Socket Zs High

[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.

 

[yellowvanman]

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 *a 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.

My names not Glenn!

OK, I see where your coming from but having a good Zs does not guarantee that the L-N impedance is lower - there could be a high resistance connection somewhere on thev neutral!

 

[Moses]

My names not Glenn!

OK, I see where your coming from but having a good Zs does not guarantee that the L-N impedance is lower - there could be a high resistance connection somewhere on thev neutral!

Thank god we r on the same page

I agree with your comment of a Hi z on the N wire.

But that's another reason why Zs is measured between L-CPC, it ensures the earth fault durations are limited.

 

[Geoffsd]

Definitions - part 2 bgb.

 

[Jonny66]

But in this instance the L/N impedance is not the problem (N end to end is ok) its the Zs values which are way off.

 

[Geoffsd]

But that's another reason why Zs is measured between L-CPC, it ensures the earth fault durations are limited.

But that's what Zs IS ​- earth fault cuurrent

The same test between L & N is called the prospective short circuit current.

The greater of these two is the prospective fault current

 

[lister]

"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)."

Will it?????
ever???
in 0.4s ??

 

[Moses]

Check the cable terminals are nice and tight at both ends going to the suspect socket.

If cable long enough, re-strip and make new terminations.

Now measure Zs at both ends of the cable.

If the cable is only 300mm, replace cable and there should be un-measurable difference in Zs at both ends.

 

[JUD]

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)

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.....

Not true!!!

R1+R2 should be the same at each socket outlet. Zs will increase towards the middle of the ring and decrease back towards the DB therefore you will get a reading of around 0.35Ω near the middle of the ring.

 

[GLENNSPARK]

Thank god we r on the same page

I agree with your comment of a Hi z on the N wire.

But that's another reason why Zs is measured between L-CPC, it ensures the earth fault durations are limited.

high Z...well that`s changed then....a minure ago it was Zs....

 

[Jonny66]

Not true!!!

R1+R2 should be the same at each socket outlet. Zs will increase towards the middle of the ring and decrease back towards the DB therefore you will get a reading of around 0.35Ω near the middle of the ring.

Agreed to a point (Hence "around" 0.35ohms)
I believe the ring in question is fairly small and the figures the op states seem way off by 1 and a bit ohms....seems alot to me, especially as he states these sockets are all on the ring final and not spurs.

 

[Moses]

"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)."

Will it?????
ever???
in 0.4s ??

Geoff, my friend, Zs is the impedance of the circuit, and not a current.

Lister, my friend, a 6A type B CB will trip in 0.4Sec with a minimum of 30A current. 7A & 8A were given as examples.

There fore 230V/30A = 7.67 Ohms is the theoretical max Zs for a 6A type B CB.

 

[Moses]

Originally Posted by Moses
Thank god we r on the same page

I agree with your comment of a Hi z on the N wire.

But that's another reason why Zs is measured between L-CPC, it ensures the earth fault durations are limited.



high Z...well that`s changed then....a minure ago it was Zs....


Yes but no, but yes but no, but yes but no Glenn

That Hi Z on the N wire is a fault, faulty connection.!!!!!!!!

 

[Moses]

Agreed to a point (Hence "around" 0.35ohms)
I believe the ring in question is fairly small and the figures the op states seem way off by 1 and a bit ohms....seems alot to me, especially as he states these sockets are all on the ring final and not spurs.

Sipmles he has not zeeored

or

he has a low battery

or

He has a faulty meter

 

[Jonny66]

Points to something along those lines agreed....where is the OP????

 

[Moses]

Points to something along those lines agreed....where is the OP????

Well confused by now i bet!!!!!:laugh3::laugh3::laugh3:

 

[GLENNSPARK]

Sipmles he has not zeeored

or

he has a low battery

or

He has a faulty meter

so if he hasn`t `zeeored`...or the batterys are low....hmm, so its a faulty meter then is it Moses?....possibly...but rather than just condemn the meter..would you not check the leads first?....

 

[Guitarist]

I had to send my Megger 1730 back a couple of weeks ago because the EFLI test at a socket (using plug adaptor OR on the terminals) was way higher than I was expecting. As has been said, it can be the internal resistance of the MCB/RCD so that would have to be deducted.
All that said, once I got my new 1730 and did the same tests, they were very close to Ze+(R1+R2), so it was definitely a faulty tester in my case.

Edit... Forgot to add. I have to do a 3-wire no-trip test, as the 2-wire is always at least 0.5 ohms higher!

 

[GLENNSPARK]

but my point was you dont just condemn the meter....its only as good as the test leads you are using....

 

[Guitarist]

but my point was you dont just condemn the meter....its only as good as the test leads you are using....

Totally agree mate.

 

[Geoffsd]

Sorry for delay - something crashed.

Geoff, my friend, Zs is the impedance of the circuit, and not a current.

Yes, I'm sorry. I should have written -

But that's what Zs IS ​- a measured value to determine prospective earth fault cuurrent

The same test between L & N is to determine the prospective short circuit current.

The greater of these two is the prospective fault current.