Any ideas?

[pushrod]

Any ideas on this - old installation (3036's) on a tn-s . When the new DP isolator is switched off there is still more than 40mA in the main earth wire going from the casing of the supply to the earth bar in the CU. And there is more than 40mA difference between the line and neutral when the CU is on.
Is this an induced current from the supply cable ? Would it have implications for replacing the CU with a double rcd unit. (Thought i would put an rcd in front of the CU temporarily just as an experiment)

I have no reason to doubt the accuracy of the earth leakage clampmeter which is accurate in other situations and has the facility to just measure in the 50-60Hz band

The supply transformer is less than 100M from the house with less than 30 houses in the street. The voltage is quite high at about 245V.

 

[pushrod]

Bumpety bump! - lot of views, no replies


Little more info - looking at it this morning with the main isolator off and the only obvious protective bond to the water supply disconnected there is anything fom 25 to >40mA in the water pipe

Is this common for an extraneous conductor to have such a large current in it???

 

[stef]

Hmm, if you have a current than you must have a potential difference as well. Can you measure a
voltage between neutral an earth when either CU is on or off? I have never used an earth leakage clamp
meter so I can't comment on it. What is the current range on this meter? I have a calibrated current clamp
which always indicates a low current in high AC current setting. Bit of a strange fault

 

[Spudnik]

If there is current appearing on the incomming water pipe then i would say there is a fault underground somewhere, assuming a PME or TNS arrangement.

 

[davelerave]

sounds like a fault on the supply side/or another installation if the isolator's off


thought it was single-pole isoloation for tn-s(i don't know much about it mind you)

 

[pushrod]

Hmm, if you have a current than you must have a potential difference as well. Can you measure a
voltage between neutral an earth when either CU is on or off? I have never used an earth leakage clamp
meter so I can't comment on it. What is the current range on this meter? I have a calibrated current clamp
which always indicates a low current in high AC current setting. Bit of a strange fault

TBH the resistance would be so low and the current is so small that a microscopic potential difference could be responsible - my voltmeter reads zero as it only reads in whole units.

The clamp meter can read to 100 amps, 40 milliamps and 4 milliamps depending on its range setting and to a 100th of a milliamp when on its most sensitive.

Certainly is a strange situation ...

 

[pushrod]

If there is a fault on the supply side, external to the installation, i could expect a similar thing on neighbour's houses???

Think i will see if i can check next doors as well.

edit : interestingly there is nothing on the metal gas pipe, although i suppose that could be plastic before it gets to the house.

 

[davelerave]

sounds like an external fault if the Ze is ok


i'm not too familiar

 

[sootyfire]

Sounds like a external fault try putting clamp meter on tails to see if leaking through isolator

 

[davelerave]

If there is a fault on the supply side, external to the installation, i could expect a similar thing on neighbour's houses???

Think i will see if i can check next doors as well.

edit : interestingly there is nothing on the metal gas pipe, although i suppose that could be plastic before it gets to the house.

i only just realised it's al DP isolation in the UK-for single phase

i'll have to observe more and talk less for a while

 

[Des 56]

I will take a guess at there being a nearby property with inadequate/none bonding of a shared water main
Their earth leakage,being a fault or just inadequate earthing/bonding (possibly non rcd protected ) finding its path via your water pipe to your met and and your supplliers earth

 

[JUD]

Any ideas on this - old installation (3036's) on a tn-s . When the new DP isolator is switched off there is still more than 40mA in the main earth wire going from the casing of the supply to the earth bar in the CU. And there is more than 40mA difference between the line and neutral when the CU is on.
Is this an induced current from the supply cable ? Would it have implications for replacing the CU with a double rcd unit. (Thought i would put an rcd in front of the CU temporarily just as an experiment)

I have no reason to doubt the accuracy of the earth leakage clampmeter which is accurate in other situations and has the facility to just measure in the 50-60Hz band

The supply transformer is less than 100M from the house with less than 30 houses in the street. The voltage is quite high at about 245V.

I will take a guess at there being a nearby property with inadequate/none bonding of a shared water main
Their earth leakage,being a fault or just inadequate earthing/bonding (possibly non rcd protected ) finding its path via your water pipe to your met and and your supplliers earth

What about the 40mA difference between line & neutral Des?

There's got to be leakage somewhere in the supply.

Seems a bit of a coincidence that the difference between line & neutral is the same as what he's getting on the main earth.

 

[pushrod]

I will take a guess at there being a nearby property with inadequate/none bonding of a shared water main
Their earth leakage,being a fault or just inadequate earthing/bonding (possibly non rcd protected ) finding its path via your water pipe to your met and and your supplliers earth

You could well be right in your educated guess Des as next door they started a massive rebuilding project and then ran out of money half way through leaving loads of things unfinished.

JUD unfortunately i can't tell if the imbalance on the Line and neutral is equal to the current in the Earth because the meter just reads "over range" when on 40mA max and reads zero when set on its max range which only measures in whole amp units - that makes me think it is below 500mA but still unfortunately a rather large range. The readings i get vary at different times and often the L-N imbalance is less than 40mA, hovering around 25 - 35mA. Maybe more than one fault??


I don't really want to start taking apart the CU for insulation tests until i am confident that i can put a dual rcd board back in as the rings have been paralleled up and i would not like to try and get all the 4 wires back in the termination once i have unraveled them for testing.


I did find a more sensitive voltmeter and got around 10mV between the water pipe and the disconnected main protectice bond. Which bearing in mind the low resistances involved is certainly in the right ball park to be pushing 30mA about. Have not yet checked Ze but as it is so close to the substation i suspect it will be very low.

Anyone got any ideas on how to proceed - don't really want to discuss it with neighbours who have the unfinished house, unless it is really necessary.