earth is floating to 117v when disconnected at distrution board - help

[jay123]

hello

Ive removed a dozen 'dead' cables from behind trunking, split the lighting earth from the radial earth, verified r1,rn,r2 back to dis-board yet when I pull the earth at the disboard it floats up to 117volts ???

can't believe the 20m of shared conduit run would cause this.

just can't understand why the earth floats high when measured at (diconnected) distribution board.

radial circuit, 10 sockets, problem persists even when only 1st socket of circuit is connected ie most of [FONT=Helvetica, Arial, sans-serif]radial is eliminated f[/FONT]rom equation.

when earth is pulled and circuit is energised it sparks on disboard earth bar.

help

 

[Bobster]

Are you measuring with a digital multimeter by any chance?

 

[jay123]

using a megger 1552 mft

2nd checked with a fluke dmm, which gave a reading=30v

calibrated both units against a traceable national standard.

 

[Marvo]

It takes very little current to make a convincing spark. The earth is functional as well as protective so if there's things plugged in to the circuit there may be standing leakage from surge arrestor circuits etc.

The 117v you're seeing with a digital tester isn't a very meaningful test, rather connect the CPC, energise the circuit and check the leakage current using an earth leakage clampmeter.

 

[jay123]

thanks for that tip.....don't have access to one at the moment :-(

the circuit in question has nothing plugged in, just can't explain the voltage appearance on the cpc

 

[Silly Sausage]

You can get a Low Impedance adaptor for your Fluke, you'll have to search yourself!
That will get rid of any 'Ghost' voltage readings.

 

[Marvo]

Have you done IR tests on the cable whilst it was disconnected?

 

[jay123]

yep IR tests >299Mohm, r1+r2 with loop at dist-board=as expected

actually this is how I noticed the issue ie sparking

 

[MDJ]

Have you done a N-E IR test on the circuits?

 

[jay123]

yep, disconnected r1,r2,rn at dis board and did IR tests

 

[sparkdog]

Floating earth...call out the Lifeboat.

 

[7029 dave]

As stated clamp it.

 

[jay123]

will try to acquire a clamp........

whats normal leakage for an energised circuit with nowt plugged in ?

20m shared conduit run

 

[Marvo]

will try to acquire a clamp........

To be honest it's bread and butter kit for a professional sparky, you can pick up a Dilog for about 30 quid I think and it get's good reports from all the guys that .

whats normal leakage for an energised circuit with nowt plugged in ?

20m shared conduit run

Zero mA or as close as makes no difference. Are you sure everything is unplugged and there's nothing sneaky still on the circuit like a TV antenna amplifier for example?

 

[jay123]

Zero mA or as close as makes no difference. Are you sure everything is unplugged and there's nothing sneaky still on the circuit like a TV antenna amplifier for example?

the room with the radial is 20m from the disboard, I rewired the radial this am, possible someone shared the cpc by T'ing out of the conduit downstream but doubt it as E-N >299ohms (TNC-S system) when disconnected at disboard - ie no parallel earth paths.

Also IR tests would have come back low if anything sneaky were connected right ?

I'll use my DMM as a series ammeter 2moz.....it's got a fuse protection should the fault current be larger than expected