Puzzling Earth Fault

[Octopus]

I would be recommending replacing the front end 30mA RCD with a 100mA and installing RCBO's for the socket circuits.....

 

[telectrix]

I would be recommending replacing the front end 30mA RCD with a 100mA and installing RCBO's for the socket circuits.....[/QUOTE

deleted due to brain not having woke up yet.

 

[richardsolway]

I would be recommending replacing the front end 30mA RCD with a 100mA and installing RCBO's for the socket

This is a 16th edition board and is original to the property. I am not asking on advice on how to bring to the 17th. I am asking why there would be a short between the service neutral and my TT earth. Can anyone shed any light on this as the distance to the transformer is quiet a way And thus should not pick up resistance to the earth point there. Has anyone had this before.

Thanks anyway murdoch.

 

[telectrix]

you have a low reading on IR test. try using continuity to get a finite value. this value would give a clue as to where it's coming from. test both with and without bonding connected.

 

[Richard Burns]

If you think about the earth fault loop impedance that you may measure on a rod this is showing a resistance of say 100ohms, this is the resistance back to the substation or nearest earth point using the line conductor and mass of earth. Since at the substation the neutral is connected to the earth point, then within a few tenths of an ohm this will be the resistance from neutral to your earth rod, using the neutral conductor and mass of earth. Measuring 100 ohms on a megohm scale will read zero.

 

[Octopus]

I would be recommending replacing the front end 30mA RCD with a 100mA and installing RCBO's for the socket

This is a 16th edition board and is original to the property. I am not asking on advice on how to bring to the 17th. I am asking why there would be a short between the service neutral and my TT earth. Can anyone shed any light on this as the distance to the transformer is quiet a way And thus should not pick up resistance to the earth point there. Has anyone had this before.

Thanks anyway murdoch.

I was not suggesting how to upgrade to 17th... I was suggesting a way to possibly minimise inconvienience!

 

[Engineer54]

I would be recommending replacing the front end 30mA RCD with a 100mA and installing RCBO's for the socket

This is a 16th edition board and is original to the property. I am not asking on advice on how to bring to the 17th. I am asking why there would be a short between the service neutral and my TT earth. Can anyone shed any light on this as the distance to the transformer is quiet a way And thus should not pick up resistance to the earth point there. Has anyone had this before.

Thanks anyway murdoch.

Maybe not, but this is the second occasion you have been to this installation, and you Have previously carried out installation work to the property. A CU change would have been prudent advice to the customer at that time, rather than 2 years or so down the line. So when you do upgrade the present CU, replace that up front 30mA RCD with a 100mA S type, to give extra protection without nuisance tripping to this TT installation. A straight 100mA RCD will not work, most faults will take out both the 30mA and the 100Ma RCD's....

As for your problem, it's just a case of breaking things down and testing until you can isolate where the connection lay's. Only methodical testing is going to reveal or point you in the right direction as to the cause!! Conduct a ramp test to find out, at what sort of leakage current is taking the present 30mA unit out.... Do you own/posses an earth leakage clamp tester??

 

[ackbarthestar]

Did you mention whether or not the RCD was tripping intermittently or whether it was a one off ?

If not the former then it might be a build up of atmospheric conditions due to lightning. That will cause spurious voltages and hence currents to appear on the Neutral / earth.

 

[plugsandsparks]

I have a 16th board on TNCS, RCD tripped at 11:30pm Friday night. I am putting it down to lightning unless Mrs P&S makes me get the tester out....

 

[rattlehead85]

I would head down the route of checking the boiler and heating system as there might be an intermittent fault on the Boiler when it fires up when the hot water is called for. RCD's are known for not tolerating boiler switching internals especially if the boiler hasn't seen a service or parts overhaul since its install. Worth a go at least!

 

[brucyboy]

Apon functional testing or circuits and then blanket testing between the neutral and earth bars inside the board I had a reading of 0.00Megohms. This is with the main switch closed (On) and all MCBS open (off).

Don't know if it will help, but can you measure N to E as a finite resistance, e.g. using the continuity range? Just wondering if there is some way an external fault e.g. in neighbours property and via gas/water bonding could be contributing in someway (not sure it's likely, but might be good to know).

Try disconnecting each earth conductor until you find the link. ??

 

[derek preece]

Hi SJD, Yeah I thinks its service side as the fault disappears when the service isolator is opened. Bearing in mind this is TT, Close the isolator and then its 0.0Megohm between the neutral and earth bars in the DB....Its weird......

Richard Burns was correct in highlighting your confusion over this issue. Perhaps you could set up a temp board with RCBO's in if you have one lying about that might help identify which circuit (if any) is causing the problem then the householder can tell you which circuit is tripping if it is an intermittent fault not showing up with IR tests.

 

[guest119]

Ref the zero n to e reading, a possible cause.
Your side is bonded to common water/gas pipes/ steelwork so is the neighbour.
Neighbour has a n to e fault, you get a zero reading to n coming from your install/bonding.

 

[Captain Custard]

If you are taking a insulation resistance reading with the main disconnecter closed you will get virtually 0M as regardless of the earthing system the neutral is always connected to the star point of the generator which is bonded to earth to stabilise the neutral referenced to earth i.e. neutral and earth are effectively the same conductor.
I had a very similar fault some years ago caused by poor earth bonding in local property's on the same overhead two wire supply
I would suggest getting a PME terminal added costs £35 around here and if the problem persists add a 100ma S type RCCB to the supply tails and obviously retain the existing 30ma.