Puzzling Earth Fault

[Kamikaze]

Now I would have picked on the original circuit to the house to see why it’s as low as 10 Mohms and done some further investigations into why it’s as low as that, and if that proved to be unfruitful moved on to the next circuit that gave you a reading lower than expected. Just keep eliminating all the most likely causes. But as Richard has said, it’s worth a stab at replacing the RCD to see what happens.

 

[Guitarist]

How regularly does it trip?

 

[Engineer54]

Change out that 30mA front end RCD with a 100mA S type and then install either a new dual 17th CU or replace the existing MCB's for RCBO's for a TT earthing system. Absolutly crazy having a single up front 30mA RCD looking at every circuit, and not particularly safe either. At least at the moment the present RCD is failing on the ''Safe'' side!! Think about what could happen, if that single RCD failed to operate at all under an earth fault condition!!!

 

[richardsolway]

Change out that 30mA front end RCD with a 100mA S type and then install either a new dual 17th CU or replace the existing MCB's for RCBO's for a TT earthing system. Absolutly crazy having a single up front 30mA RCD looking at every circuit, and not particularly safe either. At least at the moment the present RCD is failing on the ''Safe'' side!! Think about what could happen, if that single RCD failed to operate at all under an earth fault condition!!!

I agree that a 30mA front end is not ideal!, whilst this is original and to the 16th edition, there was no real issue with it. Have already mentioned a split load conversion to my customer and they seem keen to go ahead with it. I have swapped out the RCD for a spare one I had for a couple of days and it still happened at random times.

 

[richardsolway]

Hi Kamikaze, Yes already investigated and it was equipment on circuit.

 

[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!