Circuit Breaker Trip 1 & Trip 2 for a DC Load Calculation 1

[pearlbrian]

Greetings forum goers.

I have a question for you. For the purposes of battery sizing (specifically amp hour rating), I have generally used a calculation that accounts for only one trip coil per circuit breaker drawing current during the small portion of the 8 hour DC curve where circuit breakers are tripping.

My question is, should I also be accounting for the trip 2 coil when the breaker has one? I guess the real question is, does the trip 2 coil see a current when the trip 1 coil operates?

Thanks!

 

[ScottyUK]

How are your trip coils configured? Are they independent without duplication of function, or are they from one common trip signal, or is this a redundant system with duplicate protection schemes? If one fault can result in a trip on both tripping coils then you need to provide for it. You should probably look at the ability to deliver peak tripping current without appreciable drop in battery voltage as well as the battery's AH capacity.

 

[pearlbrian]

They are segregated, and run of independent DC supplies. However, I would say they absolutely could and probably do both see circuit "made" at the same time.

What I'm wondering is, based on the mechanical nature of the trip mechanism, do they both draw the full rated trip current? Honestly, the battery capacity isn't much of a concern as this is only a few cycles of an 8 hour period. It's more that I'm curious.

 

[jghrist]

If they are run from independent dc supplies, then you count the trip current of the coil run from the dc supply for which you are calculating the battery capacity.

 

[ScottyUK]

My opinion - bearing in mind that I don't design circuit breakers for a living - is that the principal factors controlling the coil current are its inductance and resistance, and the breaker operating time. I don't expect the breaker operating time to significantly decrease through the addition of a second tripping coil for the mechanisms I'm familiar with because the solenoid isn't the dominant factor in the tripping time under normal conditions. The coil current will build up at the same L/R time constant to a steady value and will be interrupted by the breaker 'b' auxiliary contact at pretty much the same period of time after trip initiation regardless of whether there is one coil or two. In some marginal conditions such as the battery being heavily depleted or the mechanism being so badly maintained that it is half-seized then the additional tripping force from the second coil might make the difference between the mechanism releasing or not, but that seems a bit of a stretch.

Where's zogzog these days? He overhauls circuit breakers for a living and might have a different perspective.

 

[cherryg]

For the sake of onerous calculation, please consider both the trip coils for DC load profile (typically in the 1st one minute, since the breaker will be expected to trip when the 8 hour cycle starts). Please note that any momentary load has to be considered for 1 min as per IEEE485. Usually the last one min load (usually the closing coil and subsequent spring charging motor load), determines the battery size along with the chosen end cell voltage which should not be the customary 1.75V/cell.

 

[crshears]

Where I work, the dual redundant tripping schemes with separate DC power supplies have provisions for being tied together to accommodate charger or battery failures [not highly likely, but possible], as well as allowing for equipment to removed from service for maintenance with a loss of redundancy only, meaning without service interruption.

And being dual redundant, the protection schemes are duplicates, meaning when a fault occurs they race to trip the protected zone out of service, hence by design they should both operate together.

Since during such equipment outages both trip coils will be energized from the same source, and also since a system disturbance could occur while the outage is in effect, would it not be better to plan for such a contingency right out of the gate?