2392 question

[jimmymack56]

Hi
Iam studying for a 2392 course and one of the questions is
State the effect of the following circuit conditions on the protective device
if they occured on the load side of a protective device that was installed
in the neutral of a single phase circuit

A short circuit
B overload
C earth fault

my limited knowledge extends as far as knowing that if the pd is installed in the neutral
that it would trip but still leave the live hot,can anyone enlighten me on what else may
happen,we only touched on the subject at college and can't seem to find much more info
on the net
Thanks in advance

 

[Lucien Nunes]

Think of the route the current takes in each case, and whether it goes through the protective device as it should, or misses it. E.g. in A, the short circuit current passes through line and neutral, so the device will respond to it as normal, although leaving the load alive when it opens, as you mention.

 

[DPG]

As a start I would draw out the circuit and follow the current flow in normal, and in each of the fault conditions. See if this helps. Daz

 

[DPG]

Lucien you posted that at the same time as me! Daz

 

[jimmymack56]

Thanks Lucien
thinking about what you said,would in the case of B and C current keep flowing until
it eventually overheated and cause a fire?

 

[Lucien Nunes]

Surely from your sketch it is apparent that the current path in B & C is different...

 

[jimmymack56]

I didn't do a sketch yet,i will when I get home,i was just running
it through my head,Obviously a rethink is needed

 

[Richard Burns]

A short circuit is a low impedance fault from line to neutral approaching the prospective short circuit fault current.
An overload is a current draw from line to neutral by a load that is in excess of the current limitations of the protective device.
An earth fault has several meanings but here they presumably mean a low impedance connection between line and earth that will generate a fault current approaching the prospective earth fault current.

From these descriptions and the help given above you should be able to work out the effects on a protective device located in the neutral supply.

remember the question is not asking if the equipment is live after the fault occurs (although this is the case), it is asking what would the effect be on the protective device. e.g. will it trip, explode, not trip, disconnect all live conductors, etc.