Fuse and circuit breaker

[stuartR1963]

Hi all I was moving my tv set and there was quite a lot of tangled unused cables which I wanted to remove, as i was in a rush I rather stupidly cut the cable to the extension lead which resulted in a loud bang and bright flash as well as a big hole in my cable snips, this tripped out the 32 amp ring circuit breaker, but when I tested the fuse on the extension lead and it still had continunity, it never blew.

So my question is why did the mcb trip faster than the 13 fuse take to blow in the plug? The rcd never tripped either, just the 32amp mcb tripped, but the fuse in the plug to the lead I daftly cut was still intact.

Just curious, and yes I know I was daft, never got shocked luckily just a bit shaken.

 

[ericmark]

The magnetic part of a circuit breaker (MCB) will operate in 0.1 seconds or less, however to operate it often needs 5 times the rated value so 160A a short circuit will allow this current to flow, however a lesser fault may not draw so much current so to work the thermal part takes a lot longer, so then fuse would rupture first.

For an RCD to trip power out needs to be different to power in, this will normally happen if some power goes to earth, however line to neutral what ever power will not cause unbalance so RCD does not trip.

 

[rsgaz]

On this chart, you can see that a 13Amp fuse, could in theory cope with up to about 65 Amps for 3 seconds before it blew...

It's not as clear cut as people think. I.e. it doesn't suddenly blow at 13.1 Amps.

 

[JohnW2]

On this chart, you can see that a 13Amp fuse, could in theory cope with up to 100 Amps for 3 seconds before it blew...

I'm not so sure about "up to 100A" - it looks more like around 26A -- 65A to me. Nevertheless, your general point is valid, and important ...

It's not as clear cut as people think. I.e. it doesn't suddenly blow at 13.1 Amps.

Indded so. As we often point out to people (and as you can see from the curves), it needs something like 22A to ever blow a 13A BS1362 fuse (even if the current flows continuously and 'indefinitely').

Kind Regards, John

 

[rsgaz]

it looks more like around 26A -- 65A to me.

Oops, I was looking at 0.3 seconds. Damn blurry logarithmic charts(or my eyes)!!!

 

[securespark]

it needs something like 22A to ever blow a 13A BS1362 fuse (even if the current flows continuously and 'indefinitely').

Like the hand dryers I saw at Leeds Armoury... two each to one 13A fuse.

 

[ban-all-sheds]

The rating of each was ____ ?

 

[ban-all-sheds]

never got shocked luckily

Or burnt.

What do you think happened to the metal that was in the places where you now have holes in your cutter?


Don't necessarily bin those, BTW, depending on the size of the holes you may find you now have a handy wire stripper

 

[stuartR1963]

Very interesting, so the short circuit was in excess of 160 amps,phew, no wonder it blew a hole in my pliers and they were quite hardened steel. Its just as well they were insulated because I'm not sure I would of survived that kind of short circuit current, even though its rcd protected it would probably a hit or miss whether it would of saved my life or not.

 

[winston1]

Very interesting, so the short circuit was in excess of 160 amps,phew, no wonder it blew a hole in my pliers and they were quite hardened steel. Its just as well they were insulated because I'm not sure I would of survived that kind of short circuit current, even though its rcd protected it would probably a hit or miss whether it would of saved my life or not.

But that current would not have gone through you, it went from L to N via your cutters.

 

[ban-all-sheds]

And if it had "gone through you" it would have been nowhere near 160A, as even a small part of you has a great deal more impedance than a short length of steel.

But if you could, with a high enough voltage, contrive to pass 160A through you your chances of survival would be nil.

I believe that the voltages used with the electric chair result in currents of 10-15A.