Low insulation reading

[haptism]

Testing a ring earlier and found 0.8M ohm at 250v between (L&N) - E.
This wouldnt be a neon causing this would it as im testing between L N (joined) to E ? I have not yet tested between the three individually.

This low reading is making me hope somethings still connected, there is a digital boiler controller that I suspect is wired into the supply side of a FCU but maybe its not; would this cause a low reading like this ? or anything else to bring back 0.8M ! I tested between (L&N) - E. Thanks all

 

[GBDamo]

Real pain low IRs in a RFC especially in old installations that have spurs all over the place. Finding everything to unplug and/or disconnect can be real fun.

Took me four hours to find a dead short that was tripping the RCD in preference to the MCB.

Much of that time was determining what was and wasn’t in the RFC, what was spurs and what had been spurred off the upstairs RFC.

Then a case of aiming for the middle splitting and repeating until I had, with a degree of certainty, found the section of cable with the faulty on.

As it turned out a drain had split and filled the underfloor with water. There was no way I was going to get under the floors, customer not intrested in the leak, just wanted the power back on ASAP.

Disconnected the suspect cable, Ran a temporary cable from socket to socket, tested the patched up RFC and powered back up, all good.

Got lucky really as no random spurs off the suspect cable.

Finally disconnected the temp cable, put the two legs on 16amp MCBs, tested and powered back up.

Left a quote for reconnecting the RFC in surface mount on a Sunday to have them back up and running.

Not a bad job really but not quick.

 

[haptism]

If you’ve conducted a global test and obtained poor results, the next step is to test individual circuits.
Whether you continue to test at 250V or change to 500V is up to you.

No spin this was an individual circuit.

 

[Deleted member 26818]

No spin this was an individual circuit.

Now it becomes time consuming.

 

[SparkyChick]

That's interesting SparkyChick. Have you seen some surge protected devices given poor IR results at 250V? Bit worrying as it means you cant assume it's not a SPD if the reading is low at 250V.

No, just thinking out loud

I'm not sure how say metal oxide varistors fail as they age, could they result in low readings at lower voltages than their original?

As I say, just thinking out load, but I'd be looking for things connected at this point and/or breaking the circuit down to try and identify the fault.

 

[haptism]

Now it becomes time consuming.

Thats it thou isnt it, customers gonna say well im not sure about that, and im thinkng well it is what it is. Customer needs an extra socket in the loft ready for the cctv guy whos installing Thursday.

 

[Octopus]

No spin this was an individual circuit.

Then you have an easier job to do then .........

How many socket circuits in the property?

I was doing a CU change recently and when I tested the "immersion" circuit I got a similarly low reading .................. I found an RCD spur that a plumber had added for a shower pump ................ removed the RCD spur and the readings went over 30 mohms

 

[Octopus]

Thats it thou isnt it, customers gonna say well im not sure about that, and im thinkng well it is what it is. Customer needs an extra socket in the loft ready for the cctv guy whos installing Thursday.

Matey ......... this is the real world - if you don't outline potential issues you won't learn ................

 

[buzzlightyear]

I had a similar problem few months back did a ir reading low ohms
found a Arial booster where some T88T had hid it under the cosy rap
right in the corner of a loft .

 

[haptism]

Then you have an easier job to do then .........

How many socket circuits in the property?

I was doing a CU change recently and when I tested the "immersion" circuit I got a similarly low reading .................. I found an RCD spur that a plumber had added for a shower pump ................ removed the RCD spur and the readings went over 30 mohms

More searching required then it looks like, before surgery.

 

[buzzlightyear]

More searching required then it looks like, before surgery.

big craw bar is needed then .

 

[Octopus]

More searching required then it looks like, before surgery.

Absolutely ........

how many socket circuits?

 

[haptism]

Matey ......... this is the real world - if you don't outline potential issues you won't learn ................

Yep Ive always got time for a cup of tea

 

[haptism]

Absolutely ........

how many socket circuits?

Er 3 I think, but its only 1 that im concerned about atm

 

[buzzlightyear]

may be a mouse chewed through the cable .

 

[Octopus]

Er 3 I think, but its only 1 that im concerned about atm

So your next step is clearly identify ALL the sockets on each of the circuits, then go hunting for additional connections ............. alarms, TV amplifiers, RCD sockets, RCD spurs, floodlights....................

tedious but has to be done

And if you can't find any extra connections then you need to split the circuit to try and find where the low reading is...