DC grounding question

[ggunn]

For an off-grid PV system with an ungrounded array, does the battery bank negative need to be bonded to ground? Why or why not? Code reference?

 

[jaggedben]

First question that comes to mind is: How are you going to have an ungrounded PV system and a grounded battery bank, assuming they are connected in parallel?

I believe the answer is you are not required per se to ground the battery negative, but you are required to have some sort of ground fault detection. I think code cycle matters a lot, at least for what the chapter-and-verse numbers are.

For the 2014 NEC, see 690.71, particularly (G).

I don't have 2017 with me right now, but I know 690.41 changed again, significantly, and they made a whole new article for energy storage systems (706).

 

[jaggedben]

For 2017, might want to check article 710 as well.

 

[Carultch]

For an off-grid PV system with an ungrounded array, does the battery bank negative need to be bonded to ground? Why or why not? Code reference?

I would hope that the charge controller, inverter, or other power conditioning equipment would specify. If you have an AC coupled system, the grounding topologies of the battery and PV system have nothing to do with one another.

 

[dkidd]

For an off-grid PV system with an ungrounded array, does the battery bank negative need to be bonded to ground? Why or why not? Code reference?

I think that this would apply:

VIII. Direct-Current Systems
250.160 General. Direct-current systems shall comply
with Part VIII and other sections of Article 250 not specifically
intended for ac systems.

250.162 Direct-Current Circuits and Systems to Be
Grounded. Direct-current circuits and systems shall be
grounded as provided for in 250.162(A) and (B).
(A) Two-Wire, Direct-Current Systems. A 2-wire, dc system
supplying premises wiring and operating at greater than
60 volts but not greater than 300 volts shall be grounded.

Exception No. 1: A system equipped with a ground detector
and supplying only industrial equipment in limited areas
shall not be required to be grounded where installed
adjacent to or integral with the source of supply.

 

[jaggedben]

I think that this would apply:

VIII. Direct-Current Systems
250.160 General. Direct-current systems shall comply
with Part VIII and other sections of Article 250 not specifically
intended for ac systems.

250.162 Direct-Current Circuits and Systems to Be
Grounded. Direct-current circuits and systems shall be
grounded as provided for in 250.162(A) and (B).
(A) Two-Wire, Direct-Current Systems. A 2-wire, dc system
supplying premises wiring and operating at greater than
60 volts but not greater than 300 volts shall be grounded.

Exception No. 1: A system equipped with a ground detector
and supplying only industrial equipment in limited areas
shall not be required to be grounded where installed
adjacent to or integral with the source of supply.

690.41(G) which I mentioned above would seem to supersede that.

 

[ggunn]

OK, some clarification:

The system will be totally off grid, with a 48V battery bank and a Schneider (nee Xantrax) inverter. The job site is in Haiti.The designer who is putting it together tells me that the PV array is ungrounded. The wiring diagram from Schneider does not show the battery bank to be grounded.

 

[electrofelon]

This is a tough question that I have pondered. Not only is the code a bit vague on off-grid, but the issue is further complicated by the topology of the components, specifically whether they are isolated or not. I believe most charge controllers are not isolated, however the two available 600V ones (schneider and morningstar) are isolated. I believe off-grid inverters typically are isolated.

 

[jaggedben]

Put it this way: if you don't want a potentially unsafe battery current, then you either need to ground one side of the battery so that an overcurrent decice trips, or you need a ground fault detection device. I beleive Morningstar or somebody made a GFDI that was basically two circuit breakers tied to each other. I believe that for quite a while most off grid 48V type systems grounded the battery negative, if not solidly then through a GFDI device.