135V DC ground fault troubleshooting

A

AaronB

Member
Location
Georgia
Occupation
Combustion Turbine Operator
Any help would be appreciated. I’ve attached links to pictures.

At work we have a 135V battery bank that powers our emergency DC lube oil systems. There is also a Primax P600 battery charger for the battery bank which also powers the DC equipment when normal AC 480V power is available. Ground reading at the battery terminals read 2.6V (+) and 125V (-), but it is showing normal ground readings at the battery charger (about 67V +/-). Why is there a discrepancy between the two? Which is correct? Do I actually have a ground fault based on these reading at the battery terminals?

I apologize if I’m not giving enough information, I’m not an expert on DC systems.

https://imgur.com/np7VDwB
https://imgur.com/pB9aoRd
https://imgur.com/fvvEvKM
https://imgur.com/e30PvjT
 
It seems like the battery is grounded in the middle of the series of cells, at least as far as the charger is concerned. My question is where are you connecting your probe to ground?
 
For the positive reading, I connected to the positive terminal of the battery bank (cell 1, top right of picture) and the battery support rack (which is grounded). Then for the negative reading, from cell 60 (bottom right) to the battery rack.
 
AaronB said:
For the positive reading, I connected to the positive terminal of the battery bank (cell 1, top right of picture) and the battery support rack (which is grounded). Then for the negative reading, from cell 60 (bottom right) to the battery rack.
Click to expand...
I have never touched such a system, but reading about them online seems to indicate that they are often either ungrounded, or grounded through a high resistance. So touching your probe to the frame may not mean anything. But if you are reading nearly full voltage from the frame to the negative terminal, that would seem like an actual fault somewhere.
 
CoolWill said:
I have never touched such a system, but reading about them online seems to indicate that they are often either ungrounded, or grounded through a high resistance. So touching your probe to the frame may not mean anything. But if you are reading nearly full voltage from the frame to the negative terminal, that would seem like an actual fault somewhere.
Click to expand...
Yeah I’m thinking that too. I need to do some more research to figure out why the charger is showing a different reading than actual ground readings. Thank you for your reply
 
Sounds like it might be a positive ground system.

Not usually typical at this voltage, but many Telco devices operate at 48 VDC and are positive ground.
 
I'd definitely inspect the entire DC wiring for a ground, intentional or not (and generally check the connections along the way). Also, unless there's a obvious bond from the rack, instead of using the rack as a reference, run a wire from a known ground.

(telco "battery" uses a positive ground for galvanic protection)
 
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