Help with Readings on circuit

[Alan Marriott]

Hi
Can anyone clear up or explain in a bit more detail what could potentially be causing the results I have. I have just tested a 32a ring main wired in 2.5mm twin on B32 Rcbo. There are 8 sockets on the ring and all are in plastic dado trunking in 2 offices. The readings I have are:
L-L= 0.76
N-N= 0.76
E-E= 1.32
R1/R2= 0.89
Zs= 0.78

Board ZS= 0.22

My R1/R2 are slightly high as should be roughly 0.52 so could that be trapped insulation in terminal instead of touching copper?

Also my zs was low at 0.78 when should be around 1.11. It can’t be parallel earth paths as everything is in plastic dado trunking.

Any help would be much appreciated in helping me further with my testing which needs brushing up more on.

Thanks

 

[pc1966]

The L/N figures are consistent with the E so it looks to be fine. However, that seems consistent with 100m or so of cable so presumably a big area?

How did you measure the R1+R2 (I'm guessing a typo for "R1/R2" ?) as you say (r1+r2)/4 = 0.52 ohms? Did you null out any 13A plug to 4mm adaptor lead before checking?

Zs is in keeping with Zdb + computed R1+R2 but then many MFT assume a test lead (at least that is what DiLOG told me about lead nulling).

 

[Alan Marriott]

Hi.
I measured R1/R2 by cross connected the lives and earths at a socket I had disconnected then when round with my MFT megger at each other socket on the ring.

 

[Pretty Mouth]

Ring final testing for R1+R2 is imperfect, and the results can be misleading.

For a ring final figure-of-8 test, results = (r1+r2)/4 only when measured at points on the ring. All spurs will give a higher result, no matter where they are on the ring. For example:

A very short spur taken from a point very close to the circuit origin would create a very short fault loop, this is obvious. However, when tested 'by the book', it would have a higher R1+R2 than a point actually on the ring, midway round the ring.

In your case, I expect you have tested correctly 'by the book'. The R1+R2 was probably taken at a spur, and the Zs measured at that spur more accurately represents the true fault path at that point.

 

[Alan Marriott]

Hi. I thought that it maybe a spur but I’ve checked all points on circuit and definitely no spurs on the ring.
I know expected readings never regularly match up but most are there or thereabouts. Which is why I was hoping someone maybe able to shed some light on it.
Appreciate your feedback!

 

[davesparks]

Did you test at the terminals of the sockets or use a plug in test lead adaptor?
If the latter did you try both sides of any twin sockets? Often you get a different reading from one side of a twin socket to the other.

 

[timhoward]

Is there a boiler or water heater on a FCU on this circuit?

 

[Alan Marriott]

Hi everyone. There is no boiler etc or spur on circuit I can confirm. When testing I did use the plug top attachment lead. I added 4 new Mk twin sockets to the ring and the reading on these were near enough the same as old ones in adjacent office.
I must also admit that I hate the megger tester I’ve been given from work. It has the confidence meter to it so takes ages to get a ZS reading due to harmonics on circuit. Give me a fluke any day of the week. The megger can take up to 2 mins per zs test.

Thanks