Two Portable Generators in Close Proximity to Each Other

[Green Electric]

Hello all; new to the group. Electrical contractor in the Rocky Mountain Region. I wish to open a discussion on the hazards of having two separate portable generators operating in close proximity to each other. An electrician I worked with at a sign company once told of an incident when he was in the US Army whereas there were two truck-mounted generators parked close to each other. A technician was stepping from the bed of one truck to the other, and grasped both metal headache racks as he did so, and was sadly electrocuted.
Since then I have been very vigilant whenever I see multiple generators in use. Especially so at park gatherings and such where children are running around. What are your thoughts on this danger? I would like input as to how exactly such a dangerous potential can build between frames of generators. My understanding of this could be called the simplified version, if that makes sense, and I wish to have a greater understanding. Thank you.

 

[petersonra]

I do not believe there is any special danger. It is likely that the incident relayed to you if it actually happened was a result of some failure of one of the generators and not because they happen to be close together.

 

[Green Electric]

I do not believe there is any special danger. It is likely that the incident relayed to you if it actually happened was a result of some failure of one of the generators and not because they happen to be close together.

Bob, thank you for the response. That is interesting, and brings up a follow up question. As I picture the operation of a generator, I am trying to understand how is it that the frame doesn't develop a potential different from, say, someone standing on the Earth near the generator. I suppose this is related to two generator frames developing similar potentials to each other. So, while generating electricity, how does a freestanding generator, or truck-mounted generator frame have a similar potential to Earth ground?

 

[LarryFine]

If there is no connection to a system grounded or grounding conductor, there is nothing bonding them.

 

[zbang]

Lets break this down a bit-

"Portable" can mean a 700w lunchbox, a 6kw "luggable", or even a 75kva trailer-mount. (We used to joke that "navy portable" meant whatever it was had handles.)
All generators should, somewhere in the wiring system, have a neutral/EGC bond. That might be in the equipment they're supplying (e.g. a building switchboard).
Sometimes, the EGC also needs to be bonded to a GES.
Multiple generators can operate quite safely next to each other, that's done all the time.
If multiple generators are supplying the same structure or equipment, then code would require that their EGCs be bonded.
Small generators via cords & plugs supplying things like bounce houses or a small PA would be the least of my worries for bonding together or to a GES.
As with everything else, poorly maintained gear is dangerous however you use or connect it.

IME, military setups often ignore safety in the name of speed or "it'll never happen here" (then it does).

 

[tom baker]

Need more info on the Army generators. Was the secondary voltage ungrounded and did they rely on ground rods?
See section 525.11 for generators at amusement parks where generators are bonded when closer than 12 ft

 

[zbang]

The few military generators I've seen had a handful of switches to do things like connect/break the N/G bond, disable the AVR, etc; all to keep the power supply working on the battlefield.

 

[Green Electric]

Excellent replies, and this has helped me to gain a better understanding. @tom baker thank you for the code reference. For me, that confirms the hazards present when power generators are grouped in close proximity to each other.
Looking forward to interacting with this group. I have been on an extended hiatus from my electrical contracting business, and I am admittedly a bit rusty. I love Mr. Holt and have used his training and books extensively over the years.

 

[zbang]

I think you're reading too much into 525.11, remember that multiple supplies to pretty much* any structure must be bonded together (see generally the early sections of 250); 525.11 merely clarifies that this applies to generators in that particular environment. Also, keep in mind the vast difference between a couple of lunch-box size generators running a few lights and a couple of 100kva generators running a job site or outdoor event (which may not be covered by 525).

*off-hand, I can't think of any exceptions