Insulation Resistance Test on Ring Circuit

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With my newly purchased Megger MFT, I will be lookin to practice a number of tests - including the Insulation Resistance Test. Initially, I will be focusing on an individual Ring Circuit. I will unplug all loads.
I know this is a dead test and I will follow Safe Isolation to terminate power at the Mains Switch. There are a couple of areas that are unclear from the reading that I have done:

- do USB sockets need to be replaced when applying 500v? If yes, can I use a 32 amp JB in their place? I assume that their resistance will be the same as a socket
- do FCU's on a ring (not a spur) need to be disconnected? The neon light is on the load side
- finally, should the CPC remain connected to the earth terminal or separated out like N and L for the purposes of this test?

Thanks in advance.
 
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- do USB sockets need to be replaced when applying 500v?
Most equipment, including USB sockets should be fine with a 500V P+N to E test. If you want to do a P to N test then USB sockets will need to be disconnected and the wires linked through.

Of course disconnecting equipment means that the equipment itself won't get tested, and disconnecting and reconnecting brings the risk of introducing faults that were not there before.

If yes, can I use a 32 amp JB in their place?
If you have enough wire-length sure, but I suspect in many cases you won't. I'd suggest using wago or similar lever terminals are probably your best bet for linking through and/or reducing shock hazards when removing equipment for testing. If you were really paranoid you might cover the wagos up with a blanking plate but I highly doubt most people would bother.

- do FCU's on a ring (not a spur) need to be disconnected? The neon light is on the load side
If it's a switched FCU then turning off the switch on the FCU should be an adequate method of isolating downstream devices (including the neon if there is one).

- finally, should the CPC remain connected to the earth terminal or separated out like N and L for the purposes of this test?
Depends on the goal of the testing.

If you are verifying the installation is ok, then the CPC should remain connected to the earthing system. This is the safest way to conduct the test, and also the way that is most likely to discover faults.

If you are fault finding you may want to seperate the CPC from the rest of the earthing system (be aware that it may be connected to the rest of the earting system at more than one point) and test the two seperately to give you a better idea of where the fault may be located, but you should assess the risks before doing so and ideally you should do it when noone else is in the property.
 
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Most equipment, including USB sockets should be fine with a 500V P+N to E test. If you want to do a P to N test then USB sockets will need to be disconnected and the wires linked through
As you say, most items should be fine with 500V - you would certainly hope so for a device designed to be permanently connected to a mains supply (that may be regularly tested at 500V).
However, in the world of PAT, we have killed a number of PSU's and other devices with a 500V insulation test.
Now we have testers with a 250V test - so far, no breakages! :)

....and turn off any FCU's on the circuit.
 
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500V P+N to E test
Can I clarify please. This is when we have P+N in one croc clip and Earth in another?

disconnecting and reconnecting brings the risk of introducing faults that were not there before.
That’s a very valid point and one I hadn’t considered. Will try to avoid this.
If you have enough wire-length sure, but I suspect in many cases you won't.
This (or wagos) won’t be necessary, given that I don’t need to remove the USB sockets with a 500v test.
If you are verifying the installation is ok, then the CPC should remain connected to the earthing system. This is the safest way to conduct the test, and also the way that is most likely to discover faults.
Sounds as though, I should start with a P+N to E (on main terminal) and then separate Earth out for further testing if there is a fault. This would remove the effect of the earth bar and all the wires connected to this.

With IR testing there seems to be many different methods and I suppose it depends on the purpose it will serve.
 
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best to have someone there to call the ambulance if you have an accident,
That's a good point.

Just remember that if you separate parts of the earthing system for fault-finding and then perform IR tests that your tests could result in 500V between different parts of the earthing system.
 
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That's a good point.

Just remember that if you separate parts of the earthing system for fault-finding and then perform IR tests that your tests could result in 500V between different parts of the earthing system.
Understood. Presumably you will only separate when the IR reading is too low when using that combined earthing terminal? That’s when we want to test with each individual CPC to identify where the issue is.
 
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I think I have this right but hopefully you can confirm or put me right.

With all 3 of these tests on a Ring Circuit:
1. Continuity
2. Cross overs (figure of 8 )
3 Insulation Resistance

If there is a fault, you could adopt the “ring halving” method. This will mean that you have a “good” and “bad” leg of the ring. You then add 1 (or a few) socket at a time to the good leg until it turns bad. This identifies where the issue is in relation to any of the above 3 tests. At this stage - unconventionally perhaps - I would add a temporary twin and earth on the surface to replace the faulty section and then repeat all 3 tests on a complete ring circuit. This should now pass.

At this stage I have two options:
- replace the faulty cable properly and this may involving channeling, etc
- or convert the ring into two radials protected by a 20 amp MCB’s.

I would appreciate your comments on this approach. Thanks again.
 
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Can I please have some comments on the above understanding. Thanks again.
 
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If there is a fault, you could adopt the “ring halving” method.
Whether that is the best, or even necessarily an appropriate, approach will, at least in my opinion, depend upon the nature and (where appropriate) 'magnitude if the fault.

This is one of those situations in which (again in my opinion) the best approach to fault-finding in a particular situation should be based on a good understanding of the underlying electrical principles, not by a 'cook book' method. In other words, if someone has a good understanding of how electrical circuits work, and if they know and understand what fault they have detected, they would (should!) not need to ask how to proceed with further fault-finding/tracing.

Kind Regards, John
 
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Whether that is the best, or even necessarily an appropriate, approach will, at least in my opinion, depend upon the nature and (where appropriate) 'magnitude if the fault.
If there is a 0 reading for a continuity test and there is no obvious visual clues, when would the ring halving method not be a plausible approach to proceed?

Also, once you have tracked down the fault and subsequently proved with a temporary cable that the tests are now successful, is there any issues with the 2 radial approach, if replacing the cable is deemed impractical?

I’m sure an expert understanding of electrical concepts will provide one with more options but I’m trying to understand if there are any obvious flaws with my suggested approach.

Thanks again.
 
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I think that there is a big difference between the case where somewhere there is leakage, and somewhere there is a continuity break. What you described in broad terms would be sensible for a leakage (lower insulation resistance than desired). But I think it would be well for you to describe in detail the steps you would take for a 'ring halving' method when trying to find a break. For one thing I would regard a '0 result' as representing continuity (or 0 ohms).
 
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Fault finding is a combination of Sherlock Holmes detective work and (often) luck.
The issue is that electrical circuits may have been installed and tested to be 100%. But subsequent work by sundry trades can add some very confusing items to a circuit.

Typical examples. Splitting a circuit often doesn’t butter any parsnips. As above, there’s no template, it’s all down to experience.
The process that you have outlined isn’t wrong. It might work. The only way to find out us to try it yourself.
 

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