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

 

[Octopus]

RCD socket or FCU ..............

But with low IR you MUST do all 3 cores properly .........

 

[SparkyChick]

It could be a surge arrestor somewhere although they are normally good at 250v and then the readings drop right off at 500v.

 

[Octopus]

AND I would spend more time unplugging everything too...

 

[HappyHippyDad]

It's quite a high reading for something connected isn't it. What's making you think it's something plugged in or a neon? I'm guessing you're hoping it is .

As Murdoch has said you really have to go round and unplug everything. I know we don't always do that but sometimes you really do have to. If you still get the result after that then you just have to start breaking the circuit in half and begin your fault finding.

Is there a reason why you don't want to test at 500V? I know it sounds daft but sometimes I've seen a lower reading at 250V and it improves at 500V, generally it's the other way round!

 

[HappyHippyDad]

It could be a surge arrestor somewhere although they are normally good at 250v and then the readings drop right off at 500v.

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.

 

[haptism]

RCD socket or FCU ..............

But with low IR you MUST do all 3 cores properly .........

It could be a surge arrestor somewhere although they are normally good at 250v and then the readings drop right off at 500v.

No theres no surge arrestor, the house is old and doubt they have one.

 

[telectrix]

damp?

 

[Octopus]

Thing is that ALL plugged in appliances will bring back lower readings ..................

 

[haptism]

So yep went in this morning and done a quick IR test on one circuit, unplugged everything I could see, and had a good rummage tto see if i missed anything.
Theres two reasons I initially do not test at 500v, firstly to save batteries, secondly to save breaking any sensitive equipment. I know in theory you can test at 500v with L&N to E, but theres always a chance somethings amiss. Once I done a 500v test (across each conductor mind you) and broke 2 pir security lights, it cost me a day and replacements. If I get a super low reading at 250v its not gonna make much difference I would have thought at 500. When the readings are good then yeh go for 500v. Anyhow yes im hoping somethings still connected and that 0.8M is typical of a likely culprit.

 

[haptism]

damp?

Theres no immediate signs

 

[buzzlightyear]

damp

 

[Deleted member 26818]

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.

 

[Andy-1960]

USB sockets anywhere?

 

[buzzlightyear]

Anyhow yes im hoping somethings still connected and that 0.8M is typical of a likely culprit.

seek and you should find it .

 

[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.