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Neutral Grounding-SDS for Standby Generator

06/21/2014 12:41 AM

If station service transformer (SST-240/120v-y/y solidly grounded) from utility and standby generator are in same substation ground grid then is it required to ground neutral at 1st disconnect of SST. As per code it shall be but most of case when neutral coming from outside not in same grid. It's separately derived system with manual transfer switch. How about bonding of transfer switch- is it only from one side if neutral is bonded at 1st dsconnect means of SST

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#1

Re: Neutral grounding-SDS for standby generator

06/21/2014 8:38 AM

If you can't interpret your local code for this situation then you must contact your local AHJ.

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#2
In reply to #1

Re: Neutral grounding-SDS for standby generator

06/21/2014 8:56 AM

Or a competent, licensed electrician.

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#3

Re: Neutral Grounding-SDS for Standby Generator

06/22/2014 12:22 AM

If the generator is an alternative to the normal utility feed don't you think it would be prudent to contact the utility company direct?

I've only dealt with UK systems where the DNO's (utility companies) are strict on earthing arrangements for standby generators. For some reason they don't seem to like their employees being killed by exported earth faults .

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#4
In reply to #3

Re: Neutral Grounding-SDS for Standby Generator

06/22/2014 12:33 AM

Hi Thanks for your reply. My generator is only emergency purpose which sitting in my substation for back up emergency power for scada system. We have station service from utility sitting same substation ground grid with separately derived system with generator. Now my concern as per NEC code if I ground neutral at 1st disconnect means of SST then from sst neutral point to that point neural would be parallel with ground grid since both are sitting same ground grid. So should we ground neutral at 1st panel of SST.

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#5
In reply to #4

Re: Neutral Grounding-SDS for Standby Generator

06/22/2014 3:38 AM

Why not simply ground the neutral always at the generator. Check that code is achieved with regard to resistance....

Never ever connect to the original mains neutral if one is supplied. I see no need. Treat is as a completely separate system, totally isolated from the mains supply.....the power company needs that as someone already mentioned....

Used modern CBs that drop your generator power if an earth fault current above say 10-20ma is found. See local code for full and correct values.

I experienced once 30ma at 230VAC, it hurt!! But it did drop the mains for the whole classroom!!! No lasting damage!!

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#6

Re: Neutral Grounding-SDS for Standby Generator

06/22/2014 7:21 AM

Your system sounds like a death trap.

The question frequently crops up "why can't I use the utilities earth rather than a nest at the generator set." As I said above, it can result in exported earth faults and some poor schmuck working on the utility system ends up dead. Likewise as Andy said the neutrals must be separated.

You asked "should you earth the utility neutral at the panel?" That isn't your responsibility so leave it alone.

To be honest, I don't think you should be working this system, I don't think you grasp some of the basics.

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#7
In reply to #6

Re: Neutral Grounding-SDS for Standby Generator

06/22/2014 8:55 AM

Good post.

You fully & correctly answered that question in that sentence of yours:-

You asked "should you earth the utility neutral at the panel?" That isn't your responsibility so leave it alone.

But do you think that anyone who could ask that question will understand fully your answer?

I don't......I don't think that he is even one grey cell more wiser than before....but what can you do???? You did your best. Thanks for that.

There is simply too much C**P spoken about neutral by too many on CR4....showing a decidedly lack of knowledge & understanding on the subject.....is it really that difficult I ask myself....?

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#8
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Re: Neutral Grounding-SDS for Standby Generator

06/22/2014 10:14 PM

It's not necessarily crap If the IEEE recommends using a common bonded neutral, is it?

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#9
In reply to #8

Re: Neutral Grounding-SDS for Standby Generator

06/23/2014 12:00 PM

As for the situation mentioned here. it does not need the generator neutral and the mains neutral bonded together, simply because as I understand it from the OP , the emergency generator does not run in parallel with the mains, ever...they are therefore two separate entities...

You need the neutral bonded to ground for safety reasons on single phase equipment, so when the mains fails, you would be bonding "generator" neutral to some sort of local ground. Bonding the two neutrals would then mean a neutral loop (mains neutral bonded twice to ground) and possible problems for the mains when it returns....

Many forget that the neutral point can have voltage on it if all phases are not exactly balanced, which they seldom are of course!! Imagine the situation if a phase fuse blows and the other two phase fuses remain whole.....this situation must be covered and the only way that I know of is to bond it to a good ground.....

If there are other better ways, please inform me.

By the way, multiple grounding of neutral is, as far as I can tell, only allowed in some regions of the USA. I could not find for instance, a single European country that allowed it......they reckon its too dangerous!! I wonder why? Please correct me if you have other information...

Generally speaking, neutral and ground should only be bonded once.....most countries agree on that it would seem...

If you are right and mains and generator neutral MUST be bonded, then I would even go as far as to say that the IEEE is wrong on that score and some linemen might of backed me up......

Can you cite the paragraph in question and link us to an online copy please?

Thanks in advance.

Maybe we need to mention to IEEE to make it clearer for ALL situations....but I would like to read what they say first.....thanks for your help.

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