Neutral to Ground in Three-Phase Generator

Have a look in the Wiki article on earthing systems.

<...asking dumb question...>

The only dumb question is the one that is never asked.

Is the generator the main supply of power or is it emergency power. If emergency, do you use an automatic transfer switch? When you say grounded, do you mean connected to a ground rod? Do you use the NEC requirements?

It is the main supply of power. It is not emergency power. It used daily as main power for powering lab equipment handled by humans continuously.

Generator's have to be ground/bonded. Motor should be grounded to frame, A/C alternator casing should be grounded to frame, if it comes in box/set that should be grounded to a rebar post box (which is in the ground 2-3ft)Neutral wire is not your ground, if exposed to the elements, it will catch on fire. Neutral wire is for energy return to generator. Remember this a/c current and if it is a diesel engine, it runs best when it runs hard, under loaded is just as bad as over loaded.

Hi PITB.

I presume you are still referring to the situation in your other posts.

Clearly grounding a mobile gen in the middle of an airfield tarmac apron would be a challenge. See Garth's answer to your previous post.

The "earth" wire on your gen should be really referred to as frame. that means, as wareagle says that all exposed metal parts are bonded together so that no one part can be at a different voltage to another part. This also extends to connected equipment. So the earth wire in the cord should connect the frame or case of your load to that of the gen.

I would like to know what the USAF manual says about the subject. Ultimately that will be the rule you will need to follow, regardless of what advice anyone here will give you.

regards

Chas

thanks chas,

I wish i could locate the manual. I found an old rescinded manual that says that the power generator must be a grounded and neutral must be connected to ground as well.

The generator is not grounded to earth, just chassis, and that neutral is not connected to ground. I think if the neutral was connected to ground (or chassis) then the power generator would have tripped and the power surge on neutral would not have occurred. ...but that's what i am trying to figure out.

Butt

If you have a phase to ground fault, then there would have been no path for the fault to return. You can not count on earth as a conductor. The fact that the neutral is not bonded to the gen fame would not have made a difference. You must rely on the bonding of the entire system all the way back to the main disconnect. Then you will have a return path back thru the neutral to the generator.

Hi Wareagle,

look at what this lass is doing. she is running routine tests &/or maintenance charge from portable gen to another portable (aviation) equipment. There is no main system here. Her "system" consists of three items, the gen, a tester, and something else.

In this post PITB mentions that the UUT as she calls the end device is grounded so, i suppose it would be practical to take a flying earth lead to the earth point of the load and connect it there before attempting to connect the gen. But what extra protection would that give? All earthed connectors I know always engage the earth pin or sleeve before the live contacts touch.

regards

Chas

Chas

If this is so, then heven help her or him. Sounds like all they need is a few extension cords.

Butt

If what Chas says is true, Then you need to get an electrician to take a sheet of plywood and build you a power distribution board that is portable with a power panel with breakers and plugs to use with your equipment. The genny then can power the distribution board and all will be well.

Hi,

First of all it is not a dumb question.

1. Generator should be grounded. It is a very bad idea to ground generator through any device or machine connected to it.
2. It is your misconception that generator is grounded through any item in true sense it is not.
3. So, what to do next? As it is a 3-phase generator it s neutral itself is acting as a ground so its neutral should be grounded to properly ground the generator.

Adam,

Electical Engineer.

so the generator is not really grounded through the item?

thanks,PITB

PITB Quote "So the generator is not grounded through the item"

What does that mean?

Chas quote

"Hi Wareagle,

look at what this lass is doing. she is running routine tests &/or maintenance charge from portable gen to another portable (aviation) equipment. There is no main system here. Her "system" consists of three items, the gen, a tester, and something else."

PITB

Is Chas correct in what he says. If so, take my advice and build a power distribution board. Then you will be set. No more problems.

War eagle, You have misunderstood. The item IS connected to ground. My statement was a reply to number 7 Guest. Guest says that even though the item is grounded, the generator is not receiving that as ground. I was asking him: "So the generator is not grounded through the item?" you forgot the "?". See guest at #7. I hope you and Guest can come to a consensus on this because I would like to know.

Yes I do have a generator, a tester, and a UUT. The tester is connected to safety ground and the UUT is connected to same. but the generator is not.

.thanks for helping wareagle! Thanks again, PITB

Thank you all so much. I need to doublecheck but I am pretty sure what I saw was the tester connected to a ground bus bar and the UUT connected to the same. I can see this info is critical and I must check again.

PITB

Here is the question that needs to be answered. Are you doing these tests inside a building that is wired and receiving power from the generator or are you outdoors doing the test using a generator to power your test equipment?

We are in a building, testing units, receiving power from the mobile power generator that is outside. We connect to the outside generator with long cables.

Thanks -PITB

You said "Yes I do have a generator, a tester, and a UUT. The tester is connected to safety ground and the UUT is connected to same. but the generator is not." What do you mean by safety ground? Physically what are you doing? What is UUT?

To get power you are plugging the cables into receptacles on the generator and then plugging the equipment. Correct? What is your understanding of grounding the generator? What is the voltage that you are using?

The tester frame is connected to a facility ground lug and the UUT is connected to same, the generator is not grounded to the facility ground, it just sits out there on wheels.

Yes we are plugging one end of the cable into receptacles on the generator and then other end to the equipment. The power is 3 phase 115 V.

The neutral terminal on the generator should be bonded to the frame. You can ground the generator frame to the facility ground. I do not mean a connection to a ground rod. It should be to the structrual steel or water pipes. You need to check to see if the receptacles on the generator have a ground pin. You can take an ohm meter and see you have a connection between the gen frame and the receptacle ground pin. Get and electrician to do this.The receptacle should look something like this.

The ground pin is the on at the bottom. This is a 120 volt rec.

You need a 3 conductor cable to connect to this rec. One is the hot wire, one is the neutral and one is the grounding conductor(usually green). This is termed the equipment grounding conductor(EGC). Make sure that the grounding conductor is connected to the grounding pin at the rec and to the equipment rec end of the cable. If this is done correctly you will not need to connect the equipment to the ground terminal. The EGC will do that back thru the generator. If this is an on going project, I believe you should take my suggestion and build a power board. That way you can ensure you have good grounding connections. The generator can feed 3 phase power to the power board. I tried to find a drawing to explain this but I could not find one.