High earth loop impedance

Nickj · May 6, 2016

Hello all,

I am studying to be an electrician but outside of the classroom I have little experience.

A friend has bought a house and during the buying process had an eicr with unsatisfactory result.

Reading the report it shows the ring and shower circuit to have high earth loop impedance.

I ran the test myself today and the earth loop impedance on the ring and radial are around 40 ohms, which is what the eicr also shows.

I have checked the wiring of all the sockets and all seem to be terminated correctly.

r1, rn and r2 for the ring all seem fine.

According to the report the Ze is also fine.

What could be causing this?

I suspect a problem with one of the other circuits, would I be going down the right alley with that assumption?

Thanks,

Nick

Sintra · May 6, 2016

Welcome to the forum. Can you add some further information..
What type of system is it?
What is the Ze?
What results did you get for r1, rn, r2 & R1+R2?

davesparks · May 6, 2016

How many ohms are there in a 'fine'?

What are the readings for L-N loop impedance? What type of test did you use, low current or high current?

Sent from my iPad using Tapatalk

Nickj · May 6, 2016

It's a tn-s, Ze on the eicr is 0.3.

R1 0.14, rn 0.14 r2 0.22. I ran out of time and didn't take r1 + r2 of the ring (it's not on eicr) I did no testing on radial, shower or other circuits.

From the eicr

shower earth loop impedance 42.3
ring 43 (confirmed with my meter today)
radial 42

Nickj · May 6, 2016

Hi davesparks low current no trip test. I had pretty much the same reading as the one noted on the eicr.

Cheers

Leesparkykent · May 6, 2016

Nickj said:
It's a tn-s, Ze on the eicr is 0.3.

R1 0.14, rn 0.14 r2 0.22. I ran out of time and didn't take r1 + r2 of the ring (it's not on eicr) I did no testing on radial, shower or other circuits.

From the eicr

shower earth loop impedance 42.3
ring 43 (confirmed with my meter today)
radial 42

Are these circuits on the same RCD and if so are there other circuits on this RCD?

Nickj · May 6, 2016

Yes all on same RCD

shower 40A
radial 20A
Ring 32A
central heating 16A
cooker 32A

not on Rcd
spare
spare
lights 6A
lights ground floor 6A
lights 1st floor 6A

Andy78 · May 6, 2016

Leesparkykent said:
Are these circuits on the same RCD and if so are there other circuits on this RCD?

Exactly what I was thinking when I saw this.

telectrix · May 6, 2016

Leesparkykent said:
Are these circuits on the same RCD and if so are there other circuits on this RCD?

see where you're thinking. had similar where the excessive reading was caused by internal "resistance" of a RCD. one way to confirm would be to take readingss by this method on hi current test.

[ElectriciansForums.net] High earth loop impedance

diagram by courtesy of richard burns.

Nickj · May 6, 2016

Hello,

thanks for for the responses!

telectrix, if I do that test and confirm it is the rcds internal "resistance" then would it need to be replaced?

thanks

telectrix · May 6, 2016

not necessarily. in the event of a fault, due to the higher current generated, the RCD impedance would probably be negated. also, there's the get-out 1667 value where a RCD is installed on the circuit/s.

Nickj · May 6, 2016

Thank you.

So if by calculation I get an acceptable Zs (and also an acceptable one using the method described above) and the rcd trip times are < 40ms (which they are on the eicr) it would be acceptable to leave the rcd and installation as is (being safe)? putting those values of 40+ down as a quirk of rcd design

nick

Andy78 · May 6, 2016

What loop readings are returned on the other circuits that share the same RCD, namely the cooker and central heating ?

Nickj · May 6, 2016

Hi Andy,

i did did not measure those, the eicr has the cooker at 0.43 and the central heating at 1.44.

the eicr has the radial at 0.83, however I recorded the radial at 42 so unsure if the electrician doing the eicr did the radial, central heating and cooker by calc.

Leesparkykent · May 6, 2016

Nickj said:
Hi Andy,

i did did not measure those, the eicr has the cooker at 0.43 and the central heating at 1.44.

the eicr has the radial at 0.83, however I recorded the radial at 42 so unsure if the electrician doing the eicr did the radial, central heating and cooker by calc.

I would check the Zs values of the other circuits on the same RCD to confirm the thinking.

Andy78 · May 6, 2016

Nickj said:
Hi Andy,

i did did not measure those, the eicr has the cooker at 0.43 and the central heating at 1.44.

the eicr has the radial at 0.83, however I recorded the radial at 42 so unsure if the electrician doing the eicr did all but the radial, central heating and cooker by calc.

Those could be worth investigating to see if the high loop readings are consistent across all the circuits that are protected by the RCD. If not then a point of high resistance common to the first three circuits would seem logical. It's not unknown for breakers to be installed incorrectly on the busbar, or for multiple sections of busbar, or even cable links, be used to make up connections between breakers for instance.

Nickj · May 6, 2016

Thanks a lot for your help. I shall check it all this weekend and see if I can find the issue.

davesparks · May 6, 2016

Nickj said:
Hi davesparks low current no trip test. I had pretty much the same reading as the one noted on the eicr.

Cheers

Try a high current test using the alternative method to avoid the RCD and see how the result differs.

Nickj · May 8, 2016

Just a quick update to thank you all for your help.

I solved the problem today and it was caused by a very high Ze. The electrician that wrote the eicr stated he measured a Ze of 0.3. This was clearly made up because when I checked the cable was tarnished and the resistance of the cable alone was > 2 ohms! (I forget the actual reading).

I used some wire wool to clear the tarnish and re-ran the earth loop impedance on all circuits. All well within allowed values now.

Thanks again

Nick

Midwest · May 8, 2016

Think its a bit late for me, but I don't understand any of that.

High earth loop impedance

Nickj

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Sintra

davesparks

Nickj

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Nickj

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Leesparkykent

Nickj

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Andy78

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telectrix

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Nickj

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telectrix

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Nickj

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Andy78

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Nickj

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Leesparkykent

Andy78

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Nickj

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davesparks

Nickj

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Midwest

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