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Minute spark on earth block in c.u

Postby remelad » Sun Apr 27, 2008 6:52 pm

Hi all,
changed a light fitting today,when i went to the consumer unit to isolate the circuit i noticed that the earth to a separate shower c.u had become detached from the earth block. I reconnected the earth and thought i would have a look inside the main c.u,i noticed a cpc had no sleeve on it so i put this right, but as i put the cpc back i noticed a minute spark. I have tested between the cpc and neutral terminals and get no readings so have assumed this to be okay.can anyone please tell me what could cause this minute spark.
Many thanks for any help
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Postby ericmark » Sun Apr 27, 2008 7:59 pm

There are many reasons why you can get a spark which are all nothing to worry about. Capacitance, transformer effect (Inductance) and even static can all allow this to happen. The most dramatic I came across was on a tower crane used in the building of Sizewell C where the wind produced 3 inch long sparks from hook to steel reinforcing bars.

Although if as regulations state you had isolated else where before removing the cover from the consumer unit it would be unusual but with all power off in the house could hardly be blamed on electrical fault? Yes I know I have also worked on things live but it is very dangerous and I will draw your attention to ionization of the atmosphere this is not restricted to lighting strikes but is a very real hazard of live working.

The saying fools rush in where angles fear to tread comes to mind. Do switch everything off next time! borrowed neutrals are not unknown! and in my experience electricity hurts?

Postby sparx » Sun Apr 27, 2008 8:10 pm

Hi remelad,
most likely to be a bit of 'pick-up' from adjacent circuits [known as induced EMF by the intelligencia].
Cables act as capacitors and can build up a small charge if left disconnected, which is why it is not recommended to leave unused cores of cables disconnected. "Capacitor [def.]= 2 conductive surfaces seperated by an insulated layer" larger surface area = highest charge.
So not to worry too much particularly if it was say 6mm2 that sparked.
Worst ones are SWA's or worse still MICS/Pyro's which is why when 'meggering it is important to release test button on tester BEFORE disconnecting test leads as meters have a built in discharge resistor.
If just disconnected with test voltage say 500v dc still applied the cable can be left holding a significant charge!
regards SPARX
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Postby remelad » Mon Apr 28, 2008 6:38 am

Thanks very much ericmark and sparks for your input.
I did wonder if this could be induced emf, as i have come across this before on a cable that was disconnected both ends.
Sometimes novices like me, like clarification from guys like yourselves that have been in the trade for some time, and have come across such instances.
As you guys will know i am part p,and am never to old to learn. Your input is much welcomed by me, and everyone else who posts here looking for answers
Many thanks Remelad

p.s....ericmark thanks to your reply on another post about the neutral and earth combined.
Regards Remelad
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Posts: 49
Joined: Thu Jun 07, 2007 8:03 am

Postby ericmark » Mon Apr 28, 2008 11:41 am

Did not realize you were a spark sorry for my comments I do worry about the DIY man going into consumer units but as a spark you will avoid the dangers. If I had realized I would have gone a little deeper but Sparx covered it anyway.

All best Eric

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