Basic domestic testing Qs

[newtothis]

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]

Safely isolate circuit under test.
Do you mean just isolate to make Line and CPC connection, then energise ?

Link Line & CPC at furthest point of circuit.

Test at the DB across the (isolated) load side of the Circuit breaker and the live supply side of the RCD.

Carry out test, record result.

Put a Q in blue

 

[Simonslimline]

Put a Q in blue

Isolate the MCB. Then test from the isolated side of MCB and Live supply side of RCD. This way you are performing the exact same EFLI test except bypassing the RCD so a high current test can be carried out which is more accurate.

The line is connected to earth at the end of the circuit. So you have a PD between the isolated Circuit breaker and the 230 on the supply of the RCD.

 

[Simonslimline]

Put a Q in blue

Don't Re-energise the circuit until you have removed the temp link and terminated conductors back to there respective terminals.

 

[newtothis]

Ye haaw, Thanks mate, I've got it !

 

[Simonslimline]

Ye haaw, Thanks mate, I've got it !

And if in doubt just draw it out. This way you will see the path the test current takes.

 

[newtothis]

what was confusing me is I knew it was a live test, and the supply L side is live even if the board isolated.

So in summary, can I say if my measured ZS doesn't exactly equal (or is a significantly out) with my R1+R2 + ZE
Using the low current ZS test can be a factor, along with user error on the testing and loose connections, poor design on the circuit ?

Failure on the IR test is not a contributor to the above, if it's down to something that should have been disconnected?

 

[Simonslimline]

Generally a low current loop test is not that accurate and it is better to perform a high current test whenever possible.

If anything, the measured Zs would normally be lower than the calculated value due to parallel paths via bonding etc.

User error could play a part and not making good contact on the terminals when carrying out the test could contribute to higher than calculated reading.

 

[newtothis]

Thanks very much Simonslimline, I'm driven to get this right. I'm going to slow down and double check everying I do.

I have mates who are sparkies and seem mega competent. They admit their knowledge of testing is a bit sketchy.
I'm also working with sparks at the moment who are very well up on testing, but don't take too seriously.
Telling me stories about skipping parts of the testing cos they didn't like the person they were testing for etc etc...
I'm laughing with them at their stories, but inside I don't agree. :teeth_smile:

 

[Simonslimline]

Thanks very much Simonslimline, I'm driven to get this right. I'm going to slow down and double check everying I do.

I have mates who are sparkies and seem mega competent. They admit their knowledge of testing is a bit sketchy.
I'm also working with sparks at the moment who are very well up on testing, but don't take too seriously.
Telling me stories about skipping parts of the testing cos they didn't like the person they were testing for etc etc...
I'm laughing with them at their stories, but inside I don't agree. :teeth_smile:

Sadly you will see this when onsite, depending where though i guess. I have seen this attitude and it all comes down to wanting to be finished faster and being a lazy T@£$%.
If it's there --- in court then its there problem i guess.

Personally i couldn't sleep without knowing i had done my job correctly and verified the safety of my work.

Chris kitcher does a great book on Inspection & testing, well worth a purchase.

 

[Richard Burns]

Remember when testing Zs normally you are also testing through the RCD and the MCB which are not included in the Ze test or the R1+R2 test and occasionally these items can exhibit a noticeable added resistance to the test. This may be what you are experiencing.

A poor design of circuit would not contribute to a variance between ZS and Ze+(R1+R2); if the circuit were poorly designed then they would both be high.

An IR test failure would not affect a Zs / ZXe(R1+R2) variance, although a really bad fail where any conductors were touching could reduce the measured value in both cases; although other factors would come into play before you could do the live test!:sorcerer:

A small diagram for the alt Zs test.

 

[newtothis]

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

 

[Simonslimline]

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]

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]

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]

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



Blimey, I coulda got a belt off that screw - must be more careful !