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Topic Title: dangerous voltage on earth conductor
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Created On: 14 January 2008 07:09 PM
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 14 January 2008 07:09 PM
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misterben

Posts: 415
Joined: 11 June 2007

Can anyone help me out? I have been asked to extend a lighting circuit in a house, on checking the circuit is healthy to proceed with work, i have found that the downstairs lighting circuit has between 50 and 120v on the cpc at all times.The property has metal switches so obviously needs sorting asap. I have broken the circuit down and carried out IR testing all is clear, when i have all earths from the four other circuits disconnected from main earth terminal and test between each of them i have around 3Megohms including each one to the main earth terminal, except for the Immersion circuit which is clear from all other earths as id expect. Any advice or experience of this problem would be greatly appreciated.

Misterben
 14 January 2008 07:25 PM
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trix

Posts: 606
Joined: 14 September 2006

Hi, you've probably got induced voltage,
picked up by digital reading meter.
best test with load i.e. vibration or lamp ?
 14 January 2008 07:34 PM
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misterben

Posts: 415
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Thanks Trix, what could be causing this in a domestic situation? I am pretty sure there arent any inductive loads on the circuit

misterben
 14 January 2008 08:10 PM
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FizzleBang

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Hi,
Against which reference are you measuring the volage on the CPC? Are you getting a voltage on the cpc while it is connected to the earth bar in the CU?

If measured between the cpc at various points in the circuit and the MET then I suggest that the CPC is open circuit at some point early in the circuit.

Have you measured R1+R2 or R2 using a wander lead?

As said already any voltage present is likely to be only induced and very high impedance. But if it is present when the CPC is connect in the CU then it indicates an open circuit CPC.

Or maybe I've misunderstood your post.

Paul

-------------------------
"I learned very early the difference between knowing
the name of something and knowing something". - Richard P. Feynman
 14 January 2008 09:32 PM
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misterben

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thanks for the reply Paul, i got the voltage when it wasnt attached to the earth terminal. I havent tried it connected to the earth terminal, does this mean if it is induced or stray voltage then it isnt harmful cos as i said its present on the metal switchplate 2feet above a radiator!

misterben
 14 January 2008 10:35 PM
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intrinsic4225B

Posts: 1623
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What sort of instrument are you measuring this voltage with?

If you are using an instrument with a high input impedance such as a digital multimeter, then the voltage may be an induced voltage of high source impedance which is only measurable due to the high input impedance of the instrument.

Try connecting an test lamp or an analogue multimeter of low input impedance across the conductors in question. You will probably find the voltage is no longer apparent - even if you connect your high impedance instrument across them with the other instrument connected.

You should, of course proceed on the basis of the conductors actually being live at the voltages measured previously, until you have proven the nature of the voltage.

You may find it instructive to have search of the forum for a variety of threads on similar issues and and related matters.
 14 January 2008 10:50 PM
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GeoffBlackwell

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Originally posted by: misterben

thanks for the reply Paul, i got the voltage when it wasnt attached to the earth terminal. I havent tried it connected to the earth terminal, does this mean if it is induced or stray voltage then it isnt harmful cos as i said its present on the metal switchplate 2feet above a radiator!



misterben


I take you do realise how dangerous it is to energise a circuit without the cpc being connected to the MET. If there is any equipment with supply line filters in circuit, and this could be anything from a discharge light to a PC, then dangerous voltages can appear throughout the installation and may be present on any class 1 appliance on the circuit.

Take a look at section 607 of BS 7671 it doesn't just apply to IT equipment.

As a general rule NEVER NEVER NEVER energise a circuit with the cpc disconnected and NEVER NEVER NEVER remove a cpc from an earth bar if the circuits are live.

Regards

Geoff Blackwell
 14 January 2008 11:11 PM
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misterben

Posts: 415
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thanks to everyone for their replys, i have read other threads and am pretty sure the voltage is a result of capacitive coupling as it only appears when i energize the supply with the earth disconnected from the MET ( i do realise that this is dangerous and only noticed the cable was live because it wasnt in its correct terminal)I have carried out an r1+r2 test and this is okay.My DMM is a Kewtwch multifunction tester but i have also used a fluke multimeter. From what i understand if it is a result of capacitive coupling then the voltage will disappear from the metal switchplates when i reconnect the circuits cpc to the MET under normal load conditions. Does this sound right to any of the more knowledgeable members? ps i will try using an analogue meter

Misterben
 14 January 2008 11:31 PM
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potential

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I have read this thread with mounting horror.
The final straw was the comment by misterben:
only noticed the cable was live because it wasnt in its correct terminal

!!!

The cpc should be reconnected immediately.
By disconnecting the cpc and leaving it disconnected you have created a very dangerous situation of which you seem to be oblivious of.
It does not sound to me that you know what you are doing and that could have dire consequences.
 14 January 2008 11:37 PM
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FizzleBang

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Originally posted by: misterben

thanks for the reply Paul, i got the voltage when it wasnt attached to the earth terminal. I havent tried it connected to the earth terminal, does this mean if it is induced or stray voltage then it isnt harmful cos as i said its present on the metal switchplate 2feet above a radiator!



misterben


As others have stated you should not assume that the voltages you measure are induced. But if R2 and Insulation Resistance are good then there is little else it can be. Perhaps the most important measurement is EFLI. If that measures to be within expected values then you can be sure that you have an effective earth. It could always be the case that the circuit passes all other tests but the Earthing Conductor could well be disconnected or poorly terminated at the MET or elsewhere up stream.

As Geoff says, it is dangerous if not thoroughly unorthodox to disconnect a CPC for the purpose of conducting live tests on a circuit.

There is anecotal evidence that some people can perceive a shock from these induced voltages. I recently visited a house where a tenant was getting "shocks" from a brass plate switch. I proded it with one end of my fluke prodometer and it showed a floaty voltage to be present. Further investigation showed that there was no cpc in the light circuit in general and the back boxes had plastic insulating screw lugs. A modification had been made to a two way switch using three core and earth. The CPC of that single run of cable had been terminated to the earth lug on the back of the switch plate. That short lenght of cpc was apprently long enough to pic up induced voltages from its associated live conductors which were then presented on the metal switch. This was the only switch which she reported getting a shock from.

Swapping all fittings back to class 2 made it safe and a report went to the landlord.

Paul

-------------------------
"I learned very early the difference between knowing
the name of something and knowing something". - Richard P. Feynman
 14 January 2008 11:48 PM
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misterben

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Message to potential-- i have not left the earth disconnected! The house was empty apart from myself. The cable was only made live to prove that i had infact found a live earth conductor, which as i said was connected into the incorrectly numbered earth terminal. Hence the reason i found it, in a very packed and untidy consumer unit.

misterben
 14 January 2008 11:58 PM
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misterben

Posts: 415
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Thanks again Paul you seem to understand exactly what situation i am facing, or maybe i havent written my post clearly enough to others. The Ze is ok R1+R2 is fine so i will go back to the circuit (that i have left de-energized) and reconnect to the supply and see whether the switchplates are still live when the cpc is connected to the MET also will try the lamp method and analogue meter

regards
Misterben
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