voltage when mcb is off

[paul b b]

evening gents,

the house i have been working on is driving me mad, today the builder is boarding over ols celing and asked me to take the light down so he can put new board up, ok i go the cu (light in room was on by the way) flick mcb off, pick up fluke t100, back to room, light off, test fitting and the voltage indication lifht is on with the buzzing sound, so back to the cu flick all lights off (16th ed split with all lights next to each other with no rcd) and re-test, all dead. this happend with the cooker circuit, i know i turned off the cooker mcb as i could see the cable with the ceiling down. now the house if very dusty and has been shotblasted, mcb have been playing up, not turning on when they should be etc, could it just be rubbish in the cu?(3 year old crabtree, house rewired 3 years ago).

i am going to change the cu but no point as the builder will be working in her room above with half of the floor up getting more crap in there.

the sockets also read 2v once but not again. oh also it only said there was something there when both live and netral wer touching the probes, t100 normaly says if you just touch the yellow led end on the live.

any idead?

 

[Lurch]

EMF.

Could be faulty MCB's, but unlikely if it's only the odd couple of volts, more likely to be induced voltages.

 

[paul b b]

EMF.Could be faulty MCB's, but unlikely if it's only the odd couple of volts, more likely to be induced voltages.

how do i test for this? and where would it come from?

 

[Lurch]

Basically, if you get a piece of cable and run it along the side of a live piece of cable the magnetic field the live cable produces creates a magnetic field in the dead cable, which shows up as a phantom voltage.

If you use a meter with a low impedance it won;t read induced voltages as it presents a decent load to the circuit and kills the voltage as it has no current to drive anything. Electronic meters are high impedance so will read the induced voltages.

 

[steptoe]

Basically, if you get a piece of cable and run it along the side of a live piece of cable the magnetic field the live cable produces creates a magnetic field in the dead cable, which shows up as a phantom voltage. If you use a meter with a high impedance it won;t read induced voltages as it presents a decent load to the circuit and kills the voltage as it has no current to drive anything. Electronic meters are low impedance so will read the induced voltages.

+1

sounds about right,

EDIT : I just meant the eddy current bit.

honest,,,,,,,

 

[kme]

If you use a meter with a high impedance it won;t read induced voltages as it presents a decent load to the circuit and kills the voltage as it has no current to drive anything. Electronic meters are low impedance so will read the induced voltages.

`scuse me.....

Am I going daft(er), or is that the wrong way round? Surely the low impedance will dissipate the induced voltage, whilst the high impedance will read it? From memory, that was why I used to use a "Vacuum Tube Voltage Meter"? ?:|

 

[Lurch]

is that the wrong way round?

Yes. Editing it now.

 

[Evans Electric]

Did you say the place was really dusty ? Shot blasting ? Builders are good at filling MCBs with brick dust etc, but they usually won't pass any voltage when ON . You may get problems later.

 

[SPECIAL LOCATION]

Did you say the place was really dusty ? Shot blasting ? Builders are good at filling MCBs with brick dust etc, but they usually won't pass any voltage when ON . You may get problems later.

oh Deke you are wrong there......

Builders do most certainly Pass voltge....

I did saw one get a shock once!!!!

he sort of went... bad day explode

 

[septiclecky]

Can also be that whilst you have isolated one circuit you can have a number of cables bunched together which are not isolated, you can get an induced voltage in the one you have isolated

 

[Evans Electric]

Its what I would wish for most of 'em TBH.

 

[Lurch]

Can also be that whilst you have isolated one circuit you can have a number of cables bunched together which are not isolated, you can get an induced voltage in the one you have isolated

Check post #2, and also #4.

 

[Graham Lister]

evening gents,the house i have been working on is driving me mad, today the builder is boarding over ols celing and asked me to take the light down so he can put new board up, ok i go the cu (light in room was on by the way) flick mcb off, pick up fluke t100, back to room, light off, test fitting and the voltage indication lifht is on with the buzzing sound, so back to the cu flick all lights off (16th ed split with all lights next to each other with no rcd) and re-test, all dead. this happend with the cooker circuit, i know i turned off the cooker mcb as i could see the cable with the ceiling down. now the house if very dusty and has been shotblasted, mcb have been playing up, not turning on when they should be etc, could it just be rubbish in the cu?(3 year old crabtree, house rewired 3 years ago).

i am going to change the cu but no point as the builder will be working in her room above with half of the floor up getting more crap in there.

the sockets also read 2v once but not again. oh also it only said there was something there when both live and netral wer touching the probes, t100 normaly says if you just touch the yellow led end on the live.

any idead?

Get a test lamp.

 

[paul b b]

thanks chaps, lots of cables bunched together! is it a problem or can i get away with it? i will be doing a cu change after builer is finished

thanks again

 

[kme]

Its not a problem at all, mate

KME

 

[paul b b]

just one more question, errrmm a bit daft i know but, would i get a shock? if so a big one? and am i right in saying that the t100 would show voltage (buzzing sound and led on) when large yelow end is placed against the live cable. with this problem it only shows when both probes are on l & n.

oh hang on thats 3 questions lol

 

[Andy™]

you could always earth the conductor in question to remove any inducted voltage

 

[Lurch]

just one more question, errrmm a bit daft i know but, would i get a shock? if so a big one?

You can theoretically get a decent tingle from induced voltages. In the normal domestic situations though there is barely any current in the induced side of things for you to even feel it, except for maybe if you jabbed your tongue across the end of the cables. I've never had a belt from induced voltages.

 

[paul b b]

and just to confirm, unless i rewire the house there is no way of geting rid of the problem? and its not a problem for the customer?

also is this common?

 

[Andy™]

even if you re-wire the house, you will still get the problem. inducted voltage is purely because of conductors running parallel to other conductors