48 volt DC

[ceb58]

Need some info on a DC system. In our communication sites they run the microwaves on 48 VDC system. To make a long story short we had 8 - 6 volt batteries go bad, as in smoking, any way. I was there to help one of the techs. change them out. We were going to check out the wiring into the panel to make sure it didn't melt. I noticed that the red positive #2 was landed on the, what would be, the neutral bar. The black negative # 2 was landed in the main breaker. I questioned the tech. as to why this was like that. His only answer was " its something that come from the way old telephone wiring was done and that is the way they were told to do it". My thought was if some one were working in the panel and did not disconnect the batteries they run a good chance of getting something between the neutral bar and the grounded case and it goes bang. Is there any good reason why this is done like that?

 

[zbang]

Telco DC systems historically use a positive ground for electrolytic corrosion reasons.

 

[GoldDigger]

Need some info on a DC system. In our communication sites they run the microwaves on 48 VDC system. To make a long story short we had 8 - 6 volt batteries go bad, as in smoking, any way. I was there to help one of the techs. change them out. We were going to check out the wiring into the panel to make sure it didn't melt. I noticed that the red positive #2 was landed on the, what would be, the neutral bar. The black negative # 2 was landed in the main breaker. I questioned the tech. as to why this was like that. His only answer was " its something that come from the way old telephone wiring was done and that is the way they were told to do it". My thought was if some one were working in the panel and did not disconnect the batteries they run a good chance of getting something between the neutral bar and the grounded case and it goes bang. Is there any good reason why this is done like that?

And, I think that if you meter it out, you will find that in this positive-ground 48 volt DC system there already is a connection between the neutral bar and earth ground/EGC/GES somewhere, even if it is just at the equipment or the racks. FWIW, if the NEC standards had been followed, the positive conductor should have been white or gray, since it was the grounded conductor (NOT a neutral!). Often inside equipment and in interconnects black for (-) and red for (+) are used even when they are not completely appropriate, since they are so familiar.

BTW, did you figure out what smoked the batteries? One or two cells shorting out internally could have caused the charger to deliver a constant high rate overcharge to the rest. (That is more likely with a lower voltage system, but it can happen with 48 too. Especially if an entire 6V battery shorts out.)

 

[ceb58]

And, I think that if you meter it out, you will find that in this positive-ground 48 volt DC system there already is a connection between the neutral bar and earth ground/EGC/GES somewhere, even if it is just at the equipment or the racks. FWIW, if the NEC standards had been followed, the positive conductor should have been white or gray, since it was the grounded conductor (NOT a neutral!). Often inside equipment and in interconnects black for (-) and red for (+) are used even when they are not completely appropriate, since they are so familiar.

BTW, did you figure out what smoked the batteries? One or two cells shorting out internally could have caused the charger to deliver a constant high rate overcharge to the rest. (That is more likely with a lower voltage system, but it can happen with 48 too. Especially if an entire 6V battery shorts out.)

Not sure on the grounding. I do know that the panel case is bonded to the halo in the building which is part of the ground ring around the building and tower.
As far as the batteries, they were old. I was there installing and starting up a UPS. I did a generator test on the building to make sure the UPS "liked" the generator. When it transfers from utility to gen. there is a 10 second time delay. When it transferred to generator power one of the batteries started rolling smoke from the top. Nothing wrong with the generator I guess it just was the fact of the batteries taking over then the rectifier coming back on. As you said there could have been a short in the battery.
Not ashamed to say I killed the main breaker and got the H*** out of the building. Have seen a TV transmitting building we are in go up in flames. Lightning strike arced from MDP to a rack of batteries. Fire took out the distribution panels, rectifier and a 125 Kw generator that was installed in the room.

 

[USC2DWR]

I agree with gold digger. Reason why they ground the positive in telco system is to prevent corrosion of the wires.There is no neutral.Red usually indicate hot , that means it goes from negative terminals to main breakers.It in fact is a positive ground system.

The other concern about generator could be that when it switched to generator batteries might have over loaded with in rush current and batteries might not have been charged completely. We used to do check capacitance on batteries every 6 months. I personally doubt anything shorted in there. It's also likely that equipment tried to pull more in rush current which the batteries could not handle.

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