Socket Zs High

A

AdieB

-
I have a supply in a log cabin, in the cabin is 6 double sockets and a couple of lights. Cables are in PVC conduit and are 2.5mm singles sockets and 1.5mm lights.

Sockets

L-L = 0.15 ohms
N-N = 0.15 ohms
E-E = 0.18 ohms
L1 with E2 & L2 with E1 = 0.07 ohms

L to E = >1000+ MG Ohms
N to E = >1000+ MG Ohms
L to N = >1000+ MG Ohms

Ze at board is 0.28 ohms 843 Amps.

Tested at socket Uc Rl 20 Amp 60898 type B

Socket 1 1.62 ohms 50V 0.04 V
Socket 2 1.29 ohms 50V 0.03 V
Socket 3 1.65 ohms 50V 0.05 V
Socket 4 1.62 ohms 50V 0.04 V
Socket 5 1.52 ohms 50V 0.04 V
Socket 6 2.12 ohms 50V 0.04 V Zs value to high.

This socket is approximately 300mm through a wall away from another socket. Only difference is that it is a waterproof socket with a neon indication. I have tried inserting the plug in and out numerous times in case of bad contact. I have taken the reading from the rear of the socket across the terminals and still the same problem. I am trying to come up with a reason for this.

Could change the mcb for 16A, but still can’t find the reason why it should be different than the others.

Any ideas?

Adie
 
take a reading at the outgoing side of the MCB. the difference between the reading you get and the Ze is internal res. of MCB.
 
Could it be due to tarnished connections on the outside socket ?

Is the outside socket part of the ring, or a spur ?

The other readings in your post look to be correct.
Sockets

L-L = 0.15 ohms
N-N = 0.15 ohms
E-E = 0.18 ohms
L1 with E2 & L2 with E1 = 0.07 ohms
Click to expand...

Are you using the no trip setting (live test), or (R1+R2) (dead test) at the S/O's ?
 
i tend to disagree with the osg figures. why? because that 0.8 factor assumes that the cable will be loaded enough to run at it's rated operating temp. of 70deg. that situation, in a domestic environment is as rare as rocking horse poo.
 
AdieB said:
I have a supply in a log cabin, in the cabin is 6 double sockets and a couple of lights. Cables are in PVC conduit and are 2.5mm singles sockets and 1.5mm lights.

Sockets

L-L = 0.15 ohms
N-N = 0.15 ohms
E-E = 0.18 ohms
L1 with E2 & L2 with E1 = 0.07 ohms

L to E = >1000+ MG Ohms
N to E = >1000+ MG Ohms
L to N = >1000+ MG Ohms

Ze at board is 0.28 ohms 843 Amps.

Tested at socket Uc Rl 20 Amp 60898 type B

Socket 1 1.62 ohms 50V 0.04 V
Socket 2 1.29 ohms 50V 0.03 V
Socket 3 1.65 ohms 50V 0.05 V
Socket 4 1.62 ohms 50V 0.04 V
Socket 5 1.52 ohms 50V 0.04 V
Socket 6 2.12 ohms 50V 0.04 V Zs value to high.

This socket is approximately 300mm through a wall away from another socket. Only difference is that it is a waterproof socket with a neon indication. I have tried inserting the plug in and out numerous times in case of bad contact. I have taken the reading from the rear of the socket across the terminals and still the same problem. I am trying to come up with a reason for this.

Could change the mcb for 16A, but still can’t find the reason why it should be different than the others.

Any ideas?

Adie
Click to expand...

Are you getting this reading at each socket or is that just what you get at the connections at the DB??
 
Sorry tried to edit my last post but it did not appear.

The socket is part of the ring circuit. The cabling is exactly the same on all sockets. It is a TNC-s system, I pushed the plug of the tester in and out numerious times so hopefully no tarnished connections. I also took reading from rear of socket. The DB is a MCG fuseboard with a 2P6330 RCD and 2 x 60898 mcb’s 1x 20Amp 1 x 6Amp. What is the table or reg that says about rcd controlled socket can be 1667 ohms.
 
Am I missing something?

