Generator 1

[pithpal]

Hi Everybody,
I was wondering, if you could comment on a suggestion made by my senior engineer. If we have an emergency diesel generator, and it is star connected but not sure if it is 3 oh 4 wire or only 3ph 3 wire, and it is connected to an ASCO power transfer switch. Now, through this switch there is a panel with disconnects which is hooked to the elevator but there are some spare disconnects available, which we can use for emergency lighting. Now this 3 ph 3 wire star is 416 and we need 240 V.
He suggested that we just use the building ground as the neutral and make it as 240 V and then use for lighting panel. Does this sound all right ?. I beleive it, but does it violate any code. Any word from you will do some contribution.
Regards,

 

[wareagle]

The NEC does not allow the grounding system to be used as a conductor. Remeber in the wiring system the EGC(green wire) is seperated from the neutral. If the generator has
no neutral terminal you won't be able to get the 240 volts except by using a transformer 416 volt to 120/240 or 416 to 240.

 

[mpparent]

A senior engineer is telling you that? Time to get ask for a new mentor.

Mike

 

[rbulsara]

Senior by a day??

This is a no- no, both by engineering and Code point of view.

Have to run the neutral from the generator to the ATS. If the ATS is 3 pole, neutrals of the both source should be connected together at the ATS. Assming your primary source is a utility with a grounded neutral, the generator neutral should NOT be grounded separately.

If the ATS is four pole (netural is switched), ground the generator neutral separately at the generator as well to meet the Code.

 

[pithpal]

Thanks for your comments and a prompt response. Here is what I heard after I explained your views to my engineer

He said, he is sure the generator is STAR grounded and the ATS is 3 pole. and we can use the ATS equipment ground and pull it in the same conduit as conductors and use this as a neutrol on Disconnect switch and make 416 V as 240 V.
Is he right now. ? as he also stressed that if the generator is grounded then we can use any gound to create phase voltage.
Please advise
Note: Comment from Rbulsara: Please put a word for: Switched Neutral, If ATS is 4 pole: What does this mean?
and I beleive if it is 416/240 gen, It should be grounded.
otherwise we cannot have 240 V.

Thanks once again for the responses.

 

[wareagle]

"He said, he is sure the generator is STAR grounded and the ATS is 3 pole. and we can use the ATS equipment ground and pull it in the same conduit as conductors and use this as a neutral on Disconnect switch and make 416 V as 240 V."

You may NOT NOT NOT use the equipment grounding conductor as a neutral. Your engineer may be usig the word "ground"
and is referring to the neutral. If you need a neutral you need to install one. Size as required.

If your ATS DOES NOT switch the neutral, you take the existing system neutral and connect it to the generator neutral terminal. Size as required. You then take the system EGC (green wire) and connect to the frame of the generator. You then remove then generator neutral to frame bonding conductor.

"Is he right now. ?" NO NO.

"He also stressed that if the generator is grounded then we can use any gound to create phase voltage.
Please advise"

Well 416 volts/1.73 = 240 volts.

"If ATS is 4 pole: What does this mean?" You don't have to switch the neutral even if its a 4 pole ATS. No need to.

 

[rbulsara]

It is true that if the generator is grounded, you can measure L-N voltage any where in the system, BUT this does not mean that you can use ground as neutral. This is Code and safety violation. Ground conductor is not supposed to caryy current in normal (unfaulted) condition.

Well, 4th pole alllow you to switch the neutral. As warengle said you do not have to use this. But if the generator is seperately grounded, then you have to use a 4 pole switch so that you do not ground the neutral of a sytem at more than one point. This makes ground fault prorection not work properly, plus it could be code violation.

If the system is not a seprately derived system or you can make it a non-separately derived by removing N-G bond at teh generator, you can use it the way I described in the last post.

 

[davidbeach]

This is a bad installation. The best way of solving your immediate problem is to install a transformer to get a proper 4 wire system for your lighting needs.

I wouldn't trust any advice out of a "senior engineer" who would suggest using an equipment ground as a current carrying conductor. Time to start looking elsewhere for other employment opportunities.

 

[pithpal]

Hi Everybody,
Thanks for your suggestions and it worked. He eventually agreed.
but I have a question now, In case of utility distribution system, when we use a pad mounted single bushing transformer, we just take one line tap and then ground the transformer primary and centre tap of secondary... Now
the same thing is happeneding we are using ground as a return path for primary. So how do we justify this installation.
Regards,

 

[davidbeach]

Grounding in medium voltage (and higher) is different than it is for the low voltage distribution system. Typically the utility will ground the neutral at many locations, while on the low voltage system the neutral is only grounded at one location.

 

[pithpal]

Thanks Davidbeach. So what I infer from your statement is that grounding principles are different in HV and MV systems. Is that correct ?
Honestly this statement confused me, but I would not question your authority over the subject but would appreciate if you could elaborate more.
Regards,

 

[pithpal]

A few more points :
Please correct me. In case of
substation transformer and then the next step distribution, the main aspect taken in to consideration is Over volatage and then the second is ground fault current which is decided by the Transformer grounding neutral. Now if we say if the distribution line neutral is multigrounded, that means overvoltage will be less duing a ground fault. This is all few concepts I know, how we can relate this concept with Low voltage system in case of grounding neutral at one point or many points.
Thanks

 

[davidbeach]

The biggest difference between MV grounding and LV grounding is that at MV, the neutral is also used as the equipment ground, and is grounded in multiple locations, while in LV systems the neutral and the equipment ground are two entirely different things and are not to used interchangeably. Ground fault sensing in MV systems almost always only looks at the three phase conductors and is set to accommodate normal system imbalance while ground fault sensing in LV systems almost always includes the neutral conductor and does not need to accommodate system imbalance.

 

[pithpal]

Thanks everybody for your valuable suggestions. As from my questions you can figure out, I am new to this area, just a doubt I have and would appreciate a word from you.

Q 1 The question comes from my thread: When from a 3 ph 3 wire disconnect, we are taking a 3 conductors out and a neutral from the automatic transfer switch from where main supply to disconnect is coming. In a EMT conduit, we are pulling 3x 1/0 AWG and 1 neutral and then taking it to next floor to connect to a single phase lighting emergency panel.

My question is a single phase light requires a line, neutral and a ground. Do we have to pull a ground(bare copper wire in the conduit also) or we can just connect a ground wire to conduit at the basement and on first floor we just connect another bare copper wire to conduit and use it as ground.
Is this allowed. if not why?

Regards,

 

[davidbeach]

By the NEC, you can use the conduit as your equipment grounding conductor. I would never do it though, I think it is a BAD idea that it is allowed by the NEC, but people with vested economic interests continually shoot down proposals to require an equipment grounding conductor in every conduit. I've seen too many conduit installations where it is obvious that the conduit won't carry much fault current.

If, on the other hand, you are talking about a hospital, the NEC does require an equipment grounding conductor in every conduit, so you could not use the conduit alone as the equipment ground.