Strange voltage on earth wire

[TonyandCheeny]

Hi, I've got a light with a ceiling rose in the kitchen. While rectifying some faults in the house I came across to the light and measure Neutral to live: 240 volts (which is fine ofc) however when measuring earth to live I'm getting 150 volts. I have checked for any touched wires with my Fluke t6 but I could not find any. I am also getting voltage on the screws of the light switch according to my dodgy neon test lamp. Cheers

 

[James]

Sounds like you have a broken earth.
this could be a very serious fault.
save a life and get a pro in to have a look.

 

[TonyandCheeny]

Cheers James. Will consult a electrician, out of curosity how can a broken earth wire give these faults? Wouldnt it just not display anything on the multimeter as its not having a return path.

 

[James]

There is likely to be a high resistance connection to earth somewhere.

 

[TonyandCheeny]

There is likely to be a high resistance connection to earth somewhere.

Yeah your right. ATM i think its fine atm because touching the screw of the switch does not give a shock but it lights the neon lamp sometimes very bright and sometimes not that bright. The light switch are old ones that get their feed direct from the swich rather than from the pendant

 

[James]

I think you misunderstood, a high resistance to earth on any cpc is a dangerous fault.

 

[Lucien Nunes]

Wouldnt it just not display anything on the multimeter

Unconnected wires that lie alongside energised ones will show a voltage to earth because of capacitive and resistive leakage between them. Even with wiring in perfect condition, a small amount of current transfers capacitively through the insulation from line to CPC (earth conductor) which would normally flow harmlessly to earth. When the CPC is disconnected from earth, the leakage manifests as a raised voltage at all points downstream of the break. The leakage in itself might be harmless as the available current is usually too low to cause a shock, but it is a tell tale that the earthing is faulty and will not provide the necessary safety when required.

 

[LastManOnline]

Unconnected wires that lie alongside energised ones will show a voltage to earth because of capacitive and resistive leakage between them.

in addition to the above you may also have some current flowing in the earth resulting from the "intentional leakage" built in to many modern day appliances .But as alluded to by the previous posters ,regardless of the type or leakage source the solution remains the same....repair the defects in the house,s earthing system

 

[TonyandCheeny]

Unconnected wires that lie alongside energised ones will show a voltage to earth because of capacitive and resistive leakage between them. Even with wiring in perfect condition, a small amount of current transfers capacitively through the insulation from line to CPC (earth conductor) which would normally flow harmlessly to earth. When the CPC is disconnected from earth, the leakage manifests as a raised voltage at all points downstream of the break. The leakage in itself might be harmless as the available current is usually too low to cause a shock, but it is a tell tale that the earthing is faulty and will not provide the necessary safety when required.

Thanks bud! Cleared my doubts.

 

[YaserThePlumber]

Unconnected wires that lie alongside energised ones will show a voltage to earth because of capacitive and resistive leakage between them. Even with wiring in perfect condition, a small amount of current transfers capacitively through the insulation from line to CPC (earth conductor) which would normally flow harmlessly to earth. When the CPC is disconnected from earth, the leakage manifests as a raised voltage at all points downstream of the break. The leakage in itself might be harmless as the available current is usually too low to cause a shock, but it is a tell tale that the earthing is faulty and will not provide the necessary safety when required.

You're technically right Lucien. However if the earth wire is disconnected in his case would it just NOT read from earth to live?. I have seen this in some properties. When he said 150 volts from earth to live this could indicate that the rest 70 volts are missing. Where are they missing? This looks pretty dangerous.

 

[Avo Mk8]

You're technically right Lucien. However if the earth wire is disconnected in his case would it just NOT read from earth to live?. I have seen this in some properties. When he said 150 volts from earth to live this could indicate that the rest 70 volts are missing. Where are they missing? This looks pretty dangerous.

If you think about the equivalent circuit of the situation (disconnected earth) you would have resistance, capacitance, inductance etc. between the various conductors as Lucien explains, which can ultimately form a potential divider together with the impedance of the measuring instrument, so you get a reading dependent on the ratio of those circuit elements. No volts are missing. It's just physics!

 

[TonyandCheeny]

If you think about the equivalent circuit of the situation (disconnected earth) you would have resistance, capacitance, inductance etc. between the various conductors as Lucien explains, which can ultimately form a potential divider together with the impedance of the measuring instrument, so you get a reading dependent on the ratio of those circuit elements. No volts are missing. It's just phys

If you think about the equivalent circuit of the situation (disconnected earth) you would have resistance, capacitance, inductance etc. between the various conductors as Lucien explains, which can ultimately form a potential divider together with the impedance of the measuring instrument, so you get a reading dependent on the ratio of those circuit elements. No volts are missing. It's just physics!

Agreed. Electrician came over today saying that its fine as the earth wire can be omitted but i did not agree with him. Charged me 40 for a callout for nothing
Can i check continuity of earth from a nearby socket outlet earth by plugging a extension lead in and poking the earth prongs of it. Its suppose to beep right as the cpc are made common in the bus bar.

 

[Avo Mk8]

Agreed. Electrician came over today saying that its fine as the earth wire can be omitted but i did not agree with him. Charged me 40 for a callout for nothing
Can i check continuity of earth from a nearby socket outlet earth by plugging a extension lead in and poking the earth prongs of it. Its suppose to beep right as the cpc are made common in the bus bar.

I suspect the Electrician is looking at the ceiling rose, seeing twin cable to the light fitting, and saying 'nothing wrong here', but I agree with you there should be cpc continuity to the rose, ready for when someone connects a fitting that requires it.

If you check continuity as you suggest (preferably power off!), based on the evidence, I'd be willing to bet you'll see a high resistance, so the next logical step would be to find the break! Given the induced voltage, it may be some way back to where the power comes from!?

 

[Lucien Nunes]

If there were continuity you could test from any earthed point to any other earthed point and read the resistance because all CPCs should be connected together as you say. However, as we suspect there is no continuity, and some induced voltage exists, your continuity tester might object to having significant AC voltage poked into it on a resistance test. So, if you want to do that, switch the whole installation off at the main switch first to ensure a valid reading.

 

[brianmoooore]

When was this house built or rewired? Earth wires in lighting circuits became compulsory from 1966, but for at least a decade afterwards there were plenty of electricians who saw nothing wrong with adding or altering pre 1966 lighting circuits with T&E, but leaving the cpcs unconnected. Add in later electricians/DIYers doing more wiring and assuming the cpcs are actually connected to earth without testing, and it used to be nothing unusual to find lighting cpcs with induced or leaked potentials on them.
Hopefully, most have been dealt with by now, but there's bound to be some still out there.