high Zs readings

[Dave the spark]

Did an EICR in a domestic property, earthing arrangement was a TN-S with a Ze of 0.51 ohms.

There are 2 ring final circuits protected by 32amp 60898 type B mcbs. Both circuits have additional protection from a 30mA RCD.

My Zs readings for the circuits are 1.63 ohms and 1.89 ohms.

My question is, with the additional protection of the RCD would it be acceptable to leave it as is and make a note of the situation? Or can I contact the DNO to ask them to improve the external earth? Other than that I'm not really sure what I can do.

Looking for some advice, rather than snide remarks.

Thanks.

 

[westward10]

Well some may say it is bad practice to rely on the rcd for fault protection on a TN system but in this case that is what they are doing so I wouldn't comment on it. Do you have any records of previous Ze readings, and did you test it on the high current range of your loop tester. Those Zs readings seem high for a domestic even with the Ze at 0.51.

 

[mhar]

Does this tally with your theoretical Zs (Ze + R1+R2)? What Ze do you get if you measure on load terminals of rcd (with mcbs off)? Looking to see if your meter reading through the rcd is giving you higher than expected results

 

[telectrix]

Does this tally with your theoretical Zs (Ze + R1+R2)? What Ze do you get if you measure on load terminals of rcd (with mcbs off)? Looking to see if your meter reading through the rcd is giving you higher than expected results

could be. i once had an oldish installation with a Ze of 0.22 ohms on the incoming side of the RCD and 349 ohms on the outgoing. that's an RCD taking the pi$$.

 

[polo1]

Doubt the DNO would do anything as Ze is within permitted limits. As others have said what's your r1,r2 etc & R1 + R2 on both circuits.

 

[Wilko]

Those Zs readings seem high for a domestic even with the Ze at 0.51.

I agree - using my busted math I guesstimate ring length of over 110m for the 1.89-0.51=1.38 ohms=(R1+R2) etc. So I reckon there are some poor joints there perhaps?

 

[Dave the spark]

Thanks for the replies guys.

My R1+R2 values are pretty similar tbh, one is 0.49 and one is 0.52. so both calculations are within max Zs.

I'm going to try it again by bypassing the RCD, I hadn't thought of that, and testing on both low and high settings. If it falls to what I'm expecting on the low setting after by passing the rcd then I'll change the RCD module. If not I'll test it on high and if that doesn't do the trick I'll have to check my connections at every socket.

Again, thanks.

 

[sparksburnout]

What sort of tester have you got? Dilog and Megger are notorious for this. You need to take a reading on the supply side of the RCD and then again on the load side, and subtract this from your Zs readings at the sockets. it is almost certain to be the RCD messing up the MFT.

 

[Bob Geldoff1234]

What sort of tester have you got? Dilog and Megger are notorious for this. You need to take a reading on the supply side of the RCD and then again on the load side, and subtract this from your Zs readings at the sockets. it is almost certain to be the RCD messing up the MFT.

Spot on. I have a Di-log MFT and if i clamp the earth probe to the main earth and test on the non RCD side i can get a reading of say 0.3ohms.Without moving the earth probe,then testing on the RCD side i can get a reading of 1.56 ohms.
This only happens with certain makes of RCD and so if this happens i subtract the amount of difference to get the true reading.
I am surprised that there isn't a bridging device on the market for bridging out the RCD without having to resort to wired links.

 

[The Ghost]

As I understand it the readings are too high to disconnect within permitted times and would require further investigation. I am talking in terms of OCPD not RCD.

 

[westward10]

As I understand it the readings are too high to disconnect within permitted times and would require further investigation. I am talking in terms of OCPD not RCD.

They may not be, I would lose the rcd and retest using the high current range. I do this most of the time anyway.

 

[Dave the spark]

What sort of tester have you got? Dilog and Megger are notorious for this. You need to take a reading on the supply side of the RCD and then again on the load side, and subtract this from your Zs readings at the sockets. it is almost certain to be the RCD messing up the MFT.

It's a Fluke MFT, can't remember the model number.

I've come across this before with a certain type of RCD, GE I think, but never with a Wylex.

 

[The Ghost]

I would lose the rcd

Yeh understood as suggested in bypassing as part of investigation, and as you say they may not be too high (Max Zs) as its a RCD problem causing the Zs to be too high. However with my MFT it does say the high current range is only for Ze. However I still use it consumer side anyway in certain circs. (KT65DL btw.)

 

[westward10]

Yeh understood as suggested in bypassing as part of investigation, and as you say they may not be too high (Max Zs) as its a RCD problem causing the Zs to be too high. However with my MFT it does say the high current range is only for Ze. However I still use it consumer side anyway in certain circs. (KT65DL btw.)

That is interesting, maybe they are assuming all your Zs readings will incorporate rcd protection.

 

[Jay Sparks]

When doing the Zs test on "no trip", it does it at around 17mA and takes longer and gives a higherror resding. But on "Hi" current, it's around 25A and it does it quite quickly but can give a lower, more accurate reading. Obviously these will be vary on different meters. But to get a more accurate reading you need to bypass the red.

Jay

 

[Jay Sparks]

When doing the Zs test on "no trip", it does it at around 17mA and takes longer and gives a higher reading. But on "Hi" current, it's around 25A and it does it quite quickly but can give a lower, more accurate reading. Obviously these will vary on different meters. But to get a more accurate reading you need to bypass the rcd.

Jay

I need to proof read before submitting.lol some bad spelling in there.......now corrected in the quote.

Jay

 

[Dave the spark]

I need to proof read before submitting.lol some bad spelling in there.......now corrected in the quote.

Jay

I got the gist of it anyway, thanks