exam puzzle

[ianiow]

Hi guys,

I am a 3rd year student and have been set a fault finding problem. The answer is probably glaringly obvious, but my poor overworked brain is not seeing it.

Circuit:- Loop in lighting circuit with a two way switch.

Info:- A nail was hammered into a wall and the lights went bang.

My test results:-

Continuity at the ceiling rose, PN PE NE all fine, but with PE and NE the continuity is not cut off when the light switches are pressed.

Insulation Resistance, PN PE NE all fine (>2Mohms) but PN and NE readings are not switched off when the switch is flicked.

On hearing about the nail thing I thought it would be an Insulation failure. But my results look more like a polarity issue between the live wires in the switch cable.

The answer I intend to give is that "the nail has damaged the cable between the ceiling rose and the first switch, causing crossed polarity between the live switch conductors". But i am not confident!

:C any thoughts?

Thanks in advance!

Ian

 

[fletchflash]

Have you done Ins res tests on all switch wires to earth?

Dont forget you need to switch both switches to test both sides of 2way switch circuit

 

[JUD]

Hi guys,I am a 3rd year student and have been set a fault finding problem. The answer is probably glaringly obvious, but my poor overworked brain is not seeing it.

Circuit:- Loop in lighting circuit with a two way switch.

Info:- A nail was hammered into a wall and the lights went bang.

My test results:-

Continuity at the ceiling rose, PN PE NE all fine, but with PE and NE the continuity is not cut off when the light switches are pressed.

Insulation Resistance, PN PE NE all fine (>2Mohms) but PN and NE readings are not switched off when the switch is flicked.

On hearing about the nail thing I thought it would be an Insulation failure. But my results look more like a polarity issue between the live wires in the switch cable.

The answer I intend to give is that "the nail has damaged the cable between the ceiling rose and the first switch, causing crossed polarity between the live switch conductors". But i am not confident!

:C any thoughts?

Thanks in advance!

Ian

I'd go with that but I wouldn't call it crossed polarity as both wires are in effect the same conductor. They have just become bridged by the nail and as a result the switches are being bypassed.

 

[sparkytim]

I would say thats a short circuit beween all the conductors going to the switch, duplicating what the switch would do, hence operating the switch does not change any readings.

It would also have made contact with the CPC to have blown the Circuit Protective device, so you would get LOW IR readings between L - E.

 

[kme]

+1

 

[JUD]

But his IR readings are fine so I would say that the nail has made contact with the CPC whilst being hammered in, but somehow is now no longer in contact with it and is just bridging the two switch lines. :C

 

[sparkytim]

Then fault must have severd cpc AND fault current melted back cpc but left OTHER conductors bridged by nail.

 

[JUD]

:Applaud :Applaud:Applaud

 

[ianiow]

Thankyou very much guys! 'Bridging' the conductors is the word I was looking for

And yes, it is strange that the CPC was not affected in some way by the nail, I guess it just squirmed away like the ball from Rob Greens hands..

I will let you know if I pass the exam with this answer. If not I will pester you again before my my resit!