Overcurrent Protection for Battery

[timm333]

I have a question about location of breaker for battery-conductors. NEC 240.21 states that the breaker has to be located at the starting point of the cable.

The breaker for battery-conductors acts both ways. Under normal conditions, the supply-source would be the charger. But when the charger is out of service, the supply-source would be the battery itself, which is on the opposite side of battery-conductors.

240.21(H) states that for battery-conductors, the breaker is permitted to be installed as close as practicable to the battery terminals in classified location. What does it mean, does it mean that the breaker can be located away from the battery (in some other room)? What do you guys think about it? Thanks.

 

[ggunn]

The breaker is usually (in my years ago experience) on the positive terminal of the battery. The charging current will only be a fraction of what the battery may be asked to deliver, and in case of a fault virtually all the fault current comes from the battery.

 

[timm333]

Yes that makes sense. Does the breaker must be in the same room as the battery? Or can the breaker be placed in some other room if it is not practicable to place it in the same room?

 

[jaggedben]

Yes that makes sense. Does the breaker must be in the same room as the battery? Or can the breaker be placed in some other room if it is not practicable to place it in the same room?

480.7(A) requires a battery disconnecting means within sight of the battery. It doesn't strictly require overcurrent protection at the same location. That said, I would probably put the circuit breaker near the battery and an additional disconnecting means at the charge controller.

There must be a third connection too, right? The load? Or is the charge controller also the inverter?

 

[timm333]

Yes there is Load (125V-DC distribution panel). Charger has three cables: first for AC input, second from charger to battery, and third from charger to load. When charger fails, the power from battery first goes to terminals of charger and then to load. In other words, the battery has only one cable which is in between charger and battery.

 

[ggunn]

Yes there is Load (125V-DC distribution panel). Charger has three cables: first for AC input, second from charger to battery, and third from charger to load. When charger fails, the power from battery first goes to terminals of charger and then to load. In other words, the battery has only one cable which is in between charger and battery.

125VDC OR 12VDC?

 

[timm333]

125V-DC

 

[ggunn]

125V-DC

Interesting. What uses 125VDC?

 

[topgone]

I have a question about location of breaker for battery-conductors. NEC 240.21 states that the breaker has to be located at the starting point of the cable.

The breaker for battery-conductors acts both ways. Under normal conditions, the supply-source would be the charger. But when the charger is out of service, the supply-source would be the battery itself, which is on the opposite side of battery-conductors.

240.21(H) states that for battery-conductors, the breaker is permitted to be installed as close as practicable to the battery terminals in classified location. What does it mean, does it mean that the breaker can be located away from the battery (in some other room)? What do you guys think about it? Thanks.

If this is a central battery station for a power plant (125 VDC for control of large OCBs/ VCBs and the like), we located the battery circuit breakers right inside the battery stations that we have. Don't worry too much about whether the installed breaker can protect the cables. These are 60-cell battery banks with each cell having a nominal voltage of 2 VPC. Assuming the internal resistance of a VRLA AGM battery cell to be in the range of 0.5mΩ, you can only expect no more than 5kA if the cable gets short-circuited. Smaller short-circuit is expected of other types of batteries because of the limitations brought about by the internal resistance of the batteries.

 

[timm333]

It would be for the control of VCB's in switchgear. It would be somewhat similar to the attached image.

Overload protection would not be an issue because when overload happens then whole cable would be heated, so breaker would trip even if the breaker is not at the starting point of cable. But for short circuit and ground fault protection, the breaker has to be located at the starting point of cable. Can cables handle small amount of short circuit currents, how much short circuit current can a copper-XLPE cable handle?