Odd one

[mort2376]

Got a ring circuit which is a ring on all legs.

With 2 breaks in the ring l-e gives 40Mohm

With the ring connected I get l-e 0.03Mohm

No usb sockets connected. All appliances disconnected.

Was tripping rcd on Thursday,but now isn't.

Any ideas peeps ?

 

[binky]

It would suggest possible trapped wire at a socket if you've had all the fronts off.

 

[Andy™]

what's voltag is your IR test at? if 500v (then clear at 250v), its probably an extension with surge protection plugged in

 

[mort2376]

Testing at 500v. Even with sockets off and wagos 50Mohm but complete the ring and take a measurement 0.03Mohm

Rcd stays in so its not a fault as such. Never had it where completing a ring does this.

I initially thought interconnection some where, but I've split it all and got no other rings.

Spent 4 hrs now and have put it all back for customer to monitor.

I'm just scratching my head at it.

 

[Murdoch]

Have you tested the IR with all the legs out of the CU ?

 

[mort2376]

Yep. Doesn't matter where I make the breaks. Still get the same readings.

 

[poni]

sounds like you are missing a hidden link between sockets that you have not found yet, you will have to do each link in the ring by themselves or have you done that already?

 

[Murdoch]

Have you ired the sockets when they are disconnected?

 

[Mike P]

I agree with an earlier post. Sounds like a blanked over/hidden socket possibly with a neon which is energising under test.

 

[mort2376]

Thanks for the ideas peeps. I couldn't find any sockets hidden but who knows potentially what's in the walls behind the cupboards.

Left the circuit powered on. Zs reading good. Trip times good. Customer is going to let me know if it trips again.

Just niggles me when you don't get to the bottom of it, but felt after 4 hours and not finding anything it was wasting money to continue.

I've explained that if it does continue to trip then it's gonna take some disruption to try further and they are happy with that.

 

[johnb2713]

Thanks for the ideas peeps. I couldn't find any sockets hidden but who knows potentially what's in the walls behind the cupboards.

Left the circuit powered on. Zs reading good. Trip times good. Customer is going to let me know if it trips again.

Just niggles me when you don't get to the bottom of it, but felt after 4 hours and not finding anything it was wasting money to continue.

I've explained that if it does continue to trip then it's gonna take some disruption to try further and they are happy with that.

It is very annoying with faults like this. My son had a similar issue in his house, it was intermittent but we finally tracked it down between two sockets. I ended up chiselling the cable out of the wall to find a screw from next door just bruising the edge of the T&E. The neighbour had mounted a TV on the wall some 18 months earlier, he insisted on paying for the plastering which was nice of him but not needed, we did it ourselves.

 

[SPECIAL LOCATION]

I agree with an earlier post. Sounds like a blanked over/hidden socket possibly with a neon which is energising under test.

Neon's will generally be connected between L & N...
Not L & E. So should not give any readings under tests to Earth.

Whereas surge protection devices sill connected...
e.g. those multi-extension leads often sold to plug all your IT gear into,
Can give low readings down to Earth.


0.03Meg aka 30,000ohms between L & E is approx 7ma (ish) of leakage,
and if this is a shared RCD, it doesn't leave much leeway for any other natural leakage from other circuits / appliances.

I can't see how you can suggest this 'known-low-insulation-reading' is compliant with 643.3.2 / 643.3.3 ?

Even if it is a hidden fault somewhere...
It is not too difficult to establish which two outlets the fault is between.

i.e. if it is all just a single ring, no spurs, branches etc..
A bit of continuity reading with L&N joined on one end of the ring and the other end all open..

Measure L to N continuity at every outlet...
noting every reading and socket position..
you will then have a join the dots picture with the increasing values at each outlet giving the order of the wiring..

Plus with a few calc's using table 11 Appendix I OSG, you can also know the approx cable lengths between sockets...

Once you know the actual order the circuit is wired in by splitting the circuit further then re-test your IR L->E unit you narrow down which two accessories are either end of the "suspect section"..

Once you know where the two ends of the suspect section connect,
this may then also give indications of the potential hidden items / damaged cable you may be looking for..

I'd be thinking partial damaged cable, or junction box with nails or damp being the top two contenders in my opinion.

 

[mort2376]

This is all fine but why does this reading only occur when the ring is complete ?

If you take the cables out of the consumer unit while all others are connected and test between L and E with the ends separate it reads 50Mohm. Which while not good is a pass.

