Grounding temporary generator installation

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david.mullins

david.mullins

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Location
Santa Barbara, CA, USA
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Electrical Engineer
Hello,

We have a large machine that's used at sea. We are developing it in our concrete paved facilities yard. It consists of up to 5 sections, each section is a control panel and a machine. There are servo drives in the panels and servo motors on each machine. It all operates from 3ph 480VAC. Each section requires about 70A continuous with 450A peaks for less than 1 second duration. At sea, it will be powered by generators on the ship with our generator as backup. In our yard, it is powered solely by our generator, a Marathon Magna Plus 280KW, 480VAC high wye, 0.8 pf, 60Hz. We connect it to each panel with 2AWG, 3c + G SOOW cord. (this was all setup before I was assigned to the project.) It is protected by GE Spectra RMS MCCBs with 80A trip rating plugs. We operate it for test once or twice a month

My question is, since this is a temporary installation in a paved concrete yard, is this legit without driving ground rods into the ground or does it qualify under "ungrounded systems" with the grounding conductors in the cable?
 
Since it's a WYE system you probably cannot operate it ungrounded. Two ground rods are all that you need. IMO a properly sized system bonding jumper is more important than a grounding electrode.
 
You are essentially a utility providing power, or a separately derived service. You will need to have a grounding electrode system and a system bonding jumper from the equipment ground to the neutral, but see below.
Your question about an ungrounded system needs more details. On board ship, electrical systems are ungrounded IE no system bonding jumper. If you are operating an ungrounded system you will need to provide ground detection. You mention grounding conductors in the cable, are you referring to an egc?
 
The three phases come out of the generator into the breaker and the green wirefrom the wye center, goes to a terminal beside the breaker. Our cable to the panels is a 4 conductor, 3 phases with a green conductor. We have all balanced loads except for a 5KVA single phase step down transformer across two phases that provides 110VAC. So, as I understand it, we should have no current in the green wire. I believe that makes the green wire our EGC.

Shipboard, we weld bases for everything to the ship deck. There are bonding jumpers that are supposed to be installed to the welded base. We do not have ground detection, currently.
 
A couple of things-
Since the bases will be welded to the deck, I'd install bonding jumpers between them to simulate the deck.

For NEC compliance, it's a separately-derived system- I agree that you need rods and a bonding jumper.

Where is the green conductor connected at each end?

But.... that's not how things operate on a ship. If the ship is an ungrounded delta system, I'd tend towards building that but with ground detectors. And add a healthy does of placards and warning signs to everything.

Got a diagram we can look at?
 
Not that I'm allowed to share, sorry.

Three phases from high wye generator windings with green wire ground from wye center to generator circuit breaker and ground terminal.
Three phases in cable from generator breaker to panel with green wire from generator green wire.
Three phases to all loads in panel. No loads from phase to ground or green wire.
Cable green wire bonded to panel ground and internal bus bar. All devices in panel grounded to bus bar.
 
Ok, I really appreciate all your help! You made me do some digging and I appreciate that, too. I wish I had as much time as I need to read code so that's where you guys are invaluable!

Or generator is not protable by definition (one person cannot move it without mechanical assistance). It's not permanent, either. BTW, NEC only defines portable under the 1000V or greater definitions section, not in Article 100 definitions, General.

Our machine is not a building or structure, either, per article 100 defs. Therefore, I don't believe 250.32 grounding requiements apply. It's not vehicle mounted although it certainly could be but then we'd have to install receptacles on the generator to meed 250.34(B).

That pushes us back to 250.30(B) Ungrounded Systems wherein the grounding requirements for ungrounded systems are defined. So, if you ground it, is it an ungrounded system or not???
 
infinity said:
Since it's a WYE system you probably cannot operate it ungrounded. Two ground rods are all that you need. IMO a properly sized system bonding jumper is more important than a grounding electrode.
Click to expand...

Actually, according to the 2014 NEC 250.20(B), this system might not need to be 'grounded'.
(1) it exceeds 150V L-G
(2) the neutral conductor is not used as a circuit conductor
(3) the system is not a 4-wire delta.

It may also fall under 250.21 (A)(3) it exclusively feeds adjustable speed drives (not enough data from the OP to make a judgement and ground detection would be required.)

But the above is not important, according to 250.4(B)(1) even equipment installed on ungrounded electrical systems must be 'connected to earth'.
 
Where did my last post go? Did I say something wrong and it got deleted? It ended with me deciding to be on the safe side and do it.

I since located the mfrs installation instructions which say it definitely must be grounded.
 
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