DC Breaker sizing for Switchgear

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timm333

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Minneapolis, MN
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Electrical Design Engineer
I have a question for sizing of 125V-DC breaker (as per NEC) which supplies 125V-DC power to the control circuits of ten (10) 4.16kV circuit breakers. The control circuit of each 4.16kV circuit breaker has one trip coil (7.5A DC), one closing coil (2A DC), one spring charging motor (4A DC), and one protective relay (0.2A DC). All 10 breakers are tripped together.

Only protective relays are constant loads, all other loads are momentary loads. For sizing 125V-DC breaker, we will have to take the constant loads and the largest momentary load. Trip coils have the largest momentary loads. We can take 125% load of one (1) trip-coil, and 100% load of the remaining nine (9) trip-coils:

[Constant Loads] + [Largest Momentary Loads] = [10 * 0.2A] + [1 * (125% of 7.5A) + 9 * (100% of 7.5A)] = 2 + 76.88 = 78.88 A.

So 80A 125VDC breaker would be required. Is it correct, what do you guys think about it?
 
timm333 said:
So 80A 125VDC breaker would be required. Is it correct, what do you guys think about it?
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If this is internal to the Switchgear the NEC would not apply.
These are trip coils not motors, do they do not need to be sized as such.
How long does the trip coil pull maximum current?

If this was an AC control circuit the supply would be sized on the continuous current draw and the momentary current draw of coils. The charging motors usually operate for several seconds, so they are sometimes considered as continuous motor loads.
 
This is not internal to switchgear, because we have a 125V-DC charger (in parallel to a backup 125V-DC battery) which supplies power to a DC distribution panel. And from DC distribution panel, 125V-DC is supplied to the switchgear.

Yes, as trip coils are not motors so we will size all trip coils for 100% load. The operating time of trip coils is not given, so I think these would be momentary (with no time delay). The charging time of spring charging motor is 10 sec. The battery manufacturer told that spring charging motor is to be taken as momentary load, but that was for sizing of battery.

Maybe for sizing of 125V-DC breaker, we should take the spring charging motor as constant load. What do you think?
 
As I recall there are some exceptions for rectifier supplied DC systems that feed switchgear.

Most OCP can carry up to 6X current for in excess of 20secs, but the NEC typically doesn't address momentary loads.
 
Actually I think the load of spring-charging-motors does not need to be included. Because trip-coil and spring-charging-motor cannot operate at the same time. So we can include only one of these two loads. And we have already included load of trip-coils which is higher than spring-charging-motors.
 
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