Selectively question

[Wilson12]

Hi I was just wondering an irons flex had been twisted until the line and cpc is visible there was said to be a bang and I can see it’s burnt between the two. my question is the 13amp fuse is fine but the rcd tripped and the mcbfor the circuit was down also. i understand there was a earth fault so rcd tripped and as short circuit the mcb tripped... But why didn’t the 13 amp fuse trip before 32a mcb and why did rcd and mcb both trip surely one is faster then the other?

 

[Strima]

Fuses take longer to operate than MCBs. When the venerable fuse was designed when we used fuses in our boards so the 13 amp would usually operate before the 30 amp fuse wire.

 

[Wilson12]

Thanks but why did the rcd and mcb trip I thought it would be one or the other e.

 

[Strima]

Thanks but why did the rcd and mcb trip I thought it would be one or the other e.

The MCB may well have tripped first then the RCD detected the earth fault and also operated, it's not uncommon for this to happen.

In short the live and neutral go pop, this damages the insulation on the neutral and earth conductors causing the RCD to then operate.

 

[ferg]

The MCB may well have tripped first then the RCD detected the earth fault and also operated, it's not uncommon for this to happen.

In short the live and neutral go pop, this damages the insulation on the neutral and earth conductors causing the RCD to then operate.

Thanks... it took me ages to type out the same thing with one finger on my tablet......
?

 

[Lucien Nunes]

It's not unknown for a B32 to trip while a 13A BS1362 stays intact (although, the fuse may be weakened, just not completely ruptured.) The behaviour of fuses and circuit breakers under short-circuit conditions can be complex and one needs to compare the time/current curves and/or the I²t values to get an idea of how different their responses will be at a particular value of short-circuit current.

Two devices can trip in series, in this case an MCB and an RCD, because tripping is not one single instantaneous event. A contact opening against a high current draws an arc, which is conductive, so even once the mechanism has done its job the current will continue to flow until the arc extinguishes, either by being split up by the chutes within the breaker or when the current waveform passes through the next zero of the AC cycle. Therefore even if one breaker is mechanically faster than the other and opens first, the arc might last long enough for the slower breaker to catch up.

There are also mechanical delays as the tripping armature accelerates, releases the toggle, then the contact accelerates etc, plus magnetic ones as the flux builds within the tripping coil. There can be a short period during the trip event where things are already on the move and the contacts will inevitably open even if the fault is cleared elsewhere first.

 

[Wilson12]

It's not unknown for a B32 to trip while a 13A BS1362 stays intact (although, the fuse may be weakened, just not completely ruptured.) The behaviour of fuses and circuit breakers under short-circuit conditions can be complex and one needs to compare the time/current curves and/or the I²t values to get an idea of how different their responses will be at a particular value of short-circuit current.

Two devices can trip in series, in this case an MCB and an RCD, because tripping is not one single instantaneous event. A contact opening against a high current draws an arc, which is conductive, so even once the mechanism has done its job the current will continue to flow until the arc extinguishes, either by being split up by the chutes within the breaker or when the current waveform passes through the next zero of the AC cycle. Therefore even if one breaker is mechanically faster than the other and opens first, the arc might last long enough for the slower breaker to catch up.

There are also mechanical delays as the tripping armature accelerates, releases the toggle, then the contact accelerates etc, plus magnetic ones as the flux builds within the tripping coil. There can be a short period during the trip event where things are already on the move and the contacts will inevitably open even if the fault is cleared elsewhere first.

Wow thanks a lot