Re: Short circuits

[Dambo]

Regarding short circuits - Would an example of this be a cable being severed at a certain point? And if so wouldn't the RCD or circuit breaker be automatically triggered to avoid anyone coming into contact with live current?

 

[Apache]

A short circuit is where there is contact between phase and neutral and the current can flow between without passing through an accessory.

In your example it would depend how the cable was severed. If you used your nice pair of VDE sidecutters then chances are you would short L-N and trip MCB or short L-E and trip the RCD. If you were careful and nibbled in from each side it may be possible to cut it without tripping anything, if the blade never contacted L&N simultaneously.

If you isolated, cut cable then re-energized the circuit then there would (likely) be no L&N contact and no short circuit.

The Vet thinks.....

 

[Dambo]

Cheers Apache!

 

[Northern Electric]

An RCD only trips under earth fault conditions, ie L-E. MCB would trip under a short circuit fault, ie L-N.

 

[Dambo]

What normally happens when a short circuit occurs?

 

[Apache]

What normally happens when a short circuit occurs?

Bang and then the MCB trips.

 

[Andy™]

In your example it would depend how the cable was severed. If you used your nice pair of VDE sidecutters then chances are you would short L-N and trip MCB or short L-E and trip the RCD. If you were careful and nibbled in from each side it may be possible to cut it without tripping anything, if the blade never contacted L&N simultaneously.

if you also short N & E, that can trip an RCD. if the circuit is isolated, but still on RCD, i try to cut L&E on one side, and the neutral seperate, to try and avoid RCD tripping

 

[ian932]

Bang and then the MCB trips.

and don't forget the FLASH and bits of molten metal

 

[Northern Electric]

if you also short N & E, that can trip an RCD. if the circuit is isolated, but still on RCD

This happens when you isolate the circuit by the mcb, but the mcb is fed via an rcd, ie, 17th edition split boards.

Technically the circuit is not isolated because the neutral is still terminated into the neutral bar. When you cut the cable enough current flows from the neutral down to earth, thus causing an imbalance in return current, thus causing the RCD to operate. This situation would not be classed as a short circuit, it is an earth fault.

Andy, I am not correcting what you are saying, just giving an explanation for the more in-experienced users of the site to understand what happens when this situation occurs.

 

[Andy™]

This happens when you isolate the circuit by the mcb, but the mcb is fed via an rcd, ie, 17th edition split boards.Technically the circuit is not isolated because the neutral is still terminated into the neutral bar. When you cut the cable enough current flows from the neutral down to earth, thus causing an imbalance in return current, thus causing the RCD to operate. This situation would not be classed as a short circuit, it is an earth fault.

Andy, I am not correcting what you are saying, just giving an explanation for the more in-experienced users of the site to understand what happens when this situation occurs.

a short circuit is a low resitance between 2 conductors.... connecting neutral and earth is a short circuit!

 

[Northern Electric]

In mains circuits, short circuits are most likely to occur between two phases, between a phase and neutral or between a phase and earth (ground). Such short circuits are likely to result in a very high current and therefore quickly trigger an overcurrent protection device. However, it is possible for short circuits to arise between neutral and earth conductors, and between two conductors of the same phase. Such short circuits can be dangerous, particularly as they may not immediately result in a large current and are therefore less likely to be detected

Well, it looks like I got something wrong! I would always have classed any fault down to earth as an earth fault.

I googled short circuit and found the text above on a website.

My appologies on that one.

 

[Andy™]

Well, it looks like I got something wrong! I would always have classed any fault down to earth as an earth fault.

I googled short circuit and found the text above on a website.

My appologies on that one.

your partly right: it is an earth fault because its a fault to earth, but its also a short circuit!

 

[Northern Electric]

Ever since day 1 of sparky school I have always classed any fault down to earth as an earth fault. Short circuits were classed as a fault on line conductors. Just goes to prove you learn something new on here everyday, even after 20 years in the game!

Cheers Andy.

 

[nic00b]

and don't forget the FLASH and bits of molten metal

Have you ever pulled out a DC knife switch? Thats a nice FLASH