Trailer mounted gen set grounding/bonding

[slipknotbmg]

Hey all, been a while got a new job w/ the county woo hoo. I am working on a 120/208v and 277/480 wye 3ph 4w trailer mounted generator. It is used to supply temporary power to sewage lift stations. Recently we hooked it up during a power outage and the gen housing gave us a slight tingle. The fella I was with said its been that way for 20 yrs. And they used to carry around a ground rod and show t in the ground. Well I was worried, lol. We closed the gate and when the gate hot the tongue it looked like someone was welding. Anyhow I went back to the shop and said we gotta fix this before someone gets killed. So it was put on me to correct this. I found that the gen is bonded to the trailer frame, neutral is not bonded to the ground, and they were connecting their ground to the neutral lug on the gen and ground at the station. All the stations are 3ph 3w and have a manual xfer sw. To connect the gen during an outage. Anyone have any ideas why the genset casing was "live" and what is the safest way to set this up. I'm thinking just bond neut and gnd at genset. Do you think that will fix the problem?

 

[e57]

What state or code cycle? Is the generator a seperately derived system? (Switched Neutral at the transfer switch?) Is there a service, or feeder from a utility for regular power? (I assume there is..)

 

[slipknotbmg]

Ohio here, and no the stations are 3ph 3w so no neutral switched. Each station has a 3 pole 2 position center off manual xfer switch

 

[e57]

05 NEC?

Hmmm.... Thats a tough one - sounds as though you would need to take a 4W w/Gng to the first disconnecting means(service), and bond the neutral there - even though you're not using it. This would make it not seperately derived, avoid this temp rod, and provide an EGC. Otherwise, I'm not sure how this is classified as 'seperately' or 'not seperately' without the neutral involvement????

I'm sure others will chime in.

 

[tom baker]

Code cycle doesn't matter, rules have not changed on this, but the 08 NEC adds a great deal of clarity.

Review the following:
Defintion of separatelty derived system
Art 250 defintion of system bonding jumper
All of 250.34 and 250.35

The generator does not require ground rods as you don't transfer the neutral. It does require a system bonding jumper from the neutral to the case of the generator, the function of this is to get the fault current back into the generator for a line to case fault.

From the generator to the WWLS inlet you need a 5 wire connection, 3 hots, neutral and ground.

Is the generator connected as a Wye or Delta. If delta, then we have an interesting situation, but your lift station is Wye so the code rules above apply.

It would help if you could draw this out hots, neutral and ground.




















Your generator is seperatly derived so no ground rods needed.

 

[ceb58]

Code cycle doesn't matter, rules have not changed on this, but the 08 NEC adds a great deal of clarity.

Review the following:
Defintion of separatelty derived system
Art 250 defintion of system bonding jumper
All of 250.34 and 250.35

The generator does not require ground rods as you don't transfer the neutral. It does require a system bonding jumper from the neutral to the case of the generator, the function of this is to get the fault current back into the generator for a line to case fault.

From the generator to the WWLS inlet you need a 5 wire connection, 3 hots, neutral and ground.

Is the generator connected as a Wye or Delta. If delta, then we have an interesting situation, but your lift station is Wye so the code rules above apply.

It would help if you could draw this out hots, neutral and ground.




















Your generator is seperatly derived so no ground rods needed.

I think I would disagree. I dont think the gen. set he has described will meet any thing in 250.34B

 

[e57]

FYI there are some changes '05-08 for instance 250.35 does not exist.... And wording changes to 250.43.

The generator does not require ground rods as you don't transfer the neutral. ~~

I disagree with this specific statement unless in the same or related sentance mention an EGC required as an alternative to rods. But that said wouldn't the neutral then be required to be bonded at the load side equipment? As defining seperately derived?From the generator to the WWLS inlet you need a 5 wire connection, 3 hots, neutral and ground.
~~~~
Your generator is seperatly derived so no ground rods needed.[/quote]Again is this not dependant on the neutral arrangement?

 

[benaround]

Recently we hooked it up during a power outage and the gen housing gave us a slight tingle. The fella I was with said its been that way for 20 yrs. QUOTE]

Maybe after 20 years, the problem is in the gen set ? Question: when you install the

ground rods, does the arking and sparking go away ?

 

[RUWired]

It appears that this gen set is supplying equipment other than the receptacles on the unit ( connection by transfer switch), and this equipment does not require a grounded conductor. Therefore the generator is a SDS and would require a grounding electrode system be installed because it does not meet the requirements in 250.34B.

 

[e57]

OK- Path and ref time....
('08 code as ref)
250.34B - requires a rod at generator.
250.34C requires the neutral be grounded/bonded.
250.30A1 requires neutral bonding be at the source or at or before the first disco if applicable. (I assume in this case at the generator - as no disco means???)

Now heres where I'm lost....

Where/what is the ground connection between the premise and generator wiring, and what size? (Which code ref?)

IMO it should be sized 250.66 based on generator size - Right? Is it a EGC, or GEC?

 

[RUWired]

Where/what is the ground connection between the premise and generator wiring, and what size? (Which code ref?)

article 250.32(A) requires a grounding electrode at the seperate building and (B) requires an equipment ground be brought to the building or structure.

IMO it should be sized 250.66 based on generator size - Right? Is it a EGC, or GEC?

It would be an equipment ground after the OCPD on the gen set sized per 250.122.

Rick