Socket Zs High

[Jonny66]

Am I missing something?

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

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.....

 

[Moses]

As its a socket should be on a RCD anyway, in which case Zs up to 1667ohms.

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:

 

[Moses]

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.....

Zs = (r1+r2)/4

 

[Jonny66]

Zs = (r1+r2)/4

Does it ??????? Zs = Ze + (R1 + R2)..........in this case Zs = 0.28 + 0.07 = 0.35ohms

 

[PC Electrics]

Zs = (r1+r2)/4

My turn to correct you, Moses:

R1+R2 = (r1+r2)/4
and
Zs = Ze + (R1+R2)

so
Zs=((r1+r2)/4)+Ze

 

[Moses]

Does it ??????? Zs = Ze + (R1 + R2)..........in this case Zs = 0.28 + 0.07 = 0.35ohms

My mistake!

R1+R2 = (r1+r2)/4

 

[PC Electrics]

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?

 

[yellowvanman]

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:

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!

 

[GLENNSPARK]

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:

your not measuring line voltage or short circuit with a Zs Moses.....

 

[GLENNSPARK]

and these `magic` RCDs should not be seen as a `cure all` for high loop......this does need investigating....

 

[Moses]

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!

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!!!

 

[GLENNSPARK]

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!!!

its always measured between CPC and L conductors because thats what Zs is....earth-fault-loop-impedence.....nothing to do with short circuit...

 

[Moses]

your not measuring line voltage or short circuit with a Zs Moses.....

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)

 

[StuSpiers]

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.

 

[Moses]

its always measured between CPC and L conductors because thats what Zs is....earth-fault-loop-impedence.....nothing to do with short circuit...

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.

 

[yellowvanman]

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)

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!

 

[Jonny66]

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

 

[GLENNSPARK]

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.

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......

 

[GLENNSPARK]

incredible...

 

[Moses]

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.