Circuit breaker question.

[Jones]

I've decided to run BUSSMAN (Buss Fuses) blade type circuit breakers instead of the one-time-use ATC or ATO fuses and so was given a bag of them to play with. Going through them I'm noticing that there are some marked 24V 10A, or 20A, or whatever-- and some marked 12V 10A, or 20A, or whatever. Now I always thought amps is amps and am wondering why they are voltage specific. Is there more to this amps thing than meets the eye; or is this just a way to sell more circuit breakers?

 

[rmgill]

They're circuit breakers. They heat up at a certain voltage AND Amperage. Basically think of a filament that's designed to trip at a specific voltage and amperage. If you run it at a higher voltage, it'll take fewer amps to trip it. Just like running a low voltage light at twice the voltage, it'll burn brighter and have shorter life.

 

[ARMYMAN30YearsPlus]

Power equals Volts times Amps so the two are thus related to what level of power they handle.

 

[OPCOM]

You can get away with using the wrong voltage as long as you don't mind slightly poorer regulation (24V units protecting a 12V system) and/or reduced contact life (12V unit in a 24V system). If you don't pop them often, they'll be fine and most equipment doesn't notice the voltage drop in the former case. Most people size the breaker for twice the expected load current.

I protect my wiring with magnetic breakers. They act very fast and require manual reset. If you are building a bank of breakers, you might wish to consider a magnetic unit rated 3-4X the expected current as the main switch, in case of a direct short.

Bussman has a nice PDF catalog at:
http://www.bussmann.com/pdf/a44055a0-4727-4a73-a55e-4d9d3bd56f74.pdf

 

[Jones]

Thanks Patrick for the link to the catalog. I ended up with over 70 24V breakers in 10, 15, 20, 25, and 30 amp ratings. I hate to throw them away but I'm unsure as to how 'off' the protection will be from the stamped marks on them; do the 20A ones now trip at 25A or 30A?. I'm converting the 24VDC system to 12VDC so components are easier to find and cheaper. Still, I would hate to see the baby HEMTT turn into a four-wheel-drive roman candle 'cause I'm too stingy to buy new ones-- so I may spring for breakers closer to the new voltage.

 

[cranetruck]

There is plenty to read on the net about fuses, breakers and fusible links.
To protect your wiring, use fusible links in critical areas, use fuses at the correct voltage and current or use electromagnetic breakers that are not voltage dependent.

 

[Jones]

I did some reading and I think that lead to my confusion. While I found circuit breakers that specified a particular voltage; none of the blade-type fuses were marked by voltage, just amp rating. While I've been up against wiring systems that had fusible links, I'm not a big fan of them as they're good at hiding if you aren't familiar with them and know what to look, and feel for. I think something that tells at a glance that it's failed or tripped is the way to go; especially when sitting in the dark with freeway traffic zipping by, mere feet from your hiney cheeks.

 

[OPCOM]

The breakers you have will be just fine. They will still trip at the same exact current. They (24V ones) will likely be more durable on 12V. Unless you are running some very special 12V equipment, you will never notice the voltage drop issue (although you might be able to measure it with a good digital meter). Don't pitch them! Use them!

If there is doubt, you can measure the DC voltage on either side of the breaker and see what the difference is. Most 12V electronic items are designed to run from 10.5 to 14.5V anyway.

Fuses do have voltage ratings, but will also blow at the correct current. The volt rating is either for minimizing percentage of voltage drop (at 12V versus 125V) or for safe interrupting at the higher voltages (600V etc). A 10A 600V fuse might drop 10 volts at 10A, but a 12V 10A fuse would only drop a fraction of 1 volt at 10 amps. With the low voltages we deal with, it's more about the percentage of overall voltage that might be wasted in the device than anything else.

but for me, nothing is as fail-safe as these DC electromagnetic breakers. they are simple to mount and they are failry cheap surplus.

http://www.fairradio.com/catalog.php?mode=view&categoryid=194
http://www.tannerelectronics.com/ -usually 20A-100A in stock
http://www.surplussales.com/Electrical/CircuitBreakers/ElecCirB-7-1.html



Please consider putting at least one single 50A, 70A, or 100A depending what you are doing and what wire size you are using, in circuit between the battery and the fusebox. if there's a short and it isn't big enough to trip, you can just 'turn it off':

 

[OPCOM]

also, Haney routinely has these real nice 30A self-resetting Klixon breakers. mil-spec, no cheapies. "THERMAL CURCUIT BREAKER, WILL AUTOMATICALLY RESET AFTER TRIPPING. USED MILITARY SURPLUS REMOVED FROM EQUIPMENT. PART NUMBER PSA-25, MADE BY TEXAS INSTRUMENTS. RATED FOR 28 VOLT DC, USED IN MILITARY VEHICLES"
check his 'place' store;
http://stores.ebay.com/HANEY-ELECTRONIC-COMPANY

this is not an ad, just a positive comment. I have always had great luck with this vendor.

 

[Jones]

I've got a circuit board with a row of the breakers you posted Patrick. I'll check and see what they're rated for. One thing I'm especially protective of is an vehicle mount FLIR system that I'd rather not set afire. I priced a replacement FLIR aperture (camera) and the quote required CPR to get my heart re-started.