High ZS on all circuits

[Dan1983]

Hi there,

Ive just tested a new installation and getting a ZE of 0.45 on a TN-S system.

I'm also getting a ZS at the board of 0.45.

Its only a small house so the R1+R2 readings are pretty low..

Sockets 0.22
Lights 0.66
Shower 0.03
Cooker 0.06

The problem im having is when taking the ZS reading they are really high.

Sockets 1.37
Lights 1.81
Shower 1.18
Cooker 1.21

So this would indicated the ZS at the board been 1.15

My testers are fully calibrated and working fine on other installations.

I've had to leave the power off today due to time to look deeper into it, but my thoughts are that it could be the RCD thats faulty.

As the ZS from the henly block and the main switch of the consumer unit are both 0.45, but i never checked the zs from the out going side of the RCD..

All earthing is correct

Anyone ever come across this..

 

[westie101]

Seeing as the ELI can be up to 0.8 at the supply terminals for a TN-S supply, what's the issue? If it is above that call the DNO to check and rectify their earthing
The figure is a DNO set value to ensure the operation of a 100A cut-out fuse for a short circuit in the vicinity of the cut-out

 

[ricicle]

Sounds like, as you imply, a high impedance between the incoming terminals and the outgoing terminals of the main switch/RCD.
Was the Ze measured at the incoming terminals of the main switch ?

Repeating the test on the outgoing side would confirm any anomalies. All outgoing circuits would need to be isolated (including disconnecting Neutrals)

Also if you are using the meter on the 'no trip' setting the results are sometimes not that accurate !

 

[JohnW2]

As the ZS from the henly block and the main switch of the consumer unit are both 0.45, but i never checked the zs from the out going side of the RCD..

That's surely the obvious thing to check - if it's not around 1.15Ω, then something pretty crazy would be going on! If it is around 1.15Ω, as you say, that leaves only the RCD (and its connections) to explain 'the extra 0.7Ω'; do I take it that you've checked that none of the RCD terminals are loose?

Kind Regards, John.

 

[Dan1983]

Seeing as the ELI can be up to 0.8 at the supply terminals for a TN-S supply, what's the issue? If it is above that call the DNO to check and rectify their earthing
The figure is a DNO set value to ensure the operation of a 100A cut-out fuse for a short circuit in the vicinity of the cut-out

The issue is is im getting a reading of 0.45 at the incomer and that added to my R1+R2 is know were near the reading im getting for my circuit ZS readings.

Also the circuit ZS reading exceed the maximum permit ZS due to this

 

[Dan1983]

As the ZS from the henly block and the main switch of the consumer unit are both 0.45, but i never checked the zs from the out going side of the RCD..

That's surely the obvious thing to check - if it's not around 1.15Ω, then something pretty crazy would be going on! If it is around 1.15Ω, as you say, that leaves only the RCD (and its connections) to explain 'the extra 0.7Ω'; do I take it that you've checked that none of the RCD terminals are loose?

Kind Regards, John.

Everything checked for tightness and also for missing the bussbarr as ive come across that before were the bar is behind and just making contact.

Gonna check the RCD tomorrow, tight on time today and its an empty house so able to leave power off..

 

[Dan1983]

Was the Ze measured at the incoming terminals of the main switch ?!

Yeah mate tested from the henly block fed from the meter.

 

[PrenticeBoyofDerry]

The issue is is im getting a reading of 0.45 at the incomer and that added to my R1+R2 is know were near the reading im getting for my circuit ZS readings.

Also the circuit ZS reading exceed the maximum permit ZS due to this

So your Ze is 0.45 and then adding this to your R1+R2 readings and expecting the same result measured.
Not going to happen.
What breakers are on the circuits, your readings look okay to me? Well some do

 

[RF Lighting]

As mentioned earlier, are you using a 'no trip' or 'trip lock' type setting on your meter to test through the RCD?

 

[JohnW2]

Everything checked for tightness and also for missing the bussbarr as ive come across that before were the bar is behind and just making contact. Gonna check the RCD tomorrow, tight on time today and its an empty house so able to leave power off..

If the Zs from the load side of the RCD is indeed around 1.15Ω, you could obviously disconnect the RCD completely and measure the resistance through its L and N paths directly,to see/confirm that it is the culprit; I guess that high resistance contact(s) of its disconnector would be the most likely fault (which gives me another thought - it might be worth operating the RCD a few times, if you haven't already).

Kind Regards, John.

 

[JohnW2]

So your Ze is 0.45 and then adding this to your R1+R2 readings and expecting the same result measured. Not going to happen.

Not going to happen precisely, no - but it ought to be close.

It's obviously not credible that a properly functioning RCD could introduce anything like as much as 0.7Ω. At, say,50A, that would mean a voltage dropof 35V across the RCD and a catastrophic 1750W being dissipated within the RCD!

Kind Regards, John.

 

[Adam_151]

Try doing a high current test on the circuit a few times, it'll trip the RCD each time, but might be able to clear up any minor corrosion within the earth path, then afterwards try your low current test again and see if its anymore realistic

 

[EFLImpudence]

Apart from the additional 0.7Ω I'm amazed at how exact all the readings are.

All exactly R1 + R2 + Ze + 0.7.


Edit - Also, separate from your problem, Zs at the board (Zdb) (and at other fittings ) should be done with all earthing and bonding connected.
This would normally give a lower reading than Ze.

 

[JohnW2]

Apart from the additional 0.7Ω I'm amazed at how exact all the readings are. All exactly R1 + R2 + Ze + 0.7.

I hadn't noticed that, but you're right - and it does rather stretch credibility, given that we live in an imperfect world!

Kind Regards, John.

 

[securespark]

Was the Ze measured at the incoming terminals of the main switch ?!

Yeah mate tested from the henly block fed from the meter.

That should read,

Was the Ze measured at the incoming terminals of the main switch ?!

No mate tested from the henly block fed from the meter.

Your RCD EFLI test will always be higher than an EFLI test without.

I would not fret too much. As long as the figures are within spec, you're OK.

After all, a calculation of ZS on paper does not always match the actual reading.

EDIT: Sorry, missed your comment about ZS's exceeding their allowed values.

You could try replacing the RCD with an incomer temporarily to check whether this changes the reading.

1. RCD gone.

2. No longer have to do a soft test.