UBM Tech
UBM Tech

Thermal vs. magnetic circuit breakers

-March 03, 2008

Because of an experience when I worked for GMC Truck and Coach 30 years ago, I have always thought thermal circuit breakers were not very good. Yet a recent experience with a thermal breaker seems to prove they work fine when properly deigned and specified. The experience I had at GMC involved the use of thermal breakers in the heavy-duty over-the-road trucks we made, the GMC General and the Astro 95. We switched to thermal breakers because they were much much cheaper than magnetic breakers. Things were OK for a while but then we had a few trucks that burned up in the field. Since these trucks had sleeper cabs and were also dwellings, this concerned GM very much. It turns out that the thermal breakers would work fine for a dead short. They would snap open for a certain time, then close and if the short was still there they would open up again. They took many thousands of these cycles so we figured they would work fine. The problem came where there was not a dead short but a cut wire or some saltwater that would conduct right about the rated capacity of the breaker. The breaker would heat up and pull off the contacts, then immediately cool down and gently start to close. This made the contacts sizzle and fry and eventually weld together. Then the wiring in the truck would overheat and catch fire and then the truck would burn up.

Yesterday I went to print a document and the plug-strip thermal breaker popped, killing my computer, a couple of TVs and the printer. I saw that the plunger on the thermal breaker had popped out. I pushed it in a few times but it would not latch, this turns out to be a good thing since it meant that the bi-metal strip was still too hot to close. Then I got the breaker to reset but the switch did not seem to work. So I cycled the switch five or ten times and pulled the plug strip out and plugged it back in and finally it started working. But it made this really sick sizzling noise when it started working. Worse yet, about 20 seconds after it was working it made another sickening sizzling sound. Not wanting to burn down the megaplex, I drove to Orchard Supply Hardware and bought another plug strip— actually a plug expander that makes a duplex outlet into 6 outlets.

I just tore the old plug strip apart and it turns out that the thermal breaker was fine. It worked like it should when the printer overloaded it. What made the sizzling noise was the switch. And it was not the contacts of the switch, but rather, the pivot point that was all burned up. The switch was made by Shinden Co. Ltd, of Japan. I am glad I replaced it and I apologize to thermal circuit breaker manufactures everywhere for assuming it was the breaker. After tearing apart the breaker, made by Mechanical Products of Jackson Michigan, I can see it is a very well designed unit and it works great. The first big difference to those breakers that burned down the trucks is that it has a plastic slide that comes in between the contacts, so once it blows, you have to press the plunger to reset it. The truck ones just would cycle by themselves. The other big difference is the large size of the bimetallic spring. It forms a Bellville washer so when it pops open it does so with a nice snap, and more importantly, it snaps closed just as assertively so there is not a sizzle or resistive contact that burns things up.

I should note that many years after the thermal breaker problem at GMC the very nice Raychem people came trying to sell us Polyfuses,. Since they work by expanding a carbon matrix in plastic, there are no contacts to sizzle. Furthermore, you have to completely remove the load before they will reset or they just stay on. These seemed great but there is an issue with automotive applications. The polyfuse is also a temperature sensor—it will trip at a lower current when the ambient air is warm. This is a real problem in cars, since they are expected to work at 120 degrees F as well as –40 degrees F. The thermal breakers using a bi-metallic spring also have this issue, although not to such a degree. They too will pop sooner when the outside air is warm. If you have a large temperature swing in the operation of your product you may what to use magnetic breakers or those spiffy electronic ones that use chips by Maxim and Linear Technology as well as TI and others.

Thermal_breakerHere is the plug-strip or duplex outlet expander that started sizzling and made me worry it would burn my house down.

Thermal_breakerIt is a SNAPIT. Yup, I snapped it right into the dumpster.

Thermal_breakerHere are the guts. Nearby is the gorgeous SPYDERCO knife I used to pry the plug strip apart. I hate products that are glued together.

Thermal_breakerThe expander had two plugs on the back to plug into a duplex socket. To my amazement only one of these was wired it. So much for reducing contact resistance. And to think that the expander I just bought has two plugs too, but I bet they are also only using one. I should have bought the expander that jut had one plug on the back and a plastic pin to locate the ground lug in the second duplex outlet.

Thermal_breakerThis is the switch that made the sizzling noise. You can see the contacts are just fine. It is the fulcrum that is burned up.

Thermal_breakerHere are the guts of the beautifully made Mechanical Products circuit breaker. Note the spring that pushed the plastic plunger between the contacts once the breaker opens. You can just make out the white tab on the right that slides up between the contacts. The metal tab on the left side of the spring is a weld.

Thermal_breakerA lighter serves as a convenient way to test the action of the bi-metal spring in the circuit breaker. It pops open so crisply, I was very impressed. Better yet, it closes nice and firmly, so you will have contact pressure when you press the plunger and let the contacts close.

Thermal_breakerHere is a close up of the bad fulcrum on the switch.

Thermal_breakerThe breaker contacts were fine. A little pitting but no burning.

Thermal_breakerHere is the circuit breaker contact on the bi-metallic spring side. It too looks pretty good, for a plug strip I have had for twenty or thirty years.

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