# DC Tripping Breaker



## wildleg (Apr 12, 2009)

tripping curve of the 5 is quicker than the 2 ?


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## ScooterMcGavin (Jan 24, 2011)

Thats why we use one of the inputs as a "control power ok input" and just hook it up to the load side of the fuse or CB. So if you do lose your 24 volts you know right away especially on stuff that has to work when needed.


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## oliquir (Jan 13, 2011)

are the 5A and 2A same type of breaker?


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## JRaef (Mar 23, 2009)

oliquir said:


> are the 5A and 2A same type of breaker?


Good place to start. That is the part number for the 2A breaker with a "C" trip curve. The "7" in the -7HG41 sequence is what identifies the curve. If the 5A is a different number, 6 or 8, that might explain it.

But it's rerally difficult to expect clear trip coordination with very small current values like this, the +- tolerances are so wide that basically everything tends to have a lot of overlap.










So looking at these 3 trip curves, the 2A breakers are a C curve, 13A is basically 6.5X the rated current, So the trip time will be in a RANGE of .01 to 7 seconds. If the 5A breaker is also a C curve, it's trip time at 2.5X rated current is a lot higher and it SHOULD have held. but how do you KNOW that the initial current flowing in that grounded wire was only 13A? In 24VDC control circuits, the power supply, if it does not trip off-line on a fault like this, will likely go into current limit, it's often a safety feature to prevent unwanted shutdowns. So that 13A may have been the limited current of the PS that you were measuring. But that current limit feature takes a period of time to come into play, so until it does, the current flow to ground could have been much much higher. If it got into the instantaneous region of the 5A breaker, then it too could have had the same wide range of trip time, and manufacturing tolerances on that particular breaker had it releasing faster than the 2A.

the point of all this was, I wouldn't worry about it. You had an even, "a" breaker prevented collateral damage, you fixed it, all is good.

By the way, there are electronic trip devices that you can place between your PS and control circuits that will trip a LOT faster than any circuit breaker. Then they give you visual indication, an alarm contact and the optional ability to reset or turn them on and off remotely. I like them better than CBs now. They cost more than a cheap breaker, but each one handles 4 circuits.


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## Big John (May 23, 2010)

As usual, Jraef covered the bases. The only thing I'd be concerned about is that if you actually need supplementary overcurrent protection, I am not sure that an electronic protector can be a substitute for a UL 1077 circuit breaker. You may end up still needing the breaker and just using the protector in addition.


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## JRaef (Mar 23, 2009)

Big John said:


> As usual, Jraef covered the bases. The only thing I'd be concerned about is that if you actually need supplementary overcurrent protection, I am not sure that an electronic protector can be a substitute for a UL 1077 circuit breaker. You may end up still needing the breaker and just using the protector in addition.


You're right BigJohn, technically it can't substitute for the branch circuit protection. So they have fuses inside. They are too big to ever blow and the electronic trips will always act faster anyway, they are there only to satisfy UL.


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