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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.

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Most are surprised to learn that Gold is not the most conductive material, but is significantly superseded by Silver!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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by CCostanzo

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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by CCostanzo

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 = αd^3/2) 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.

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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by CCostanzo