Puzzling Earth Fault

[richardsolway]

Hello all. I have a really puzzling fault at a property. There is a 16th edition Dis Board onsite with a 100a 30mA rcd protecting the front end. The Installation is TT. I wired a new extensions to the property in 2005 and oversized all earthing to adhere to the prospected 17th edition. There have been no alterations or additions since its completion. Recently the property has experienced the main 30mA trip operating and with this, there seems to be no specific circuit causing it. I have been over and completely tested every circuit. The lowest IR reading was still above 10 MegOhm and was an original circuit to the house. The RCD its self tests OK with it not being to sensitive or beyond maximum disconnection times. The ZE reading a the the board is 95 ohms (TT). 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). That's the background and now for the question. If this were a TN-C-S Supply you would expect a reading like this as the neutral and earth are connected within the cutout and at various intervals tapped to earth on the lines. However this is a TT supply, I can seen the main earth for its entire length to the Rod outside and has no physical connection. Has anyone else experienced this? The overhead supply pole is 40m away and then passes underground before rising up the neighbours property and across the facia. The poles are not marked PME and the supply authority will not comment on it either (Western Power Dist) as the general state of the services down here in cornwall is poor and they will no longer guarantee an earth to a domestic property. The neighbours property is TT also and their Rod is 30m away from this installations rod. The transformer is 3/4 mile away also. Thanks in advance to anyone that can give any hints or tips to try. (I have also carried out IR with equipment installed and lighting in the on position (naughty I know) and this did not indicate any different readings from the norm). Thanks Again. Richard.

 

[Sintra]

Could be accumulative earth leakage from the appliances within the property. Can you test with a clamp.

 

[Guitarist]

As Sintra says, clamp across LN to see how much leakage you have, then try switching various appliances on. Also, get information from the owner about when it happens. May be something such as boiling the kettle with the washing machine on. Another "usual suspect" is outside lighting, especially after it's rained.

 

[richardsolway]

Hi sintra, yes already done that and its minimal, like hardly noticeable. PAT tested everything......Most appliances are new 12-24 months. Hi Guitarist, yes already been down that road. No specific time of day or night, or weather conditions as it happened in the middle of a 3 week dry spell (3 or 4 times) and now (chucking it down). Again, been and checked all appliances, circuits (including switched lines) and basically done a full PIR (sorry I mean EICR) on top. All appliances checked, then load tested and then checked again (i.e. elements cold and then when hot). I am really clueless now. I am guessing its external (I have had this once in the past on a council contract and it was the streetlight outside). 15 years of testing and fault finding experience and this still got me stumped. Thanks or your input though , any further suggestions welcome.

 

[Paultrim123321]

Had a similar fault on a work colleagues property which turned out to be the dish washer at a particular point of the cycle.

 

[richardsolway]

Hi Paul, yeah monitored all washing appliances today (for 3 hours) and this has happened with none of them in use.

 

[Richard Burns]

We had one fault on here some time ago where the cable was damaged as it passed under the floorboards and walking over that point caused the fault. Worth estimating cable runs for this sort of thing.

 

[richardsolway]

Hi Richard, Thought of that too, but I know the cable runs and it will not be that. (old side - open beam, newside - installed by me.) This has also refused to reset with ALL MCB's off, All appliances unplugged, All Lamps Removed, All Fixed appliances isolated by DP switches (i.e. shower, cooker). Thanks anyway.

 

[SJD]

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).

 

[Sintra]

Are there any motorised valves on the heating system? Have come across this before where during the summer the hot water is used more than the rads and only when the room stat was made and the valve tried to open it took out the RCD. Might be worth a try.

 

[richardsolway]

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......

 

[SJD]

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......

But measure it on an Ohms scale, it won't be 0.0 Ohms, the actual Ohms value might tell you something.

 

[richardsolway]

Are there any motorised valves on the heating system? Have come across this before where during the summer the hot water is used more than the rads and only when the room stat was made and the valve tried to open it took out the RCD. Might be worth a try.

None onsite I am afraid. I always check anything with a motor, element and alike,

 

[richardsolway]

Thanks SJD, will try it if I have to pop over there again......any more idea's?

 

[Richard Burns]

For all the testing and checking you have done have you tried swapping out the RCD just in case there is an internal fault that is not noticeable.

 

[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.