrity connection?  I think that the manufacturers of automatics would agree that is the case – as they also offer the compression option – but nobody would argue against the importance of the “quick” means afforded by the automatic splice.

Because so many people ask, it seemed appropriate to share some of the inner workings of automatics.  While most people who use them know what is inside, perhaps it will assist them when installing these connectors, to understand the purpose of the components, and possibly give them a bit more appreciation for the manufacturer’s instructions!

The following is a picture of the “guts” of one side of an automatic – the other side is the same.

Beginning at the upper left and moving clockwise, we have (a) the spring, which provides a forward thrust to the jaw assembly during installation, (b) two jaws which interlock and make up the “jaw assembly”, (c) the pilot cup, which captures the ends of the conductor strands, and maintains their position (keeps them together) during the installation, and at the bottom, the yellow component is the “funnel guide” which serves to hold the pilot cup inside the assembly and guide the stranded ends of the conductor into the pilot cup – all within the tapered body of the splice.

To illustrate the action of the pilot cup, the following photo shows how the conductor, once inserted through the funnel guide, fits within the confines of the pilot cup.

Prior to insertion, having this side cut out from the splice, one can see the position of the jaws, urged forward by the force of the spring, awaiting insertion of the conductor.

Upon insertion of the conductor, having picked up the pilot cup as it passes through the funnel guide, the “resistance” that is felt during installation is the force required to push the jaw assembly backward against the spring.

As can be seen, the pilot cup serves to maintain the conductor’s position, centered within the jaws as it is pushed toward the center of the splice.

Properly installed, the conductor will carry the pilot cup completely throughthe jaws, to the center of the splice.  Because the pilot cup must contain the entire conductor, it is obviously of larger diameter than the conductor, and as can be seen in the following photo, upon passing the pilot cup completely through the jaws, the jaw assembly will “spring” forward, and the taper of the body will force the jaws into intimate contact with the conductor!  Additional tension on the device will serve to further “seat the jaws” and urge them into increased forced contact with the conductor.

Without question, 75-80% of the problems with automatic splices occur due to installation error, and about 80% of those are “partial insertion” errors, where the conductor is simply not inserted to its full intended depth such that the pilot cup has not “cleared” the jaws!

The result is that of the following photos. In the instance of the first photo, insufficient force was provided to drive the pilot cups into the jaws, and the conductor was simply not gripped.  If this occurs, the lineman will immediately recognize there is a problem, and take action to correct that, by installing another splice.

However, in the instance that follows, the pilot cup has passed almost to the end of the jaws, which is the most dangerous situation, as the tips of the jaws may capture the conductor, leading the installer to believe that the splice is made adequately, and the conductor may withstand the initial tension of the line, but the jaws are prevented from closing completely on the conductor by the invading pilot cup!  In such instances, the splice may be left in service for days, weeks, or even years, before some event jostles the line sufficiently or components of corrosion allow the conductor to slip out!  This condition may be impossible to detect with infrared techniques, depending on the current load on the line.

A second type of insertion error occurs from time to time, when upon inserting the conductor, the lineman pulls it back a bit (possibly to get a better grip because it was obvious that the conductor did not go it far enough – such as the condition in the previous photo and the pilot cup does not come back, as it is held in the jaws.  The purpose of the pilot cup is to keep the ends of the strands together, and assure that they all pass completely through the end.  In the scenario where the conductor is retracted as much as ½ inch, the strands can escape from the pilot cup, and one may erroneously slip through the side between the jaws.  In this instance, although the conductor may be fully inserted, the errant strand between the jaws prevents them from closing completely, and because it is not within the bundle, the overall diameter is reduced and again, the conductor can slip from the grasp of the jaws.

The critical concern is that neither of these situations are visible or otherwise readily detectable, and while the splice may hold the line under the stringing tension of a few hundred pounds, additional tension or other disturbance during work on the line may be enough to allow it to slip.

The old method of putting a set of grips and come-a-long on the line, could in its own movement, cause the line to slip before it is secure.  If that occurs, there is an arc flash hazard, along with the fallen conductor.  If one puts the MAC cable (ground set used as a jumper cable) on first, it can eliminate the arc flash, but may in itself drop the line.

Placing a ClampStar on the line in a gentle fashion allows both the electrical and mechanical safety to be applied at the same time, and is faster and safer to install.  If the splice appears bad, i.e., has burnt funnel guides or has evidence of obvious expulsion of contaminants (looks like black grease) running out of it, perhaps it would be wise to simply leave the ClampStar in place!

However, if there is sufficient confidence concerning the integrity of the splice, the ClampStar can be removed, and used again.

Have you ever looked up as you were driving, and noting a splice in the conductor passing over the road, wondered if that splice was one of many that, although it is there today, may have been improperly installed, and could “let go” at any moment with no warning?  Perhaps one might sleep a little better if they saw a ClampStar covering that splice, knowing that one connector had been “corrected”!

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