Zs on a ring question

[Deleted member 9648]

In other words anything on the ring will have (or should have) identical r1r2 readings whereas a spur will have the ring r1r2 reading plus the r1r2 for the legnth of cable on the spur.

 

[pevvers]

In other words anything on the ring will have (or should have) identical r1r2 readings whereas a spur will have the ring r1r2 reading plus the r1r2 for the legnth of cable on the spur.

Indeedy!

 

[WayneL]

If the circuit is prorected by a 30ma rcd then the max allowed zs is 1667 ohms....so you are well under with 1.21.

edit....note that the 1667 ohms maximum zs applies to all circuits/mcb ratings where they are protected by a 30ma rcd....saves looking up the max zs readings for each protective device when filling in the schedule .

What a load of *£L@+#~^&

Might as well throw away half the regs then - be easier to carry around, any rate.

30mA RCD s are for Additional protection against electric shock, not fault protection!

You should still try for a Zs within the max limits of the MCB given in the Regs.

In extreme circumstances, with good reason, you can use an RCD for fault protection on a TN system, but not just when you feel like - coz you can't be bothered looking up the maximums!!
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Also, are you telling me that you put 1667 ohms in the 'Max Zs allowed' column for each circuit on the schedule of test results - done your 2391 have you?

 

[Deleted member 9648]

QUOTE 'Also, are you telling me that you put 1667 ohms in the 'Max Zs allowed' column for each circuit on the schedule of test results - done your 2391 have you?' QUOTE........................................................................................................................................................................................................................ Did the 17th edition update course and got 100% in all sections bar one where I slipped up and got 80%. Yes I do put 1667 ohms in the max allowed Zs column on the schedule of circuits BECAUSE THAT IS THE MAXIMUM ALLOWED Zs WHERE A30ma RCD IS PROTECTING THE CIRCUIT. Of course on a TN system the expected Zs would be infinately lower than this,and a high reading would warrent investigation....but it remains a fact that 1667 ohms is the maximum allowed value and that is what should go on the schedule

See regulation 411.4.9 and table 41.5 on page 50 of the BRB

2391?...Maybe you should have paid more attention when you did yours.

 

[Spudnik]

Easy now fellas.

This is a very grey area of the regs and is open to many different interpretations.

 

[WayneL]

If you are correct, then why don't we just put a 200mA RCD main switch up front on every install -

We wouldn't have to learn about earthing systems .........it would be irrelevant

We wouldn't have to worry about disconnection times, Zs, even Ze and R1 + R2 wouldn't matter.

The Regs would be about 30 pages long - happy days.

And we'd only have to test R2 continuity, IR and polarity.......oh yes, and the functional operation of our RCD!

Wonder why the IET haven't thoughht of this???

You are correct in what you are saying, though, Wirepuller, it is less of an issue when there is RCD protection - you've just got it the wrong way around:

If you have trouble meeting the required Zs value for a circuit, you can use an RCD for earth fault protection

Not:

If there is RCD protection, you don't have to worry about meeting the required Zs value

 

[Deleted member 9648]

Ok, take on board what you say up to a point. Of course you would not expect a zs reading anywhere near the max value on a TN system. Problems with a high zs would normally become apparent before the zs is even tested...poor r1r2,or high ze,which would set alarm bell ringing. But If I do a PIR on an existing circuit,which probably has been spurred off and extended to the point where the measured Zs slightly exceeds the max value for the protective device,as long as other readings are acceptable and there is 30ma rcd protection it passes because the zs reading becomes irrelevent within reason.

Regarding the points you made.
A 200ma rcd up front on all installations would not comply with the requirement for other circuits not being affected in the event of a fault on one circuit.

I dont see how R1R2,Zs,Ze and disconnection times become irrelevent....all need to be measured whether or not an Rcd is used to provide earth fault protection.

Whether or not an RCD is used for earth fault protection a reliable means of earthing has to be in place,the use of an RCD doesnt change that.

 

[WayneL]

Ok, take on board what you say up to a point. Of course you would not expect a zs reading anywhere near the max value on a TN system. Problems with a high zs would normally become apparent before the zs is even tested...poor r1r2,or high ze,which would set alarm bell ringing. But If I do a PIR on an existing circuit,which probably has been spurred off and extended to the point where the measured Zs slightly exceeds the max value for the protective device,as long as other readings are acceptable and there is 30ma rcd protection it passes because the zs reading becomes irrelevent within reason.
This is a reasonable point

Regarding the points you made.
A 200ma rcd up front on all installations would not comply with the requirement for other circuits not being affected in the event of a fault on one circuit.
This happens with TT systems and doesn't seem to be an issue - as does one 80A cutout protecting the whole system

I dont see how R1R2,Zs,Ze and disconnection times become irrelevent....all need to be measured whether or not an Rcd is used to provide earth fault protection.
The readings become irrelevant when you have 250 to 1667 ohms to play with (depending on RCD rating), because as you stated, TN readings aren't gonna get anywhere near that. Also, to disconnect in 0.4 sec you could even have more play than that.

Whether or not an RCD is used for earth fault protection a reliable means of earthing has to be in place,the use of an RCD doesnt change that.
This is true - so all you have to do is confirm the earth path - readings wouldn't matter again.

I think that we can agree that you just have to apply a bit of common sense where RCDs are concerned

 

[Deleted member 9648]

Fair enough

 

[james101080]

I know it was a while ago you replied to this thread but i just had a question as to where 1667 ohms maximun Zs reading was in the regs book, have had an issue recently where im over the 1.16 ohms but am still protected by a 30mA rcd! Any advice will be appreciated!

 

[andyemms]

I think you will find this on Page 50 of the regs, table 41.5.

 

[andyemms]

also read 411.4.9 page 50.

 

[poheZ]

i wouldnt bother measuring a zs, calculate it from your r1 r2 figure as an accurate zs measurement is not always possible.

 

[andyemms]

I would of thought getting an accurate measurement on a ring main would be fairly easy and certainly better than calculating r1+r2+Ze.