DC Distribution & Circuit Protection

[ICman]

As an EE, this is embarassing, but I don't work with electrical circuits much so ...

I am designing a DC distribution circuit for a meter house. We have a 24 VDC battery charger (Input: 120 VAC) that feeds DC circuits and a set of batteries. If we lose AC to the building (thus, the charger), the batteries back up the 24 V power.

My predecessor used a standard Square-D Load Center, with the charger tied to the Main (30 A breaker) and the batteries off one of the circuit breakers (30 A). If the charger lost power, the batteries would feed back thru the breaker to power the main bus. It works, but the Load Center and breakers are not listed for DC.

I tried to redesign it using DC circuit breakers, with fuses for each circuit. I routed power (+24VDC) from the charger, thru a 30A DC breaker, to the fused terminal strip. This is also connected to a second 30A DC breaker and then to the batteries (+). Thus, when both breakers are closed, power comes from both the charger and the batteries. If you open CB1, power is fed from the batteries, but backward thru the breaker. If you open CB2, power comes from the charger.

Here's a rough idea:

CHARGER-------|CB1|+------------TERMINAL STRIP
|
+-----|CB2|------BATTERIES



My questions:
1. Is it permissible to feed back thru the Circuit Breaker?

2. If the batteries short out, won't they take both Circuit Breakers with them? Or, should I size the CBs differently?

3. Is this exercise fruitless? Is an AC Load Center suitable for 24 VDC?

TIA,
. . . Steve

 

[Skogsgurra]

Hello ICman,

I think that it is best to answer qestion # 3 first: A DC circuit should have DC breakers. Even if your voltage is about twenty percent of the nominal AC voltage of the breakers, I think that the breaking capacity of the breakers is very questionable. It may work, but if it doesn't - you will have some legal problems. Is there a DC rating for the breakers? If so, is it in line with your expected short circuit current?

#1 The current direction is of no consequence in an AC breaker. If it can break the DC, it will do so regardless of current direction.

#2 Yes. Choose different ratings so that you get some selectivity in the circuit.


Gunnar Englund

http://www.gke.org

 

[resqcapt19]

Square D says that their standard QO and QOB breakers are rated for 48VDC in sizes 10-70 amps. The interrupting rating is 5kA when used on DC. See page 4 of this document;

http://ecatalog.squared.com/pubs/Ci...rs/QO-QOB Circuit Breakers/QOT/0730CT9801.pdf

Don

 

[ICman]

Thanks! I could not find that page on their Website, though I did see some reference to 48 VDC.

Their FAQ seemed to indicate that these were not suitable.

. . . Steve