Dead Testing

[Iggifer]

Doing some dead testing on my house today, need some input on my results.

Downstairs RFC.

r1 = 0.55
rn = 0.54
r2 = 0.87
L+N Interconnect = 0.26

So those results are all fine. Very similar to the upstairs rfc and smaller than the shorter kitchen rfc

However, when interlinking L+CPC or N+CPC i get 30-50 ohms!

Would a still connected appliance cause these results? Because as far as I can tell I've unplugged everything but can't see another reason for such a high reading, especially with the end to end being fine.

 

[riveralt]

What type of tester are you using and did you null it?
How did you obtain the end to end figures?
How did you conduct the interconnect test.
What is its purpose?

Was the 0.26 ohms reading found at all the sockets or just one?
Were the 30/50 ohms readings found at all the sockets or just one?
Did these higher readings rise or fall along the ring final circuit?

If you are in college as I think you have mentioned on a previous post, you really should be making a stab at answering the questions you raised yourself - first.

 

[Iggifer]

It's a Megger MFT 1552, test leads nulled, double checked with a second set of test leads.

End to End in the C/U with cables out of bars/mcbs no croc clips

Interconnect L1+CPC2 / L2+CPC1 into connector blocks (tried a couple and even tried Wago to check there was nothing giving me a false reading)

Purpose is having the C/U changed in a couple of weeks, spark is happy for me to dead test for experience and he will double check results if he needs to, then he will live test once the c/u is changed.

0.26 was roughly the same throughout the ring, 0.26 was the highest.

30/50 was at all sockets on the ring, none was significantly lower than an another. And again, pretty similar the whole way round the ring, although I'll be honest, I didn't write them all down, I just checked they were all similar, and then tested and re-tested.

Yea my best stab at answering the question is an appliance, I'm thinking something with an element of some sort to cause such a high reading.

Because the downstairs is split into two rfcs, there aren't a great deal of socket outlets, 5 double outlets in the living room, then 4 double outlets in the diner section of the kitchen, kitchen side of the kitchen is on the kitchen rfc

 

[PrenticeBoyofDerry]

You have some added resistance somewhere, check W/M D/W, dryer etc are unplugged, also any FCUs for power points and lights?

 

[Iggifer]

The tumble dryer!! I unplugged absolutely everything inside, totally forgot about the outside sockets, believe them to be spurred off the downstairs rfc.

You legend. I will retest tomorrow with some light.

 

[riveralt]

End to End in the C/U with cables out of bars/mcbs no croc clips

I assume you mean at the the CU - and that you followed the pictures in the Onsite guide? If you didn't use croc clips what did you use?

30/50 was at all sockets on the ring, none was significantly lower than an another. And again, pretty similar the whole way round the ring, although I'll be honest, I didn't write them all down, I just checked they were all similar, and then tested and re-tested.

There is a big difference between 30 and 50 ohms - so there not really similar are they? How the figures change between sockets can give you a big clue as to what the problem is.
Also I take it you don't have any spurs on the circuit.
By the way I will assume that you used some form of plug tester connected to your test machine for each socket rather than taking readings from the cables themselves. If you did it is worth pushing the plug in and out a few times to ensure good contact. I don't use a meggar personally but I assume your probes went into the right pins in the plug tester.

Yea my best stab at answering the question is an appliance, I'm thinking something with an element of some sort to cause such a high reading.

I thought you said there were no appliances connected - don't think that would be the reason. Probably an issue when conducting insulation resistance tests though.

Given your (L/N) interconnect was 0.26 ohms if there is an issue it would appear to be with the earth - possibly a disconnect somewhere on the ring.
You could split the ring and test either end of the two radials to see what reading you get - this may point toward the problem socket/area

 

[Electrifying]

I'm not getting how something 'plugged in' is causing a rise in the resistance reading on a R1 + R2 test.

Anyone care to explain??

