NEC code vrs Military handbook

[Bruce Lorraine]

Question, which is right?

NEC
When operating multiple vehicles at the same location and each is operating off their individual onboard generators, never connect the vehicles to the same ground electrode subsystem because of differences of electrical potential. (NEC section 250.34, page 70-103)

Military (A Guide to Proper Earth Grounding and Bonding Methods for Use with Tactical Systems CECOM TR-98-6)
Interconnecting the ground rods for different shelters helps to create an earth grounding electrode system having an overall lower grounding resistance, and results in other benefits as well. Therefore, where electronic equipment shelters are located within 25 feet of each other, it is recommended that the individual earth grounding electrodes be bonded together using a heavy copper conductor (6 AWG minimum) run along the ground.

If more than 1 truck (each with own gen set) are operating within 25 feet of each other, do they need to use a common bond/ground? (Radio and IT systems operating)

 

[tom baker]

But in Article 525.11 it states
Where multiple services or separately derived systems supply rides and attractions, all sources of supply separated by less than 12 ft must be bonded to the same grounding electrode system.
And Mike Holts comment is: I have no clue what this is supposed to accomplish.

 

[Bruce Lorraine]

More info,

Lets say that each truck has it's own gen set supplying power to its components, now lets hook equipment (radios, IT, Video) between the trucks, should the trucks share a common ground or bond of some kind?

 

[zog]

I know this dosent answer your question but if you are in the military and go against the procedure, you know what happens to you if something bad happens.

If you are questioning the procedure thats one thing and you probally should but if I were you I would follow the military procedure, CYA!

 

[dbuckley]

As long as we're leaving what the regulations say out of this, and assuming you are not connected to shore power at all, so each truck is truly running off its own individual generator, and theres no silliness like ground rods...

Under the circumstances listed above there is no power electrical or safety benefit to be gained by connecting the trucks together, as each one is an isolated SDS.

If you hook video and/or IT between trucks, assuming it's not fibre, and excepting UTP Cat5 ethernet (which has isolation transformers) then they are sharing a common "ground" through the signal wiring; STP or coax or audio XLR cables all have screen continuity.

There are shock possibilities under certain conditions involving people on the ground touching the metalwork of interlinked trucks, but they are multiple fault scenarios, so unlikely to occur in practice, however, having GFCI protection on the genset output eliminates those possibilities completely.

But... a dirty great ground wire between trucks may (or may not) improve the technical performance of the interlinked equipment. Video is often run unbalanced, so having no currents flow through the screen (or at least, no current the phase of which is not constant with respect to the phase of the video signal) is quite import and prevents hum bars. Audio is always run balanced, so (under almost all practical situations encountered) doesn't care about the screen. So it all depends. If the equipment in the trucks is designed to operate with a clean common ground between the items, then extending that between trucks may be mandatory for satisfactory equipment performance.

 

[jghrist]

Question, which is right?

NEC
When operating multiple vehicles at the same location and each is operating off their individual onboard generators, never connect the vehicles to the same ground electrode subsystem because of differences of electrical potential. (NEC section 250.34, page 70-103)

I don't find this in NEC-2002 or NEC-2005. NEC-2005 250.34 (B) states:

"The frame of a vehicle shall not be required to be connected to a grounding electrode ... under the following conditions:"

Not being required is not the same as "never do it". Basically 250.34 is just saying that for serving loads on the vehicle or from cords connected to the vehicle, you only have to bond to the frame and a grounding electrode isn't required by NEC. If it is required by the military, the NEC doesn't prohibit it.

 

[iwire]

I don't find this in NEC-2002 or NEC-2005.

I am guessing it is from the handbook?

If so it is not code, it is opinion and I don't see an NEC rule that backs that opinion up.

If it is required by the military, the NEC doesn't prohibit it.

I agree

 

[brian john]

using a heavy copper conductor (6 AWG minimum) run along the ground.

The word heavy should be dropped, when I first saw the word I thought #2 or larger...Each persons definition of heavy would be different, why not say.


Number 6 copper or larger.

 

[nakulak]

I think you have misinterpreted NEC 250.34, it does not say it is prohibited.

If you are doing a milspec job or are in the field of operations then those rules must apply - they are the ahj and they make the rules, the NEC would simply not apply even if it did say what you think it does.