UK Zs readings unusually high on new installation, what is the best option to lower them?

[Joemile]

I've just finished an installation on a large outhouse around 50 meters from the main property, but I'm having issues with unusually high Zs readings within the new installation..

The nature of the installation is a TNCS system with 25mm tails & 16mm cpc Henley blocked from the main supply to a 60A sub-board with a 25mm2 SWA running 50 meters to the outhouses own 10 way RCBO consumer unit. (I was only able to ground the SWA at the supply end due to the SWA routing at the other end not alllowing me to gland into the consumer unit as it was laid in by the builder.)

My Ze reading at the sub-board came back around 0.19 and my Zs at the outhouse Consumer reading is 0.25 but when testing the circuits within the outhouse installation they all seem to come back unusually high for example 4 socket 16a radial - 3.89, 6 spotlights lighting circuit - 4.12 etc..

My R1R2 results come back absolutely fine so why are my Zs readings coming back so high and how can I possibly lower them?

I've been toying with the idea of installing a TT Rod at the outhouse to try and lower the readings but I'm not 100% sure if this would be sufficient.

Could anyone give me some advice?

 

[SparkyChick]

If you are using a 3 lead test through and RCD or RCBO, those devices could be adversely affecting the Zs readings. The way to see is do a 3 lead test at the input terminals of the RCD/RCBO and then at the output terminals. If they are wildly different, the device is the issue and I would use calculated Zs values for the certificate (this is perfectly acceptable).

 

[ipf]

For peace of mind, replace one circuit RCBO with a temporary mcb (for test only) to give you the true circuit Zs. Compare with calculated for further proof.

 

[Vortigern]

First off gland the other end of the SWA, run a supplementary 10/16 mm earth? As you are above max Zs for the 16a you will have to remedy that to acheive ADS within specified times. Did you have AT on? What is your maths on the situation? i.e. what was R1/R2+Ze (Zdb?). Did you somehow not clamp the crocs on properly (I know, but I am scaping the barrel for reasons) Was your Ze acutally Zdb? or was that at the point of origin? Lot of questions but I am curious as to how that could be. I think the uplift factor off the top of my head for RCD is .5 ohm.

 

[pc1966]

If your DC measured R1+R2 is fine and in line with expected values but the AC Zs is way too high it sounds a lot like the "RCD uplift" problem where your MFT is seeing the impedance of the RCD's sense transformer.

Some MFT are better than others at suppressing the effect, and some RCD/RCBO are possibly bigger causes of it.

The suggestion by @SparkyChick to test Zs at both the immediate input and output of the RCD/RCBO should show if it is due to that reason as then you have no final wiring, etc, to consider.

Another trick (which is a bit more hazardous) is you short the end of the circuit L-E with the RCD open and do the Zs measurement "over" the device (e.g. MFT's L to RCD input L, MFT's E to RCD output L, and MFT's N (if 3 wire) to neutral bar). Just be careful not to close the device on to your deliberate short!

 

[GBDamo]

Sounds very much like RCD interference.

It has made my arse drop out on more than one occasion, happen infrequently enough that I forget about the issue before it happens again.

 

[SparkySy]

I have experienced RCD uplift on a number of occasions but usually much less than 1ohm, I would definitely test either side of the RCBO'S to confirm if this is the problem.
If it is I would test the installation as @pc1966 suggests.
I still prefer to test on high Zs setting, the confidence meter on my Megger gives me no confidence at all!
Have you checked your test leads and crock clips for consistency? I had a dodgy live lead a while ago and sometimes it would read fine but get it in just the wrong spot and it would give wildly inaccurate readings!
Sy

 

[pc1966]

I still prefer to test on high Zs setting, the confidence meter on my Megger gives me no confidence at all!

Very good point!

The "over the RCD2 method can be done at normal high current mode, not low non-trip mode with more uncertainty.

Have you checked your test leads and crock clips for consistency? I had a dodgy live lead a while ago and sometimes it would read fine but get it in just the wrong spot and it would give wildly inaccurate readings!

Yes, also worth checking probes are OK.

 

[telectrix]

If your DC measured R1+R2 is fine and in line with expected values but the AC Zs is way too high it sounds a lot like the "RCD uplift" problem where your MFT is seeing the impedance of the RCD's sense transformer.

Some MFT are better than others at suppressing the effect, and some RCD/RCBO are possibly bigger causes of it.

The suggestion by @SparkyChick to test Zs at both the immediate input and output of the RCD/RCBO should show if it is due to that reason as then you have no final wiring, etc, to consider.

Another trick (which is a bit more hazardous) is you short the end of the circuit L-E with the RCD open and do the Zs measurement "over" the device (e.g. MFT's L to RCD input L, MFT's E to RCD output L, and MFT's N (if 3 wire) to neutral bar). Just be careful not to close the device on to your deliberate short!

I do same any time I have this problem. Olď MÈM RCD giving a Zs

 

[telectrix]

I do same any time I have this problem. Olď MÈM RCD giving a Zs of 351ohms!!! At a socket .Actual Zs then camè in at 0.25,TNCS

 

[telectrix]

Where has the edit button gone???

 

[Robert P]

Have you double checked your busbar connections to make sure you haven't accidentally got the finger behind the main switch terminal?