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How dangerous is it to leave 35KVA diesel generator ungrounded?

This generator is connected to temporary metal buildings for workers, AC and other electrical devices.

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  • Depends, is that a 1V, 35kA generator. Also are you sure it won't float to silly voltages, also you sure accidents won't happen. How good is your insurance? – JonRB Aug 10 '17 at 13:37
  • temporary structures and (cheap) workers - permanent graves... This should be properly earthed or grounded as per the manufacturer's instructions and / or relevant codes in the area. – Solar Mike Aug 10 '17 at 13:39
  • Is the generator the sole source of power for the buildings? Is there a utility electrical system that is grounded? – ArchonOSX Aug 10 '17 at 15:51
  • If you are providing your own private power to your own private buildings via your own private lashup, you should seriously consider grounding the metal buildings also. – A. I. Breveleri Aug 10 '17 at 15:53
  • If there is already a grounded electrical system and the neutral is NOT transferred through a transfer switch, then it does not need to be grounded. – ArchonOSX Aug 10 '17 at 15:54
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"not grounding things" has been tried. Gets people killed. The problem is, what happens in case of a ground fault? That is where current leaks from one of the hot wires to a surface you can touch. What keeps a ground fault from energizing the entire side of the building?

Without a proper ground, you leave to Providence what happens next. And lots of people get electrocuted this way, and it starts fires. Your generator has a steel skid or landing gear, yes? What happens when it rains? Now the impedance between your building and generator case is much lower, and now electrocution risks exist that weren't there in the dry. With a ground,

  • surfaces and structures are prevented from floating at high voltage,
  • a bolted fault will have high-current path back to source, assuring a circuit breaker trip.
  • a smaller ground fault will complete its loop through the grounding system, resulting in a reliable trip of GFCI/RCD (ground fault aka residual-current) protection.

Why not an isolated system? Two problems. #1, you need a maintenance program wherein staff electricians regularly checking for your first ground fault, or else you'll be blindsided by the second that completes the circuit. #2, you must engineer it to be an isolated system, which means you can't use common, off-the-shelf gear intended for grounded systems - it is disallowed to use gear contrary to its labeling and instructions (since it was only tested and listed for uses conforming to the instructions). The world of the Electrical Code is based on hard-won experience.

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    This answer is good, Harper, except the first paragraph doesn't sound quite right. With an ungrounded generator you have an isolated supply - similar to an isolating transformer. What happens in the case of a ground fault is "nothing" - for the first one, at least. The problems occur with the second fault. – Transistor Aug 10 '17 at 18:09
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    In Codeland, everything is based on the lessons of field experience. Isolated systems should have a facilities department actively looking for the first fault. Otherwise you are blindsided by the second. Also you need to architect the system to be isolated - not use COTS gear intended for grounded systems. That generator has a steel skid or landing gear, yes? What happens when it rains? – Harper Aug 10 '17 at 19:23
  • Exactly! Add it into your answer. – Transistor Aug 10 '17 at 19:25
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Temporary structures and (cheap) workers - permanent graves...

The generator should be properly earthed or grounded as per the manufacturer's instructions and / or relevant codes in the area.

And so should all the structures and any other electrical devices.

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It depends on the generator but they do not recommend grounding 'for fun'

I would strongly suggest you ground it, so you don't get all kinds of random stuff happening, or worse, people getting killed.

Some reading: http://www.ecmweb.com/content/solid-grounding-your-generator

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