Circuit Breaker general info

Old 01-16-03, 04:30 PM
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Circuit Breaker general info

Here's the application:

I am making test cords for service techs to jump start 1/2 and 1/3 refridgeration compressors. This is to rule out electrical problems leading to the compressors. The cords we used to use had a fast blow fuse in the hot line in case the compressor windings shorted to ground or the piston is locked up causing increased amp draw. These cords are hard to find now a days.

I have the bright idea to use circuit breakers instead of fuses but am unsure if the breaker can be made to function the same. I have no knowledge of home electrical applications.

Does a circuit breaker trip by increased amps across the bottom and top poles? There is no grounding through the cord.

Any info on how the breaker trips would be welcomed.
Old 01-16-03, 07:11 PM
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A circuit breaker will detect an overcurrent condition in the circuit based on the amperage of the circuit breaker you choose. (Of course you want the circuit breaker's amperage to be the weakest link in your circuit that is made up of the 1) load, 2) the wiring and the 3) circuit "breaker". Most circuit breakers operate (or trip) using a pre-set (amperage) magnetic sensor inside the breaker's housing, while others use both thermal (heat) and magnetic (amperage) sensors.

A circuit breaker can be used just like your fuse; in-line on one of the conductors feeding the compressor. A standard circuit breaker, like is used in residental and commercial loadcenters and panelboards, are designed to allow an inductive load (like a compressor) enough "time" to go from a static or stopped condition to its full speed without tripping even though such a load often pulls 6 times or more its normal running current for a brief few seconds when it is first energized. The fast-blow fuse that you have been using, on the other hand, is designed to blow (melt the current carrying link) pretty much as soon as the amperage exceeds its current carrying design. Fast blowing fuses should not be used for inductive-type loads...but instead resistive-type loads like incandescent lamps, heaters, etc. Therefore, unless you sized the fast-blow fuses that you have been using a goodly amount higher in amperage than what the compressor actually pulls when it is up to its full speed (known as full "load" amperage i.e. FLA) they would have blown when the compressor was first energized since, as mentioned above, it can pull 6 times or more its normal amperage when first energized. By the way, fuses are made for various applications with various degrees of "time-delay" capabilities in a huge range of voltages and amperages.

Sorry for the long answer...but I felt an explanation was needed for you to understand a little better.

In my opinion, your best bet would be to make up a small loadcenter (with circuit breaker) or an enclosed circuit breaker with the appropriate size and length of cord and plug needed for you to test compressors. My choice would be to use a Square D "QO" type circuit breaker that has both thermal and magnectic protection as described above. It would offer full protection while allowing a "good" compressor to start but trip (take off-line) if the compressor was defective (locked-up). Plus, you would never have to replace the circuit breaker like you do a blown fuse.

Since you are testing 1/3 and 1/2 horsepower 120 volt motors a single pole 15 amp circuit breaker will work perfectly and offer all the protection needed.

I hope this helps!

Old 01-16-03, 08:15 PM
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Thank you for your informative answer. The fuses that I use are 20 amps with the compressors pulling about 16 amps with both windings energized. I will try the 15 amp breakers.

Hopefully this will ease my workload. I have a lot of machines sent to my shop with the dreaded "Bad compressor" as the sole description of a problem.
Old 01-16-03, 08:21 PM
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If you are in the shop you could just put a 15A breaker on the circuit you are using as test. I really dont see what the reasoning is to make a seperate cord is if you been using 20 a fuses anyway,,, the building wiring is already fused. If you wanted further protection you can get a little thingy with a fuse and a recept that fits in a regular box, just plug in your test cord to it.
Old 01-17-03, 05:39 AM
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The fused cord is so that my (street) service techs do not trip a business's circuit. Not a major problem there but you know how people can be. We are supposed to be on dedicated circuits anyway.

The original cords date back to the '50s unchanged. They have been on back order for the last two months from the only vendor that I could find. I just thought that a circuit breaker would be easier that having to supply twist fuses ( dont know what they are really called).

These street service techs don't work directly for me. I am just the guy that has to fix everything they can't or won't. So no excuse about they were out of fuses and could'nt find any!

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