Help required, odd problem

[prideofengland]

I was called to a local college where we do some maintenance work, a guy who had been installing an overhead projector had drilled into the wall to fix some mini trunking. hit a cable, flash lights gone out.

When I arrived he showed me where he had drilled, I checked the DB, lighting mcb tripped. The wall was plasterboard partition so cut out a reasonable size hole, no cable to be seen! He had drilled through a tin upright which I presume had caused the spark, but definately no cable anywhere near by.

Carried out IR test L-E and N-E 12 meg. Reset the Mcb lights working zs fine. Still - concerned I may have missed something and confused as to why the lights tripped, checked voltage at the tin upright - 30v between the tin and the earth on nearby socket.

Now i am aware that metalwork concealed in the fabric of the building wont be bonded and so may not be at 0v but 30v seems very high, could this have something to do with the lights tripping? should I bond the Tin? Any advice comments welcome. My brain is cabbaged going down the pub!

 

[Spudnik]

When you carried out the IR test are you sure all the lights were isolated and you werent including them on the test?

With regards to the tin, did the voltage disappear after switching off the MCB?

 

[Guest123]

Hey.

You wouldn't happen to have suspended ceilings in the classroom would you??

 

[prideofengland]

No didnt have time to disconnect all the ballasts and lights not on click roses so just tested to earth not L-N. Didnt think to check for voltage at the tin with the mcb off - doh! well it was late on a friday!

Lennytheloon

No its an old building with concrete ceilings.

 

[Guest123]

Well thats my theory blown out of the water then.

 

[Adrian Rawlings]

The 30v you measured might be high impedance. I had a parallel experience to yours; using a volt stick, the whole plasterboard wall appeared to be 'live', because the thing glowed red within an inch of it, anywhere. What the volt stick does is to pick up the electrostatic field (high impedance).
The cause of the problem was the cable behind the plasterboard had no CPC, so the electrostatic field could just leak out.
If you used a low input impedance voltage tester, such as a Fluke 1000 (wierd shape, built-in ammeter, yellow), I bet you'll get only a low reading.
Solution to the problem is to earth the metalwork.
Hope this helps, Adrian

 

[maddfridge]

I was called to a local college where we do some maintenance work, a guy who had been installing an overhead projector had drilled into the wall to fix some mini trunking. hit a cable, flash lights gone out.

When I arrived he showed me where he had drilled, I checked the DB, lighting mcb tripped. The wall was plasterboard partition so cut out a reasonable size hole, no cable to be seen! He had drilled through a tin upright which I presume had caused the spark, but definately no cable anywhere near by.

Carried out IR test L-E and N-E 12 meg. Reset the Mcb lights working zs fine. Still - concerned I may have missed something and confused as to why the lights tripped, checked voltage at the tin upright - 30v between the tin and the earth on nearby socket.

Now i am aware that metalwork concealed in the fabric of the building wont be bonded and so may not be at 0v but 30v seems very high, could this have something to do with the lights tripping? should I bond the Tin? Any advice comments welcome. My brain is cabbaged going down the pub!

hi there

if you have metal walls one must assume they are bonded (?) assuming pme tncs system to main earthing terminal neutral bonded any live to earth could have been enough to trip out the mcb, loose light wire etc.

i had a similar fault which turned out to be a defective lighting capacitor. i just wondered if he had caused a capacitor to discharge assuming you have florescent lighting in your class rooms


would cause a volt stick to glow if it is discharging out of interest see what values the caps are and if they all look good if they are browning they are breaking down
cheers

 

[johnnyb]

Could there possibly be another fault say another cable thats going through the same piece of steel somewhere lower down or something. We have been to rip outs in the past where the builders have ripped partitions down and we have seen cables there going through holes with no grommets or the like, you may be picking a leak off another circuit possibly.

 

[maddfridge]

Could there possibly be another fault say another cable thats going through the same piece of steel somewhere lower down or something. We have been to rip outs in the past where the builders have ripped partitions down and we have seen cables there going through holes with no grommets or the like, you may be picking a leak off another circuit possibly.

hi there

Thats a really good point

It could even be a transient voltage from one circuit damaged going back to the main earth terminal to the origin of the installation.
Try taking a wander lead from the consunit and see if the wall is actually connected to earth and see if you have a voltage from the wall to the met

cheers

 

[Spudnik]

 

[prideofengland]

Thanks everyone for your help, wether its an electrostatic field, or a fault on another circuit running in the wall will be hard to tell. I think I will recheck for a voltage with the lighting circuit isolated to see if there is a direct link - if there is I will have to investigate further. If not I will just try bonding the tin stud to earth.