Join Ring L1 to Ring L2 and Ring E1 to Ring E2 and then test between L & E you get 0.03Mohm.

This doesn't even follow the 1/4 measurement when doing a crossed r1+r2 test.

I have split the cables and the low reading is on the leg going from the mains thru the garage to the kitchen.

The customer has had no work done in the last 3 years so this fault potentially has been there for that long and maybe there are other appliances leaking more now causing more frequent trips.

You have to balance cost to customer to locate the fault, then rectify the fault and put right any damage done during the proved with living with an occasional trip.

While this may not be a regs compliant situation, the circuits is mcb and rcd protected.

My next advice if tripping does become more frequent would be to change to an rcbo rather than a shared rcd.

Sometimes money is a factor. If I thought it was dangerous and a risk to life I would be telling the customer that more work is needed straight away but as it is I don't feel this is the case.

 

[binky]

This is all fine but why does this reading only occur when the ring is complete ?

If you take the cables out of the consumer unit while all others are connected and test between L and E with the ends separate it reads 50Mohm. Which while not good is a pass.

Join Ring L1 to Ring L2 and Ring E1 to Ring E2 and then test between L & E you get 0.03Mohm

Have you done any testing N to E or L ?

I have split the cables and the low reading is on the leg going from the mains thru the garage to the kitchen.

Kitchens are frequently the work of bellzeebub ( builders) lots of potential issues...

 

[Andy™]

have you got the earths disconnected from the DB and your test to earth is not actually to earth, just the earth conductor of the circuit? if so, faulty cable or something to earth but not the circuit cpc

 

[Fleeting]

BS7671 does not suggest a minimum value of insulation resistance for an existing installation so if it no longer trips don't worry about it. I agree that the scenario you have would be an annoyance not to find.

 

[UNG]

The customer has had no work done in the last 3 years so this fault potentially has been there for that long and maybe there are other appliances leaking more now causing more frequent trips.

You may have to be a bit more probing on the work front, an odd nail or screw to fix a creeking floor board is easily forgotten, picture hooks are another one I've had problems with and to a customer isn't classed as work

 

[SPECIAL LOCATION]

IR tests must be done with the CPC's connected to the electrical earth other wise you would have difficulty reading earth paths through other bits of building fabric..

e.g. consider some metal capping buried in a traditional plaster wall, with a nail that cuts the insulation against the live conductor, without breaking it.

So the earth path is live conductor, metal nail, & meal capping in contact with wall, plaster, mortar, building fabric back to electrical earth and/or via other bonded parts..

So if your CPC is disconnected at the CU you are breaking the path from electrical earth for your test readings.

{ Side Note:- Best practice guide 4 (pg13) suggests an IR below 1meg between L+N to E would be a C2 if you were doing a EICR. Where obviously C2 is considered "Potentially Dangerous". }

When I have encountered similar problems in the past, I ensure I give written communication to the customer, detailing my concerns with ref to IR testing reg's from BS7671 and Best Practice 4.. Advising it would be sensible to get it identified and fixed ASAP, due to fact that industry recognised guidance does sugges there is a potential danger. Basically make sure you have your back covered should anything go pear-shaped, before the true cause is actually resolved.

The last one I had to fix took 2 and half days labour to investigate and fix with a new 9m section of cable from the CU to a box under the lounge floor.

Other instances where a fault was part way round a circuit I have done a temp split into a pair of 20A radials until the ring can be fully fixed.

 

[SPECIAL LOCATION]

BS7671 does not suggest a minimum value of insulation resistance for an existing installation so if it no longer trips don't worry about it. I agree that the scenario you have would be an annoyance not to find.

Not sure I would agree with that...
651.2 periodic inspection and testing does refer to the tests in chapter 64...

Where chapter 64 includes the table 64 values (min 1.0meg)...
Which would be a pretty crap value on an New Installation!!!

Very much doubt 1.0meg would be a good value to write onto a new installation or rewire EIC..
More realistic values in those situations are either off the scale or well into the multiple hundreds of MegOhms.

 

[mort2376]

No one has yet managed to answer why the reading is different open ring and closed ring.

Even with the break at a socket not at the CU 40Mohm v 0.03Mohm.

That is the part that's doing my head in.

And if I split it into 2 radials great the readings are >200 and 40 but that doesn't solve the other issue.