DC System Grounding

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timm333

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Electrical Design Engineer
I have question regarding the grounding of 125V-DC system, which is used for the protection of switchgear in an industrial facility. The DC system consists of two chargers and one battery in parallel, which supply power to a 125V-DC panel. This panel supplies DC power to all the DC loads (relays, trip coils of breakers, etc.)

It looks that this 125V-DC system is not required to be grounded as per NEC, is it correct?

Also is the ground fault protection (for personnel and equipment) required in the 125V-DC panel for main and branch breakers? Thanks.
 
Not sure that the rule in 250.162(A) applies to that type of system, but it does require DC systems operating at that voltage to be grounded.
 
petersonra said:
Would not exception 2 apply?
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All of the switch gear DC power systems I have seen were supplied by batteries and not a.rectifier-derived dc system. Sure there are battery chargers, but I don't see that as being covered by Exception 2.
 
don_resqcapt19 said:
All of the switch gear DC power systems I have seen were supplied by batteries and not a.rectifier-derived dc system. Sure there are battery chargers, but I don't see that as being covered by Exception 2.
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(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.
Exception No. 2: A rectifier-derived dc system supplied from an ac system complying with 250.20 shall not be required to be grounded.

I don't see how you could claim the controls for a piece of switchgear constitute premises wiring.
 
petersonra said:
(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.
Exception No. 2: A rectifier-derived dc system supplied from an ac system complying with 250.20 shall not be required to be grounded.

I don't see how you could claim the controls for a piece of switchgear constitute premises wiring.
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Most of them have the batteries and chargers external to the actual switch gear and even have distribution panels. I see that as falling into the very wide ranging code definition of premises wiring. If it was all contained within the actual switchgear, I would not say that.
 
In normal operation, the charger feeds 125V-DC to the DC loads. And at the same time the charger charges the backup DC-battery. Charger has rectifier, so I think in this case Exception 2 of 250.162(A) would apply.

If charger fails, then backup DC-battery would supply the DC loads. DC-battery does not have rectifier. But maybe exception 2 would still apply because maybe we should design the system for normal condition when charger is working (not for the abnormal condition when the charger fails). What do you guys think about it?

How about ground fault protection; is ground fault protection (for personnel and equipment) required for the main and branch breakers of 125V-DC panel?
 
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timm333 said:
In normal operation, the charger feeds 125V-DC to the DC loads. And at the same time the charger charges the backup DC-battery. Charger has rectifier, so I think in this case Exception 2 of 250.162(A) would apply.

If charger fails, then backup DC-battery would supply the DC loads. DC-battery does not have rectifier. But maybe exception 2 would still apply because maybe we should design the system for normal condition when charger is working (not for the abnormal condition when the charger fails). What do you guys think about it?
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I wonder what the rectifier logic is? Perhaps that makes it not a separately derived system.
timm333 said:
How about ground fault protection; is ground fault protection (for personnel and equipment) required for the main and branch breakers of 125V-DC panel?
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210.8 Mostly applies to receptacles. if your not powering DC receptacles or crawlspace lighting outlets with DC I would not be concerned.
 
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