High Zs Reading on Megger MFT 1552

[phawk]

Can somebody help me please?

I'm just finishing a rewire, and I've installed a GF radial cicuit in a large domestic property and am in the process of testing it. It's a new radial circuit, 2.5 T&E, 13 sockets, 20A type B breaker on an RCD protected board. My R1&R2 = 1.40 and Ze = 0.20, so my calculated Zs should = 1.60 or just under (just within maz Zs).

The problem I have is that my Megger is giving me a Zs of 2.33 which is way over the max permitted of 1.85. It's even just over the 2.32 max permitted Zs of a 16A breaker which rules out dropping the breaker size.

As I am using this installation as my NAPIT assessment it needs to be spot on, so I really need to know why my meter is reading so high! I know I can note a calculated value of Zs, but if the assessor asks me to measure the Zs it will just be a flat fail.

I have been told it could be the impedance of the RCD, but I have also been told that Meggers are notorious for giving high Zs readings. Can anybody shed any light on this?

Points to note:

I don't have access to another tester to compare against.

My Megger has just been calibrated.

There are no loads in the circuit.

All connections are tight (R1+R2 = 1.41, R1+Rn = 1.40, R2+Rn = 1.14).

Thanks!

 

[Edd]

I get high ZS readings on my megger 1730 non RCD circuits fine then on the RCD side over high readings. Found to be worce on crabtree boards. Swop your circuit to the non RCD side(if there is one) and re test. (or link out RCD, but shhh dont tell anyone) then note it on EIC. or try a new RCD and charge for it. Rember as long as you note these things on the sheet then you cant be cault out.

Note I think your radial could have been done in 4mm (greater CPC lower ZS) so as not to push the max ZS limit.. but hindsight is a wonderfull thing.

 

[Sintra]

Try doing a Zs both sides of the RCD on the no trip setting and compare results.

 

[phawk]

Thanks Edd, as I was installing it I actually thought hmmm there might be a bit much on here for 2.5 but as you say, hindsight etc.

Good to know I'm not the only one with this problem.

 

[phawk]

Sintra, even if I did this and it gave me more favourable results, it wont be a true Zs, so I won't be able to note it, or will I?

 

[Silly Sausage]

There is the argument that the max Zs for an RCD protected circuit is 1667ohms.
So you're well in!

It's not an excuse to get away with a high Zs though!!!

 

[Sintra]

Sintra, even if I did this and it gave me more favourable results, it wont be a true Zs, so I won't be able to note it, or will I?

No but it will tell you if your meter is inaccurate or you have a high impedance across the RCD.

 

[MarkieSparkie]

Your radial circuit is RCD protected, so for a 30mA RCD the maximum Zs is 1667Ω, therefore your measured Zs is comfortably compliant. BS7671:2008+A1:2011 Table 41.5

Your Zs measurement is probably higher than the calculated Zs value because the low current employed in the No-Trip loop test method cannot break-down the oxide and tarnish of in-service joints and contacts. Normally one would expect fortuitous parallel earth paths (eg. via bonding) to reduce the measured Zs to a reading similar or less than the calculated value. However, if the non-electrical services (water and gas etc.) are provided via plastic or plastic covered piping, or not available, and there is a lack of structural steelwork, fortuitous parallel earth paths may be minimal or non-existent.

You should not blame you Megger MFT, using the No-Trip test on a Fluke or any other tester would likely as not produce similar results. It’s most likely the combination of the test method employed and nature of that specific installation that has caused your doubts.

In laboratory conditions both No-Trip and Hi loop test measurements are likely to produce similar results, certainly within the calibrated accuracy of the MFT, but add in the oxidized joints and contacts of a typical real world installation and result is likely to be completely different. My money is on the Hi current loop test giving the more accurate result because the higher current is likely to be able to break down the oxide or tarnish film.

If you're using a Megger MFT17xx MFT and it has a firmware version earlier than 2.01 installed it would be well worth getting it calibrated and the latest firmware update (usually inclusive in the UKAS calibration charge), as this has addressed virtually all the previous variability and RCD test issues. Two wire loop testing is still less than successful, but each firmware update seems to produce a slight improvement.

 

[electric will]

I never record max Zs as 1667, I always record the value in osg, (80% of actual max Zs)

 

[MDJ]

Hey Will the NICEIC inspector who visits me every year doesn;t like the 80% value, in fact he throws his toys out of the pram and insists on the real value haha

 

[electric will]

How odd, the inspector from elecsa told me to use 80, just can't win! He also said that recording the max value in the cert was something to do with the NIC and not actually required by the regs, not got my bgb here or would look.

 

[MDJ]

Perhaps it is just my inspector then bud, and yes he is odd.

 

[Amp David]

Sintra, even if I did this and it gave me more favourable results, it wont be a true Zs, so I won't be able to note it, or will I?

You asking or telling here boss?

 

[phawk]

Some great answers people, especially markiesparkie, what you said about the loop hi test is true. I tested the upstairs radial and for some reason the rcd didn't trip, and gave me a more sensible reading. But the only question I now need definitively answered is.....do I note the 'maximum allowed value for Zs' as 1667 or 1.85?

 

[phawk]

Asking, always asking!

 

[Sintra]

The way I look at it is on a TN system you are using the RCD as additional protection not fault protection so I would use the 1.85 figure.

 

[thekingiam]

on tn 1.85 on tt the 1667 figure

 

[Amp David]

I'd be putting 1.85 for the breaker and 1667 for the rcd