National Electrical Installation Standards

Standards as High as Your Own

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Tuesday, March 5, 2019

Question:

Dear CQD: Most of our manufacturing equipment is supplied by a 480V ungrounded delta system. The busways have only the three phase conductors, with no grounding conductor in the bus. The manufacturing equipment is "grounded" (?) by properly-sized equipment grounding conductors that are connected to field-installed ground lugs screwed to the metallic bus plug enclosures, using whatever type of machine screw and nut is handy (few of the bus plugs, if any, have a factory-installed grounding means in them). I don't know how the busway enclosure is grounded, but the bus plugs are clamped to the painted busway via painted clamps. Almost every machine comes from the build shop with a 120V duplex receptacle (sometimes GFCI protected), supplied by a 480V x 120V step-down transformer, and protected by a 10A or smaller OCPD (receptacle is intended for powering a programming terminal). The receptacle's ground wire AND grounded circuit conductor are both connected to the X2 of the transformer. In turn, the X2 of the transformer is connected to the backplane/enclosure of the manufacturing equipment. This is a very common arrangement is manufacturing plants.

With regard to grounding and bonding, what code violations come to mind with this type of installation, for both the 480V and 120V circuits described, if any?

Thanks, Wes Nichols

A

Answer:

Hey Wes thanks for your question. You don't mention when the equipment was installed but many thoughts come to mind and here are a few of them.

For listed busways, the enclosures and, in some cases an additional ground bus, are intended for use as equipment grounding conductors as stated in Busways and Associated Fittings, CWFT.

http://productspec.ul.com/document.php?id=CWFT.GuideInfo

Check your busway manufacturers literature, it that is available, or contact them if they are still in business to determine if the busway enclosure is intended to be used as an equipment grounding conductor.

A fault current path is required for ungrounded systems as stated in 250.4(B)(4). A requirement for ungrounded systems to have ground detectors was added in the 2005 NEC in 250.21. The rules in 250.8 and 250.12 might not have been the same when the installation was made but they are very important.

If you meet the conditions in 250.30(A)(1) Ex. No 3 and 250.30(A)(5) Ex. No 3 for Class 1 circuits, the 120 volt circuits might be acceptable.

Beyond the NEC, OSHA has some retroactive rules that can apply such as in 1910.304(g)(1)(ii), (g)(1)(iii), (g)(1)(iv), and (g)(1)(v) and 1910.304(g)(5).

Finally, when a fault occurs will people be exposed to hazards, that is a concern that should be considered.

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