Zs what if

[the pict]

If Zs exceeds the table value for a circuit what situtation exists, disconnection times cannot be met I know, but why, the mind says surely it would have trouble staying closed in normal service IE if a Zs value is to high for a 32A but ok for a 20A but if the connected load is 25A the 20 will break, what am I missing here, I mean if Zs is high why is it not tripping constantly
Pict

 

[malcolmsanford]

Are you on medication Pict, if your not perhaps you might think about it...................why would an high Zs trip a healthy circuit??

 

[PB1]

The higher the Zs, the lower the fault current that can flow around the fault loop. So if Zs is too high the fault current won't be large enough to trip the breaker.

 

[telectrix]

the fault current will be high enough to trip ther breaker, but not within the specified time ( 0.4secs in most cases )

 

[the pict]

When I re-read my post I could not understand it either, and the thought I had this morning is evaporating I was thinking about a job where the Zs is off the table despite trying to get it down, any way it'll come back to me later
Pict

 

[Deleted member 9648]

You can work out the earth fault current using simple ohms law....V/R=I
So a Zs of 1ohm on a 230v circuit will result in a EFC of 230a
Then look at App 3,which will tell you how long OCPD's will take to operate at the fault current you have calculated.
It is really the earth fault current that we are interested in,measuring Zs is simply a means of finding out what that earth fault current will be.

 

[Dan the spark]

Is this job of yours on a domestic install or a commercial/industrial?

 

[malcolmsanford]

Dan would it matter??

 

[Dan the spark]

Not really, but was thinking that if Zs is off the chart and it was a domestic install then this would be covered by an rcd and the max Zs allowed would be 1667ohms

 

[malcolmsanford]

Think old Pict was suffering from a Saturday night to be honest, as he came back on later and said his post didn't make sense to him when read back, most unlike old Pict he is normally spot on, think it was a blonde moment

 

[Dan the spark]

Haha, we all suffer from those time to time!

 

[the pict]

Right where I was going with this was, suppose a 4mm radial has been extended with 2.5 on a old 30a fed elect OPD and the efc at the original radial end is ok but at he extended end it the efc is lower much lower, then the table for a 32A (board change involved) the Zs drops of the scale, I know why it will not trip and I cannot for the life of me think how i thought it should with a low EFC, beer probably who knows, now for the time being I have whacked a 20 on the circuit but its a big radial 21 sockets on it 3 hotel rooms I know the logical thing to do is rip out the 2.5 or ring it back to the board but would it make any differance if I went with a32 as it,s now RCd'd and a shed load of bonding was done to ECP,s

thoughts anyone
Pict

 

[telectrix]

if the 20A OCPD holds, and the Zs is within limits for that device,leave it at that.

 

[tony mc]

A little confused at what your asking but.

If you have extended a 4mm radial in 2.5 cable your ZS is going to be higher than at the original socket due to reducing the csa and extending the lenth of the cable.

If your EFLI test is now too high for the OCPD to disconnect in the required time 0.4 secs (TN) system then you need to do something about it. You say a RCD is fitted on the circuit so you max ZS could be 1667 ohms but I would be dead testing and making sure the R1 + R2 was acceptable for the cct in question making sure there are no faults or high resistive joints etc!

 

[oldtimer]

Right where I was going with this was, suppose a 4mm radial has been extended with 2.5 on a old 30a fed elect OPD and the efc at the original radial end is ok but at he extended end it the efc is lower much lower, then the table for a 32A (board change involved) the Zs drops of the scale, I know why it will not trip and I cannot for the life of me think how i thought it should with a low EFC, beer probably who knows, now for the time being I have whacked a 20 on the circuit but its a big radial 21 sockets on it 3 hotel rooms I know the logical thing to do is rip out the 2.5 or ring it back to the board but would it make any differance if I went with a32 as it,s now RCd'd and a shed load of bonding was done to ECP,s

thoughts anyone
Pict

Why dont you improve the circuit so it does comply is that not what this is all about I was called to a job with a high Zs turned out there was a poor main earth contacted DNO who came 2 months later (because there was a RCD) and put a good earth in now Zs complies

 

[wildgoose]

To clarify this is how I understand it:-

If the Zs meets disconnection period spec, then it will trip the CB within the alloted time period when a Live to Earth fault occurs (eg: 0.4s). However, if it can't meet it, then there must be additional protection from an RCD, to eliminate the risk of the earth becoming live because of a high ZE. This is the basic TT protection mode.

For normal ADS without RCD protection, the disconnection time is of course mandatory.

 

[Engineer54]

There's a big difference between a TN and a TT system!! The use of an RCD on a TN system is for ''additional protection'', It' shouldn't always be considered a cure all, for covering a wiring circuit that doesn't comply with required Zs when it can quite easily be made to comply. Whereas, on a TT system, especially in UK, it is the prime and only protection for earth faults

The use of RCD devices for cure all situations seems to be, becoming the norm in the UK. Get the fundamentals right first, before applying RCD additional protection