I am currently working on a test fixture that functionally tests a PCB by applying 120VAC to it's Line and Neutral input. The device under test consumes about 600mA @ 120VAC and should never go above 1A. I have a 1A 240VAC thermal circuit breaker in series on the line input. To test the breaker I created a direct short across the power terminals that apply power to the DUT and got the breaker to pop. However, resetting the breaker does not work. It got pretty warm and I believe it melted internally. This happened with two breakers.

I understand that a direct short pulls an infinite amount of current through that breaker faster than the mechanicals can switch - causing a bunch of heat. So would an inrush current limiter with a steady state value just above that of the circuit breaker (say 1.5-2A) be an adequate solution to this problem?

How can I effectively protect my circuit and circuit protection in this case?

  • \$\begingroup\$ Given that this is a thermal breaker, have you waitied for it to cool off before trying to reset it? \$\endgroup\$ – Jack B Jun 8 '17 at 16:29
  • \$\begingroup\$ I have to ask..... you are applying 120V AC yes? \$\endgroup\$ – Trevor_G Jun 8 '17 at 16:45
  • \$\begingroup\$ Jack: Yes, but good question. Thanks for asking! \$\endgroup\$ – NFranklin Jun 8 '17 at 16:48
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    \$\begingroup\$ Trevor: Thank you for pointing that out. I am usually pretty good about specifying but missed it there. \$\endgroup\$ – NFranklin Jun 8 '17 at 16:49

Thermal breakers rated at 1A will interrupt the circuit after having 1A for a period of time. They're not instantaneous in the sense that as soon as you exceed 1A they'll interrupt.

That's why you normally get thermo-magnetic switches. This interrupt after a while 1A but much faster in case you get a serious short-circuit. This is done by the magnetic ones. This type of protection is the one normally seen in houses around the world.

This circuits also have a maximum interrupt current. You can't throw everything at them and expect them to work.

If failure is not expected very often you can use a normal fuse. But you'll have to change it every time you burn it.

BTW, never test things just shorting them. If the protection didn't work what could have happenned? Make sure you have a second line of safety and first start with a resistor that passes a bit more than 1A.

  • \$\begingroup\$ Thanks for the response, Andres! When you say "This circuits also have a maximum interrupt current." Are you referring to the circuit breaker? I'll look into thermo-magnetics. I understand just replacing a fuse would be the easy way to go here (failures should be extremely uncommon) but I prefer using a circuit breaker, it is simpler for the user. Thanks for the tip about about using a resistor to test. \$\endgroup\$ – NFranklin Jun 8 '17 at 16:47
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    \$\begingroup\$ A thermomagnetic has a maximum current it can break. If it's for example 500A, it will be able to stop any current less than that. If your short circuit were to be 600A, the current wouldn't be interrupted (probably there's an arc that has formed). This applies more to households. If the electric company tells you that the short circuit current is 1kA, you have to make sure you're able to stop that because that's the current you'll get if you short-circuit the two cables given by the electric company. You shouldn't have problems with this since there should be more protections upstream. \$\endgroup\$ – Andrés Jun 8 '17 at 22:49

If you did indeed damage your 1A breaker, then I suspect that you exceeded the maximum interrupting current specified by the manufacturer. For example, if you have a 120VAC circuit, and 10A of load brings it down to 118V, then you have 600 Amps of short circuit current so your interrupting current on the circuit protection device needs to be at least that big.

To answer the question other than telling you to get a better circuit breaker: put a resistor in series with your AC supply to reduce the short circuit current of your supply. It will have to very high wattage resistor. One or more large several hundred Watt light bulbs works great for a resistor.


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