How to protect a 1A 120V circuit breaker from direct short?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

Typically household circuit breakers (ie inside your electrical panel at home) have an interrupt rating of 10,000A. That is because a 240V 60hz AC source can draw that much current in seconds. The circuit breaker must be certified to safely handle that much current in order to interrupt the circuit (fault condition) safely. Without a properly rated short circuit current (interrupt rating), bad things can happen. The CB can fuse closed (melt together), and then the current flow will only stop if more components melt (catastrophic failure of components); this is usually the point at which the building gets very warm, and everything smells like s'mores... but with more plastic!

One simple method to protect yourself, such as when designing projects like this, is to use a transformer. This will isolate your circuit from the mains supply. Short circuit current is easily calculated for a transformer, and then your circuit protection can be implemented safely. An isolation xfmr also can help you reduce other components to save money on the project, but limiting the power capacity of the circuit/ device. It is an extra cost, and weight, but it's a huge step forward in safety, and thus worth a thought in the planning stages.

Likewise, you can use other sources of power supplies, but this post has rambled on long enough.