 

[Iggifer]

End to end yea, similar technique to the OSG. No clips = holding cable with thumb against probe tip. Instructor at college is not a fan of croc clips so taught us this way.

I will confirm the readings re: 30-50 ohms tomorrow.

I used the probes into the outlets themselves for the old MEM outlets that let me do it that way, and for the new MK ones, I had to use the plug in adapter. It was inserted numerous times to each outlet and the probes were inserted into it numerous times.

I was also thinking a disconnect, but it would presumably give me >299 ohms for my end to end if there was a total break, or if it had just come out of one outlet, a >299 at that outlet. It seems the electricians that rewired the house were fans of T+E twisting, so it would be logical to think that the cpc could have come out of a socket outlet but would only give me a higher/zero reading at that outlet.

And I thought I had unplugged all appliances, but as per my previous post, apparently not

 

[riveralt]

End to end yea, similar technique to the OSG. No clips = holding cable with thumb against probe tip. Instructor at college is not a fan of croc clips so taught us this way.

Wow - I suppose he asks you to do the same when conducting insulation resistance tests as well

I used the probes into the outlets themselves for the old MEM outlets that let me do it that way, and for the new MK ones, I had to use the plug in adapter. It was inserted numerous times to each outlet and the probes were inserted into it numerous times.

Probes in the pin holes is not a good way to test - you need to be consistent either use the plug in adapter or take the front of and test using the rear terminals.

I was also thinking a disconnect, but it would presumably give me >299 ohms for my end to end if there was a total break, or if it had just come out of one outlet, a >299 at that outlet. It seems the electricians that rewired the house were fans of T+E twisting, so it would be logical to think that the cpc could have come out of a socket outlet but would only give me a higher/zero reading at that outlet.

If there is a break in the circuit on the end to end test you will not get any continuity zero ohms - if there is a disconect and the wires are twisted together then you will still get a reading.

And I thought I had unplugged all appliances, but as per my previous post, apparently not

As Electrifying has already commented I'm not sure I understand how this will make any difference.

 

[PrenticeBoyofDerry]

I'm not getting how something 'plugged in' is causing a rise in the resistance reading on a R1 + R2 test.

Anyone care to explain??

It shouldn't really, unless there is a fault at that appliance or equipment.
But still good manners to unplug/off load everything prior to testing.

 

[Iggifer]

He is a she actually. No she is pretty good actually. However the probes into the socket is also her technique.

By >299 I meant no reading / open circuit.

I initially thought it could be my megger playing up so I went back an re did the L-N interconnect and then L-CPC on the upstairs rfc and all the same.

There is a spur off this ring - the outside socket I didn't test that has the tumble dryer plugged into it.

An appliance is the only think I can think of. As end ends are all fine, this suggests no break and L-N is fine also.

I'm unsure why the appliance would add resistance, but there's not much else I can think of hence my OP. Stumped

 

[Iggifer]

I'm not getting how something 'plugged in' is causing a rise in the resistance reading on a R1 + R2 test.

Anyone care to explain??

It shouldn't really, unless there is a fault at that appliance or equipment.
But still good manners to unplug/off load everything prior to testing.

In that case we can probably rule out an appliance as the circuit is RCD protected this kind of fault would pop the RCD surely?

 

[PrenticeBoyofDerry]

Providing the RCD is functioning correctly, you'd hope it would.
Don't rule anything out until you have proved it!

 

[Iggifer]

I tested the RCD recently and its trip times were fine. Will re test the RCD in a minute as there's no CU disassembly required I can do that in the dark

 

[Iggifer]

RCD Test Results

1/2 - No Trip
1 @ 0 - 53.8
1 @ 180 - 59.9
5 @ 0 - 29.7
5 @ 180 - 38.4

Getting pretty close to 40ms at 5 @ 180 but it passes. Not the best test results I've ever seen on an RCD but it's going in the bin fairly soon so I'm not overly concerned.