Testing 5x20mm fuses

terrypingm · 2018-10-20T17:17:26+00:00

I was curious about the actual current needed to blow my newly purchased se t of 250V 20mm fuses. Half a dozen experiments with 0.5A, 1A and 2A samples indicated a multiplier of 3.0-3.5. For example, 3.4A to blow a 1A fuse. Is that typical? P.S: While googling about fuses I came across this scary incident on a rath er larger scale: I?d recommend watching from the start for atmosphere, although the incident is at 08:49. Terry, East Grinstead, UK

Oct 20, 2018 24 Replies

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Steve Wilson 7 years ago

Circuit breakers seem to do the same thing. I have a situation where I have to draw more than the 15 Amp circuit rating. If I do this for 15 minutes or so, the breaker pops. So I limit the overload to 5 minutes and the breaker stays on.

This is an undesirable and temporary situation. The parts needed to fix the problem arrived yesterday. All I need is the time to put it together.

But I think you may be able to overload a circuit for a limited time without blowing the breaker. Don't overdo it. You might burn the house down!

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Phil Hobbs 7 years ago

Thermal breakers do that on purpose. You can get thermal-magnetic breakers as well, which are much faster but iirc have the annoying habit of tripping when some motor starts up. (My joint has all thermal breakers.)

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com

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Phil Allison 7 years ago

** Thermal-Magnetic breakers (aka MCBs) are standard items on distribution boards. Intended to protect building wiring much the same way as old fashioned fuse wire did, with the distinct advantage of be easily resettable.

The internal design is non-trivial but how they behave is simple enough.

Used at or below the nominal current rating, the breaker should never trip.

Exceed the nominal amps figure and wait long enough and it will, a high ambient temp will make waiting time less.
A larger overload (5 times or more) causes the breaker to trip almost instantly by virtue of a magnetically operated latching switch in series with the bi-metal one.

10 or 20 times overload reduces the delay time to a few milliseconds or a small fraction of one AC cycle.

Like HRC fuses, MCBs are rated to break very large currents, thousands of amps if need be.

BTW:

MCBs work just as well in 12V DC circuits as they do with 120/240V AC ones.

Saves carrying around and using up lots of wire fuses in some situations.

.... Phil

(My joint has all thermal breakers.)

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tabbypurr 7 years ago

MCBs pass almost 50% over rated current indefinitely

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I struggle to believe that's safe. I don't see how it can be. A hard short would surely melt & set fire to things before they acted.

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Phil Allison 7 years ago

** Seen far too many open much closer to the rated number to fall for that one.

Allow for a fairly high ambient due to warm weather and adjacent breakers that are running near limits. The bi-metal is affected by any heat source.

** Yeah, thermal only sounds unsafe to me too.

Thousand amp+ fault current would probably weld the contacts together.

... Phil

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