DC Battery Rack Grounding

[trallio]

For a standard substation DC battery rack, I am having trouble determining whether a ground is required to be installed along with the wires between the battery disconnect switch and the battery rack. It's 125VDC. My usual approach is to include a ground until I can prove that a ground is not useful or is detrimental to the system. I have seen installations done both ways. 2/C with a ground and also 2 wires without a ground. When the ground is included, it is usually bonded right to the battery rack.

Is the ground required?
If not, and the rack is grounded to the substation ground bus, then is there still value in including a ground wire?

 

[ron]

Guessing this isn't utility (not covered by the NEC), the circuit needs an equipment ground conductor run with the ungrounded conductors.

 

[Besoeker]

For a standard substation DC battery rack, I am having trouble determining whether a ground is required to be installed along with the wires between the battery disconnect switch and the battery rack. It's 125VDC. My usual approach is to include a ground until I can prove that a ground is not useful or is detrimental to the system. I have seen installations done both ways. 2/C with a ground and also 2 wires without a ground. When the ground is included, it is usually bonded right to the battery rack.

Is the ground required?
If not, and the rack is grounded to the substation ground bus, then is there still value in including a ground wire?

Aren't all battery racks DC?
Sorry, couldn't resist..........
I don't know what your code requires but we normally ground all metal parts. It shouldn't add too much to the installation cost if done at the same time. Maybe a different story if you have to do it as a retrofit.

 

[petersonra]

Guessing this isn't utility (not covered by the NEC), the circuit needs an equipment ground conductor run with the ungrounded conductors.

I am inclined to agree that an EGC is required to be run with the circuit conductors, whether they are grounded or not.

I also think that the battery rack is probably something that could get energized thus the rack needs to be bonded to the EGC.

I am not sure it would do much good since it would appear the main GF hazard here is a battery terminal to the rack and even if it tripped the main CB it would not clear the fault. Might be a case where ungrounded is safer.

 

[meternerd]

In my years as a substation electrician we never grounded battery racks. All substations I'm familiar with have ungrounded DC because a ground fault on either polarity will not cause a fault, which increased reliability. DC is what provided power to all of the protective relay systems and control circuits. The potential for a grounded rack or other DC equipment allowing a ground fault is greatly increased if there are intentional equipment grounds installed. Ground alarms were installed on every DC system. Safety hazards are relatively small, since contact from ground to either ungrounded polarity does not constitute a circuit. But....we, as a utility were not governed by the NEC, so ours procedures may not apply.

 

[Sahib]

In my years as a substation electrician we never grounded battery racks. All substations I'm familiar with have ungrounded DC because a ground fault on either polarity will not cause a fault, which increased reliability. DC is what provided power to all of the protective relay systems and control circuits. The potential for a grounded rack or other DC equipment allowing a ground fault is greatly increased if there are intentional equipment grounds installed. Ground alarms were installed on every DC system. Safety hazards are relatively small, since contact from ground to either ungrounded polarity does not constitute a circuit. But....we, as a utility were not governed by the NEC, so ours procedures may not apply.

Grounding of DC battery system enhances safety really because, if ungrounded, any short of power conductor to battery system would not be cleared by OCPD and personnel doing routine maintenance on the batteries are liable to be shocked or to suffer worse than that.

 

[Tony S]

Guessing this isn't utility (not covered by the NEC), the circuit needs an equipment ground conductor run with the ungrounded conductors.

It’s the same this side of the pond. I’ve never grounded a substation DC system. The ancillary equipment (charging units, etc) would be grounded.

 

[meternerd]

Grounding of DC battery system enhances safety really because, if ungrounded, any short of power conductor to battery system would not be cleared by OCPD and personnel doing routine maintenance on the batteries are liable to be shocked or to suffer worse than that.

You'll find that in this country, at least in my experience, ungrounded systems are often used when reliability is a signifigant factor. In substations, it's protective relaying equipment and on a grounded DC system, a ground fault of the ungrounded polarity would create a short and trip the OCPD and remove any relay protection. Plus, if the fault were to happen at the battery rack ahead of any OCPD, a dead short of a 125V or 250V DC system would be catastrophic. Any equipment that has a typical grounded AC source such as battery chargers would have EGC's installed. On navy ships, it's an ungrounded AC system because if any single conductor goes to ground (due to some unfortunate things like depth charges or torpedoes), there will be no fault path back to the OCPD. Ground alarms are, of course, installed.