Regulation 643.3.3 (ii) clarification

Line and neutral are both live conductors, the test should really be carried out at the meter tails with line and neutral joined together to earth with the installation completed and all earthing and bonding in place.

In the IET GN3 it says on page 69 “An insulation resistance test of line and neutral (L&N) connected together to Earth (E) will ensure all the circuit conductors are tested.”.

But that is not what is shown in the Figure 2.21 at the bottom of the page.

When I started work the Electricity Board meter installer working for the Midlands Electricity Board would do an insulation test at the meter tails before connecting them into the meter to test the entire installation to earth, but these days the meter operative just installs a main switch and it’s the electricians responsibility to test and make the final connection, absolving the meter installer and the electricity supplier of any responsibility it there is a big bang when the new installation is energised.

The IET OSG page 120 is much clearer when explaining the three-phase installation insulation resistance test.

Quote “Test to earth from all live conductors (including neutral) connected together.”.

Thank you, that's very helpful. The reason for my confusion is that I'm coming across increasing numbers of  fixed plug sockets with USB, Zigbee modules, cheap LEDs, etc and, whilst they are SELV components, they are part of a non-SELV circuit. So I'm never sure how to treat them. Especially when competing a minor work certificate or EICR and it isn't always reasonable (or possible) to disconnect every socket or light fitting. 

There may be a slight typo, but I assume that the reference is to 643.3.1. 

There are two tests.

First, between the live conductors, so L-N in SP and L1-L2, L2-L3, L3-L1, L1-N, L2-N, and L3-N in TP. Once it has been established that those values are satisfactory, you do the second test.

Second, between live conductors together and PE. PE is connected, but fairly obviously, the live conductors are not. This is a shortcut because if the IR is low, you would then have to test each live conductor individually to see where the problem lies.

I remember: SP 3 tests: L-N, L-E, N-E; TP 10 tests: L1-L2, L2-L3, L3-L1, L1-N, L2-N, L3-N, L1-E, L2-E, L3-E, N-E.

HTH.

Thanks Chris, the main thing I'm getting bogged down in is the 250v vs 500v issue for the second set of tests, and whether, in a situation where you have SELV components in a predominantly 230V circuit, and sensitive equipment cannot be removed, is it OK to conduct the test at 230V with L-N connected together? Or in other words, how likely is the probability that modern circuits could contain ultra-sensitive SELV modules  or circuitry that could be damaged by 500V? In which case, is 230V just being over cautious. 

Something a bit odd here because when I posted 20 min ago your posting of 30 min ago was not visible.

I see exactly what you mean. Clearly for an EIC, nothing will be plugged in, but for an EICR, you may have missed something. So IR is low and you have to find it, remove it and test again. Testing at 500 V could fry the appliance, which is why the test voltage has been lowered.

The problem with a BS 1363 socket with added USB ports is that there is nothing which you can unplug, so the default voltage has been reduced.

In my simple mind, if a device is powered by 230 V AC, 250 V DC should not harm it. Should not as opposed to cannot!

In my simple mind, if a device is powered by 230 V AC, 250 V DC should not harm it.

generally, but things that rely on inductors to limit the current will only be happy if as part of that test you reverse the 250V dc before the current has built up to a significant value and continue to do so, perhaps at 50 or so times per second ;-) 

In practice if you cannot with confidence, remove all loads from between L and N, or L1 and L2/3 then there is no point in trying a DC test between those conductors, as the test will fail as the resistance presented by the load will be a lot less than the expected insulation, and there is a good chance some equipment will not appreciate it and may not work afterwards.

In such a case, the only sensible test is L+N on one side of the meter, and E + bonding and electrodes  on the other. (this then also finds faults from L to the heating pipes or whatever, as well as to the CPC of the circuit with the L in it.)

That way  no current flows  in L-N connected loads and as both L and N should be isolated from E, it is a sensible test. Even so tests at 250V where surge limiters are involved, which is not only the ones we can see in the consumer unit, but also those in the connected equipment, as you will probably trigger them with 500V.

In an EICR there is no risk of missing an L-N short, as you are testing a circuit that was until recently energized, and the ADS would have 'indicated' such an issue had it been present, by operating.

In a new and as yet un-energised circuit it does make more sense to test L-N and L_L as well this avoids the embarrassment of the main fuse failing on first power up, but at that point there should be no connected loads as it is under your control.  Which may mean usb sockets must not yet be fitted.

Mike.

Chris Pearson refers to the "second test" which is all lives connected together to earth as a short cut, it is not a short cut, it ensures the entire installation has a satisfactory insulation test value. This part of the testing should be done at 500 vDC before sensitive loads are connected and again at 250 vDC when the sensitive loads that cannot be isolated are connected.

The mininium test result value of one megohm is for the entire installation and the entire installation should be tested all at the same time to earth.

Mike said “In a new and as yet un-energised circuit it does make more sense to test L-N and L_L as well this avoids the embarrassment of the main fuse failing on first power up, but at that point there should be no connected loads as it is under your control.”.

Been there, done that, a completely new installation failing the insulation test, because whilst the job was awaiting completion a TV aerial installer connected a TV aerial amplifier into the back of the front bedroom light ceiling rose, I found the same issue on a few installations as well when altering lighting circuits and preparing EICRs, it doesn’t blow the main fuse, but it does leave you standing in a completely empty house or flat trying to work out what could be the issue.

The L-L and L-N at 500 vDC tests are the first set of tests, the second test is the global 500 vDC of the entire installation to check the plasterers or other trades have not damaged cables, and the third test is the 250 vDC global test of the entire installation after completion to ensure you have not pinched any conductors putting covers back on or the like your self. 

Note that the 1meg limit is a bit arbitrary, as what to expect  is rather installation dependent Of course large rambling installations on farms with lots of damp sheds or anywhere with a lot of pyrotenax mineral insulated cable (churches  are good candidates), are likely to fail with no fault found, while a short PVC lighting circuit in a small bedsit flat ought to be a hundred meg or more really and should raise an investigation at more like 10meg. So don't be too black and white about it.

Mike

PS to pass the farm/ damp church it may be necessary to review it in zones each of which individually pass.