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Mobile 80VDC System Grounding

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mgmonett

New User
Location
Michigan
Occupation
EE
I am designing the controls for a mobile system. The system consists of three interlocked carts, the first of which provides power to the next two carts. I have scoured NEC and the internet and I am having some difficulty in finding a solution to my design question. Brief system overview:
  • Power source is an 80VDC battery
  • Battery powers two (2) inverters
  • Battery powers two (2) DC-DC converters
I am trying to wrap my head around how to adequately ground the system. Because this system is over the 50V threshold, according to 250.162(A) this system must be grounded. This is the best that I have come up with so far.
  1. Bond -B (battery -80VDC) to the frame of the first cart.
  2. From this bond, run a ground wire to the main control panels ground bus, which resides on the second cart. Both inverter ground connections and both DC-DC converters ground connections will share the ground bus bar.
I also currently have 2-pole circuit breakers on every device. I believe that since there is not an earth ground, this 2-pole breaker arrangement is required. Is this correct?

Is this ground bus arrangement the proper way to fulfill the requirements of 250.162(A)?

Thank you.
 
J

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
This probably doesn't fit under the scope of the NEC, since it is not 'installed'. A product standard for portable power supplies might be more relevant. (Even if it's a one off that you aren't trying to get listed or tested, you could perhaps still try to design to a standard.) Also you're not really 'grounding' by bonding the cart chassis, although it serves a similar function.

That aside, I don't see anything wrong per the NEC. Whatever plug connects the carts should probably ensure that the 'ground' is make-first-break-last.

I don't think you need two-pole breakers, if you're bonding one conductor to the chassis and treating it as grounded.
 
D

david

Senior Member
Location
Pennsylvania
From your description your power source is suppling loads on the 2nd and third cart, you make no mention of supplying any additional loads. for example the NEC does not apply to your alternator on your vehicle. Just mentioning that for clarification.

There is a difference between bonding the carts metal to your power system and grounding your power supply
 
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