Electrical – find information on slow blow breakers

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In a previous question, the comments mentioned slow blow circuit breakers. I have done a web search and went to Square-D's website(the maker of my current breakers) and came up empty. There is a lot about curves and such but nothing that makes sense to this laymen. Can anybody help?

Best Answer

I don't know about "slow trip circuit breakers", but maybe I can help you understand trip curves (and circuit breakers) a bit better. Then you'll see that all circuit breakers are "slow trip".

Your basic, everyday, run of the mill circuit breaker offers two types of protection. Short-circuit protection is provided using a magnetic trip function, while overload (over-current) protection uses a thermal trip function.

Short-circuit Protection

To provide this type of protection, a circuit breaker relies on the fact that current traveling through a wire creates a magnetic field. The breaker basically uses a solenoid to open the circuit, in the event that there's a large fault current flowing. If there's enough current flowing, the magnetic field is large enough to pull open the circuit. This is often known as an instantaneous trip, because it can react in just a cycle or two (0 - 0.05 seconds).

Overcurrent Protection

This type of protection is provided using a thermal device, that works based on the fact that current traveling through a wire generates heat. As the current flowing through a wire increases, so too does the temperature of the wire. A circuit breaker uses a bi-metal strip to trip the breaker, if the temperature (current) gets too high. This is known as "long trip", or "slow trip", because it takes time for the breaker to react (0.5 - 1000 seconds).

Trip Curves

If you look at the trip curve of a breaker, you can see both types of protection in action. You'll notice a slow arcing motion leading down the chart, this is the "long time trip" portion of the chart. As the current increases, the time required for the breaker to trip decreases. Eventually, the breaker hits the instantaneous trip threshold. At this point the breaker trips within a cycle or two. This can be seen at the bottom of the chart, where the curve levels off and goes straight across the bottom of the chart.

Now that we've covered the basics, let's take a look at an example. Here's the trip curve for a 20 ampere Square-D QO circuit breaker.

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Click for larger view

The vertical axis is time in seconds, and the horizontal axis is multiples of current. I've highlighted in orange what two times the breakers rating looks like (40 amperes). As you can see, even at 40 amperes this breaker shouldn't trip for about 9 to 35 seconds. This is because it's relying on the thermal protection, and the bi-metal strip has to heat enough before it pulls the circuit open.

I've also highlighted in red what it would take to trip the breaker in one second, since an observer might consider a second "instantaneous". You'll notice that it should take between 4.5 and 8 times the breakers rating, before the breaker trips in under a second. That means your saw would be pulling 90-160 amperes, when the breaker tripped.

If your saw is pulling that much current, the problem is not the breaker, it's the saw. Increasing the size of the breaker is not the solution.

Check the saw for faults, including a short-circuit. You may also want to test/replace the breaker, as breakers can go bad.

After some more research, there do seem to be what are known as "High Magnetic (HM)" circuit breakers available. Schneider Electric describes them as

High magnetic trip circuit breakers are recommended for applications where high initial inrush may occur and for individual dimmer applications.

One such device would be the QO120HM Miniature Circuit Breaker. I couldn't find a trip curve for this device, but rumor has it the instantaneous trip function may not kick in until 20-30 times the rated current (400-600 amperes for a 20 ampere breaker). However, I don't think these are applicable to your situation, since there's no way the saw should draw 400 amperes when starting (or at any time for that matter, unless there's a short-circuit).