Shouldn't the reading be 0.35 (or even less with parallel paths) at all the sockets?


1666 ensures the RCD will disconnect before voltage rises above 50V.
It should not be used to ignore maximum Zs values - unless unavoidable (e.g. TT)
 
table 41.5, p50 in the red book. not sure where in bgb.
 
pushed the plug of the tester in and out numerious times so hopefully no tarnished connections
Click to expand...

I meant where the cables terminate in the back of the SO too, as your end to end readings were ok.

What is the table or reg that says about rcd controlled socket can be 1667 ohms.
Click to expand...

Table 41.5 (BGB) p57.

It actually says anything above 200 may be unstable wrt Earth rods.

Edit: Tel beat me to it, again! lol
 
Last edited:
the 1667Ω figure is for earth faults, and not for live-n faults. I would carry out a Zp-n, which means that you test between live and neutral, not live and earth. If your using a fluke, its just a case of swapping the leads over, but if its a megger, then youll have to use the probes. If this comes up ok, then youve identified that it is indeed a problem with the earth path, and not the live. If it comes up bad, then it means that you have problems wiuth either your live or neutral.
 
Geoffsd said:
Am I missing something?

Shouldn't the reading be 0.35 (or even less with parallel paths) at all the sockets?
Click to expand...
Totally agree, should be getting a Zs of around 0.35ohms at each socket outlet. Sounds like a loose connection at one of the sockets.....
 
yellowvanman said:
As its a socket should be on a RCD anyway, in which case Zs up to 1667ohms.
Click to expand...

Not Correct!

RCD is backup / supplementary protection to the main protection.

Will the RCd trip the circuit? when there is a shortcircuit? and the cable burns the house down?
because the CB did not trip, because the Zs was too high?:laugh3:
 
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This log cabin, does it have it's own metered supply? Or, is it fed by a distribution circuit from another building?

If the latter, then what is this Ze you quote? Ze at the supply or at the cabin board (ie Zdb)?

Looks to me like we're missing some info.

Anyway up, those Zs figures (assuming that's what they are, you don't state explicitly) are somewhat astray. Poor or varying Zs at sockets can be due to bad connections at the pins or dirt/corrosion on the switch contacts. Have you tried flicking the switches a few times to 'clear' the contacts?
 
Moses said:
Not Correct!

RCD is backup / supplementary protection to the main protection.

Will the RCd trip the circuit? when there is a shortcircuit? and the cable burns the house down?
because the CB did not trip, because the Zs was too high?:laugh3:
Click to expand...

I'm not disagreeing with the need to try and find out why the Zs is high but:

411.4.4 - An RCD can be used for fault protection with an overcurrent protective device also in place.

When does anybody measure the Z between neutral and line to ensure that an MCB will trip? The only measurement you make is Isc at the origin!
 
Moses said:
Not Correct!

RCD is backup / supplementary protection to the main protection.

Will the RCd trip the circuit? when there is a shortcircuit? and the cable burns the house down?
because the CB did not trip, because the Zs was too high?:laugh3:
Click to expand...
your not measuring line voltage or short circuit with a Zs Moses.....
 
and these `magic` RCDs should not be seen as a `cure all` for high loop......this does need investigating....
 
yellowvanman said:
I'm not disagreeing with the need to try and find out why the Zs is high but:

411.4.4 - An RCD can be used for fault protection with an overcurrent protective device also in place.

When does anybody measure the Z between neutral and line to ensure that an MCB will trip? The only measurement you make is Isc at the origin!
Click to expand...

Zs is always measured between CPC and L conductors.

This is because, in UK we use a reduced size CPC, so if the Zs is ok when measured with CPC-L fo a fault current, then its obviously OK between L-N for a short circuit current!!!
 
Moses said:
Zs is always measured between CPC and L conductors.

This is because, in UK we use a reduced size CPC, so if the Zs is ok when measured with CPC-L fo a fault current, then its obviously OK between L-N for a short circuit current!!!
Click to expand...
its always measured between CPC and L conductors because thats what Zs is....earth-fault-loop-impedence.....nothing to do with short circuit...
 
