grounding?

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tsloan

tsloan

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this is my first post so please be gentle. I had a friend ask me a question today that for some reason cramped my brain. A contractor was using an enclosed trailer with numerous power outlets for saws and other power tools. the trailer has a 30 amp twistlock, I can only assume 4 wire, hooked to a portable generator due to an outage at the jobsite. the question was is this trailer safely grounded through the generator? where would the reference come from for the ocp to function properly? Since when its hooked up to the utility it is bonded but what about the generator? Im sure it happens all the time at rv parks and the such.may seem like a silly question to some, but i wanted to give him an educated answer and fell short in my attempt:blink: any input is appreciated.
 
tsloan said:
this is my first post so please be gentle. I had a friend ask me a question today that for some reason cramped my brain. A contractor was using an enclosed trailer with numerous power outlets for saws and other power tools. the trailer has a 30 amp twistlock, I can only assume 4 wire, hooked to a portable generator due to an outage at the jobsite. the question was is this trailer safely grounded through the generator? where would the reference come from for the ocp to function properly? Since when its hooked up to the utility it is bonded but what about the generator? Im sure it happens all the time at rv parks and the such.may seem like a silly question to some, but i wanted to give him an educated answer and fell short in my attempt:blink: any input is appreciated.
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If the generator is the only power source to the trailer, then you have to have a ground to neutral bond either in the trailer panel or at the generator (but not both). The NEC also requires a ground rod (or two) at one end or the other unless some other ground electrode is available.
The functioning EGC as a fault current return path is more important than the earth ground for safety, but NEC requires both.
 
The ocpd operates on line-to-ground fault through the EGC in the cords. The requirements assume the only non-current-carrying conductive parts that could be energized by a fault are connected to the system EGC. The entire system is essentially floating with respect to anything not electrically connected. Even though the system has a "chassis" ground, there is no reference to earth ground, so the entire system is comparable to an ungrounded utility supply.

For NEC reference see 250.34.
 
thank you for the reply! i guess the paperwork (or website) for the generator will tell him whether or not its bonded and the proper way to do so if necessary?
 
tsloan said:
thank you for the reply! i guess the paperwork (or website) for the generator will tell him whether or not its bonded and the proper way to do so if necessary?
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With only a 30A twist lock receptacle on the generator, I'd bet the neutral, EGC, and generator frame are bonded from the factory.
 
GoldDigger said:
If the generator is the only power source to the trailer, then you have to have a ground to neutral bond either in the trailer panel or at the generator (but not both). The NEC also requires a ground rod (or two) at one end or the other unless some other ground electrode is available.
The functioning EGC as a fault current return path is more important than the earth ground for safety, but NEC requires both.
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250.34 Portable and Vehicle-Mounted Generators.
(A) Portable Generators. The frame of a portable generator
shall not be required to be connected to a grounding
electrode as defined in 250.52 for a system supplied by the
generator under the following conditions:
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According to the OP

A contractor was using an enclosed trailer with numerous power outlets for saws and other power tools. the trailer has a 30 amp twistlock, I can only assume 4 wire, hooked to a portable generator due to an outage at the jobsite.
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thanks Smart $ for the code ref. ive been inside an industrial fac. for a while now and its amazing how fast you lose your ability(or aleast double the time) to find things in the code.
 
GoldDigger said:
... The NEC also requires a ground rod (or two) at one end or the other unless some other ground electrode is available.
The functioning EGC as a fault current return path is more important than the earth ground for safety, but NEC requires both.
Click to expand...
For portable and vehicle-mounted generators, a grounding electrode is not required under the conditions specified in 250.34.
 
But does that also exempt the trailer from any earth ground requirement? I did not think I said that it had to be at the generator end of the cord.
 
GoldDigger said:
But does that also exempt the trailer from any earth ground requirement? I did not think I said that it had to be at the generator end of the cord.
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The NEC does not cover utility trailers in general (i.e. Chapters 1 through 4). Article 250 only covers portable and vehicle-mounted generators. IMO, a portable generator mounted on a trailer would not qualify as vehicle-mounted.
 
SunDogAz said:
In this application wouldn't it also be likely to fall under 590.6, temporary construction :cool:
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It would. But how does it affect anything we've discussed here thus far? Most of that article just reiterates issues already covered in earlier chapters.
 
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