WILEY

WEEBLug Story...

Problem – no existing lugs meet UL467 when bonding to PV modules

Extensive testing was required to develop the new WEEB family of bonding products. During the testing, it was necessary to connect wires to the power supply which produces the current levels called out by UL467, the standard for grounding and bonding equipment. There is a style of lug popular with solar installers which is available from ILSCO and Burndy. John Wiles, in a report sponsored by Sandia National Laboratories, http://www.nmsu.edu/~tdi/pdf-resources/PV=NEC_V_1.3.pdf, suggests mounting the lug to a module with a 10-32 stainless steel screw. When the lug was used in the suggested manner, it failed before the bonding equipment under test.

Test Results:

During testing we were able catch some interesting video footage (You will need Quicktime to view the video) which showed where the lug failed. The photo sequence below shows snap shots at the critical points in the video.

Both the video and the photo sequence show the lug bolt glowing red hot after 2 seconds and completely shearing off at approximately 4 seconds. If you watch the video closely you can see the lug shift to the left just before 3 seconds have passed, indicating that the aluminum around the bolt threads has melted and allowed the bolt to move.

Under repeated testing we witnessed cases where the bolt head actually flew off the lug a landed on the floor, engulfed in flames. Can you imagine this same situation happening on one of your roof mount installations?

Investigation of the test results produced the following facts:

1. UL467 section 3.6 limits the wire size used with a 1/4 inch screw to 6 AWG so the lug should only be rated for 14-6 AWG.
2. In the suggested mounting method no current travels through the tang of the lug. All the current is carried from the top of the tang, through the length of the screw and finally from the screw threads into the aluminum.
3. Since the screw is stainless steel it has fairly high resistance. The screw is able to conduct 470 Amperes (UL467 rating for 12 AWG wire) for 4 seconds, but fails when carrying 750 Amperes (UL467 rating for 10 AWG wire). The combination of lug and stainless steel screw should therefore be used with a maximum wire size of 12 AWG.

Further investigation with ILSCO produced additional facts:

1. For UL approval testing, the lug was mounted on a steel plate. The suggested mounting method is therefore not in accordance with the listed use of the lug. To use the lug properly then, the aluminum oxide should be scraped off the mounting surface before the lug is mounted.
2. For UL approval testing, the mounting screw was tightened to 110 inch pounds. This torque level is far in excess of what a thin PV module frame will support. Again the suggested mounting method is not in accordance with the listed use of the lug. There does not appear to be a way to obtain this level of torque with the suggest mounting method.

Conclusions:

1. Lugs popularly available are not being used in a manner consistent with their listing and therefore do not meet code.
2. A 10-32 stainless steel mounting screw should only be used where the required equipment ground wire is 12 AWG or smaller.

Additional comments:

The current levels called out in UL467 are fairly high, though of short duration. One might argue that for PV installations the current will never be so high because the modules are a current limited source; however, for those installations where the inverter is located near the modules, there is a possibility of AC faults which can generate this level of current. In addition, a good low impedence connection is desirable to give the best protection from lightning induced currents.

Solution - the WEEBLug

The patent pending WEEBLug consists of a washer and lay-in lug. The WEEBLug is mounted using a 1/4-28 stainless steel screw. Specialized teeth on the washer are constructed to embed into anodized aluminum and establish a gas-tight electrical connection. To obtain the lowest impedence path for the bonding connection, the washer is constructed of thin stainless steel. The current path through the aluminum, the teeth, the washer body and into the lug is therefore very short. The lug is constructed of tin plated copper for corrosion resistance. Wire is clamped by a stainless steel screw on the lug, which is horizontal to the tang for easy access when mounted under a PV module. The screw is a 1/4-28 size for easy wrenching. The WEEBLug comes with hardware for mounting to a rail or 1/4 inch through hole.

How is this safer?

The WEEBLug and other members of the WEEB family are design to conduct thousands of amps safely. When they do fail - at extremely high currents - the WEEB washers fail in a safe manner. Most of the heat is generated right at the teeth so they and the surrounding aluminum melt and open just like a fuse. Because the washer is sandwiched between two large metal pieces, the small amount of melted material is safely contained.

For more information, check out our page on rail electrical splice kits.

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