Basic domestic testing Qs

newtothis · Jan 9, 2016

I'm just look for confirmation thoughts on this first point.

I think when IR testing on an existing installation and it’s not practical to remove all vulnerable equipment. I can connect L to N to earth and test at 500V on the circuit I’m testing.

The question is if I do the above, could it result in, or contribute to; the measured ZE + R1+R2 not equalling the measured ZS ?

In my case it’s the downstairs RFC it includes the kitchen. It's hard to without causing a bit of damage to access a FCU with a neon to the cooker hood which has an extractor fan and 4 light bulbs.

Thanks in advance for any help, and there's no need for undue diplomacy, or softly softly language, if I'm barking up the wrong tree. :bucktooth: I'd rather know straight !

newtothis · Jan 9, 2016

Thanks Richard I will study that and digest it tomorow, been watching Man Utd and it's driven me to drink.

Simonslimline · Jan 9, 2016

Can't believe i forgot to mention some of what Richard has said.

In the past we have had issues with RCD's creating a high reading when carrying out EFLI testing. Quite common with MK RCD's in particular i found.

Calculating Zs is acceptable but IMO a direct measurement should be carried out. This will ensure that any high resistances through protective devices etc are picked up and can be looked in to and rectified.

A calculated Zs will not highlight this possible issue.

newtothis · Jan 10, 2016

Simonslimline said:
In the past we have had issues with RCD's creating a high reading when carrying out EFLI testing. Quite common with MK RCD's in particular i found.

Calculating Zs is acceptable but IMO a direct measurement should be carried out. This will ensure that any high resistances through protective devices etc are picked up and can be looked in to and rectified.

A calculated Zs will not highlight this possible issue.

It's an MK board. I shoulda mentioned that

I agree with measuring everything, that's what I'll always do unless ordered not to.
I've worked out I'd need to test a hellova variety and quantity of houses with all different 'challenges' and the brands of CU
to move towards being a competent tester.

Richard your diagrams and text leave me with no questions unanswered.

The only elephant in the room is Ring FCs how could I get around the having to use the low current no trip test.
Thinking out loud, if I establish there is a consistent variance between using the Low current test and the high current test, I could apply that to the ZS reading I get using the low on the RFC.

Richard Burns · Jan 10, 2016

A ring final circuit would be more difficult to test in this manner.
The figure 8 testing provides the worst case scenario for R1+R2, this would be the point at the mid point of the ring.
It is unlikely that there would be a socket exactly at the mid point of the ring or that you would necessarily be able to identify this socket if it were there, as you would have to know the exact lengths of cable in use and most would be inaccessible and the routes unknown.
However if you were able to identify a socket that would be close to the mid point and since the Zs measurement is a real life measurement where the worst case in practice would be the socket closest to the mid point (but does not take account of putting nail through the exact midpoint) then you could make your line cpc connection at that point and test, if you were not sure you could also test by putting a line cpc link instead at each of the sockets either side of this one and take the highest reading.
However in practice it is probably easiest to use the low current test and be aware of the level of inaccuracy that may be present by comparing the high and low current readings for other circuits.
A supply with very little noise on it would give the closest match between high and low current tests.

newtothis · Jan 11, 2016

Carried out that test earlier, ZS on the 2wire Lo Gave me 0.66 Ohms and using the alt method read 0.27ohms.
Now my results make sense. Happy as larry !

Feel a bit bad saying my mate made up some of the results, what he did was calculate ZS from measuered ZE and R1+R2.

Here's a pic showing the awkward postion of the cooker switch in a quarter sized unit. LOL