Battery Earthing 3

[alingstone]

What are the advantages & disadvantages of solidly earthing one pole of a 48VDC substation battery? I have a system that i'd like to tie the positive to the substation earth mat, is this a good or bad idea?

 

[Marmite]

In my experience earthing of 48V substation batteries follows telecomms practice of +ve pole earthing to minimise electrolytic corrosion. The +ve pole is earthed via a solid link and the -ve pole is fused.
The other options are battery biased, and battery centre point earthing, normally used for 110V batteries. The centre point earthing method is predominant in new installations and allows the detection of earth faults on either pole upto about 50k Ohm resistance.
Regards
Marmite

 

[davidbeach]

Substation battery systems should always be ungrounded. Apply ground detectors, but leave the system ungrounded so that you can continue to operate properly in the presence of a single ground fault.. Center point grounding ensures failure of the control system for any ground fault. Grounding the positive or negative leaves some ground faults you can continue to operate through.

 

[Marmite]

I agree with Davidbeach that unearthed systems are best. The centre point earthing system I'm familiar with is not a solid earthing system. It uses a potential divider made up of 2x 6k Ohm resistors across the battery with the centre point of the divider connected via a 10k resistor to an earth fault relay. It therefore detects an earth fault on either pole, rather than causing complete failure of the control system. I should have made this clear in my original reply.
Regards
Marmite

 

[busbar]

You might want to review this report from the IEEE Power Systems Relaying Committee…

http://www.pes-psrc.org/k/rlytrckt.zip

 

[slavag]

Hi.
Im agree with David, and with Marmite too.
110/125V DC and 220V DC is always ungrounded, but 48V DC in substations for telecommes, + is grounded and it is separated bank.
If you would like use 48V DC for the "small" SS, usually it is MV SS, you can use ungrounded system.
Of course in ungrounded systems you must use ground detector fault. See systems like to Bender, Startco.

Best Regards.
Slava

 

[alingstone]

Thanks for all the responses, but here is my predicament, the systems we buy come negative (not positive as i said eariler) earthed, and only have MCB protection on the positive distribution rail, i.e. no negative protection. If i used a floating system then i'd have to fuse the positive and negative distribution sides (i.e. for a 100 W load i'd simply put in a 4A MCB on the positive side and the negative wouldn't be protected). The cells connected to the system have positive & negative MCBs on them and the grounding is at the dist board. If there is a positive side earth fault then the distribution fuse would trip, so i'm confortable with this approach, any comments?

 

[RRaghunath]

Are you talking of Diesel engine cranking battery, Alingstone!

Only automobile battery systems come with negative grounding. I will be surprised if you are talking of negative pole grounding for a substation or telecom battery.

Would like to hear more details from you.