GLENNSPARK said:
your not measuring line voltage or short circuit with a Zs Moses.....
Click to expand...

Where have you bin man?

Zs is measured at the furthest point in the circuit and not at the CU, as some have suggested. Ze is measured at the CU.

Zs is measure of how much short circuit current will flow, when any circuit is short circuited at its furthest, and that this short circuit current will trip the CPD in the requisite time(0.4/5.0 sec)
 
Last edited by a moderator:
According to your readings that circuit warrants further investigation.

Wip the sockets off there is only 6!

your ring final doesnt really add up.
your Zs readings are way out.

where did you perform your ring final? - at the board or at a socket?

If you do ever do it at a socket make sure you disconnect earths from earth bar and put in a chock block same goes for neutrals


this prevents any parallel paths.
 
GLENNSPARK said:
its always measured between CPC and L conductors because thats what Zs is....earth-fault-loop-impedence.....nothing to do with short circuit...
Click to expand...

For simpletons, yes you r r8

But before the RCDs

CB's used to protect us against earth faults and short circuit faults. They did then and they do now even without the RCD's.

RCD's are there because of their sensitivity and speed.

As I said before, Zs is measured between L-E

1.because CPC is a smaller size conductor, i.e. R2 is larger than Rn, i.e. Zs measured with R2 is a higher value than measured with Rn, giving you the worst case Zs.

2.because it ensures the continuity of CPC.
 
Moses said:
Where have you bin man?

Zs is measured at the furthest point in the circuit and not at the CU, as some have suggested. Ze is measured at the CU.

Zs is measure of how much short circuit current will flow, when any circuit is short circuited at its furthest, and that this short circuit current will trip the CPD in the requisite time(0.4/5.0 sec)
Click to expand...

You'd better understand more clearly what Zs is!

Zs is measured between Line and earth, so if a fault to earth happens it ensures enough current will flow through the fault to trip an MCB.

For a short circuit to trip a MCB there needs to be an overload or L-N fault but the impedance to ensure the MCB trip in overload is never measured!
 
Zs is a measure of the earth fault loop impedance path, not short circuit impedance (L/N)

A measure of how much short circuit current will flow, when any circuit is short circuited (perhaps at its furthest point) is PSCC
 
Moses said:
For simpletons, yes you r r8

But before the RCDs

CB's used to protect us against earth faults and short circuit faults. They did then and they do now even without the RCD's.

RCD's are there because of their sensitivity and speed.

As I said before, Zs is measured between L-E

1.because CPC is a smaller size conductor, i.e. R2 is larger than Rn, i.e. Zs measured with R2 is a higher value than measured with Rn, giving you the worst case Zs.

2.because it ensures the continuity of CPC.
Click to expand...
well firstly in answer to your question where have i been....well...over st helens way today Moses......and simpleton?...i think youv got me confused with someone else fella....oh, by the way....when did i mention Ze? another thing n all.....you refer to the CPC being a smaller CSA than live conductors...is this always true Moses?...what about metal conduit with 2.5mm CSA single live conductors......
 
AQUOTE=yellowvanman;533036]You'd better understand more clearly what Zs is!

Zs is measured between Line and earth, so if a fault to earth happens it ensures enough current will flow through the fault to trip an MCB.

For a short circuit to trip a MCB there needs to be an overload or L-N fault but the impedance to ensure the MCB trip in overload is never measured![/QUOTE]

Vanman my friend

I think you must agree, that if 6A a breaker will trip with a 7A fault current(CPC-L), then it surely will trip with a 8A current short circuit(L-N).

As Zs between L-E has a higher value, than Zs between L-N, only Zs for L-E is measured.

CPC wire on a 6mm T&E is 2.5mm, and N wire is 6mm.
Therefore current through the L-CPC path( 6mm+2.5mm) will be less, than the Current through L-N path(6mm+6mm).

If there is no earth continuity then its potentially fatal, but if Neutral is missing or non continuous then it no big deal, it comes apparent in the functional test.
 
Last edited by a moderator:

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AdieB

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Socket Zs High
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Periodic Inspection Reporting & Certification
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