Very low insulation resistance on ring?

[Johnny-G]

Hi all, first post so be gentle with me

I replaced a single s/o with a double today and got some scarily low I/R results, this is at home by the way.
Here's the test results:
S1 - all < 0.3ohms
S2 & S3 both < 0.6ohms
I/R - all 3 < 0.5Mohms

I removed everything from all sockets, removed the new s/o & the conductors from the CU terminals and tested them alone but still got the same results.
It's not an old installation by any means (about 10yrs max at a guess) & the general condition of the cables and insulation seems fine.
Have I just missed something daft or should I start looking for the fault?
If so, how?
Remove all s/o's and I/R test individual legs?

Hope that makes sense

 

[not clever]

S1 S2 & S3 ?

Does the circuit include the kitchen? if so check for under worktop items that are plugged in.

If a fault is present do as you say..........thats devide & conquer.

 

[Johnny-G]

Sorry, I meant Steps 1,2 & 3 for continuity of the ring ie. end to end & both crossovers.
The circuit does include parts of the kitchen and there's a beer fridge which I forgot about and didn't unplug
Is that the sort of thing that'd give me those readings?

 

[bugsy]

Well you didnt want cold beer did you!

 

[Des 56]

Insulation resistance
Before carrying out the test disconnect blah blah etc etc

Forget this first action at your peril

 

[uksel]

also, do a test on the circuit before you do any work

always always always cover ones ****

 

[SirKit Breaker]

Blew the brains out on my mother in laws brand new fridge freezer doing an IR test a couple of years ago.
She rang the supplier and told them it had just packed up, so they gave her a new one got me right out of jail.

Did something similar to a Plumber, but that was hilarious

 

[uksel]

also, do a test on the circuit before you do any work

always always always cover ones ****

It was late when i made that post, what i actually meant to say was


also, I always do a soft (250v) IR test on on every circuit first - this usally brings anything you have missed out of the woodwork and saves you kersploding any of your clients treasured electricals

 

[SparkTech]

Hi

Had a similar issue with an existing ring when performing IRT, it turned out that one s/o had a crushed / peeling insulation.

Discovered when all s/o were removed to identify the low IRT point

Must have happened when the last person screwed it back catching it between the back of the s/o and the metal back box.

Regards

 

[Spudnik]

Must have happened when the last person screwed it back catching it between the back of the s/o and the metal back box.

Regards

Thats a good one to catch you on a CU change too.

 

[5-Week-Wonder]

Have you done an insulation resistance test with line and neutral at equal potential?

Doing that tends to eliminate a lot of false readings you might be getting from the spurs and bits and pieces that are plugged in. Also stops stuff from being damaged by 500v d.c. test voltage.

 

[rocker]

I don't know if anyone answered this but YES having a fridge plugged in will give you a low low reading.

Also, are there any neons in the S/Os? I bloody hate neons.

Gotta agree with UKSEL, I always do a soft test first, then whack it up to 500v. It takes literally four seconds and could save you ballsing up a customer's £4000 plasma.

 

[Johnny-G]

Thanks all, it was the fridge after all

 

[spadxiii1918]

Readng through the comments, have you removed items that may contain a neon indicator or fused spur etc.

 

[5-Week-Wonder]

I keep saying this, but any false readings you might get from Neons, fridges, spurs etc. etc. can be bypassed by simply doing your IR tests with Live and Neutral temporarily shorted, which is standard practice as per Guidance Note 3.

Just done a CU change and had my own low IR readings.

Before I touched the old rewirable board I did an IR test, which showed 0.31 Meg!

Narrowed it down to the downstairs ring. Split the ring into 2 halves, and then started working back towards the board on the side with the low IR, which led me to a socket where the conductors had been tightly twisted around each other. Just separating the cables seemed to fix it... bizarre.

Anyway, with every circuit now showing > 299 Meg I went ahead and did the CU change. Then I repeated the IR test across every circuit... back to 0.31 Meg!

So, I unplugged an extension lead from the problem socket (which had been switched off at the socket anyway) and tested again, and it's back to > 299 Meg. Very odd - socket should have isolated any fault with any connected stuff.

Completed my dead tests and powered up. RCD held OK. Plugged in the extension lead expecting it to pop the RCD but it didn't. Half expecting it might do at some point in future though, so have forewarned customer.

 

[Des 56]

I

So, I unplugged an extension lead from the problem socket (which had been switched off at the socket anyway) and tested again, and it's back to > 299 Meg. Very odd - socket should have isolated any fault with any connected stuff.

.

5-W-W
A possible reason could be that the sockets are single pole and a low insulation reading on the extension cable was being picked up via the neutral of the socket, then clear again when it was unplugged

 

[5-Week-Wonder]

could be that the sockets are single pole and a low insulation reading on the extension cable was being picked up via the neutral of the socket

Blimmin good point. Damn those single pole socket outlets and the people who turn stuff off at the socket.

 

[uksel]

low resistance N-E perhaps ?? these aren't isolated at the S/O

and a Resistance of three hundred and ten thousand ohms (o.31 meg) wouldn't pull enough current to trip an RCD

would be looking at about 0.0007A ?

 

[Johno]

i have had low results before (other than forgetting to unplug appliances) it was the actual socket /spur/switch itself that was the problem. to make sure you could put cables into connectors and retest. if there has been decorators or plasterers you can bet they are to blame.

 

[Jason Smith]

Just installed a max ring, (100 meters) loads sockets but very low max demand, and on doing R1+R2 at the sockets found that one side of a socket was reading 1.0, the other 0.41. o.41 being the expected. One of the phase wires was badly connected in that half of the DSSO. This would of caused havock when testing ir later.So being inexperienced on falt finding ( getting better and better ) I consider that day to be a good onefor experience on DSSO's.

 

[skinback]

Just installed a max ring, (100 meters) loads sockets but very low max demand, and on doing R1+R2 at the sockets found that one side of a socket was reading 1.0, the other 0.41. o.41 being the expected. One of the phase wires was badly connected in that half of the DSSO. This would of caused havock when testing ir later.So being inexperienced on falt finding ( getting better and better ) I consider that day to be a good onefor experience on DSSO's.

What is DSSO's ????