Exceeding Max Zs

[Donkey_Dave]

Hello, first time poster, long time viewer. Testing a school which on the socket circuits have BS 3871 type 3 breakers with a 30ma RCD attached. Some of the socket circuits exceed the 0.72 ohms max zs for the breaker. Should I be using the max zs as 1667ohms? I seem to remember that if max zs cannot be achieved then an RCD could be used, or have I got that wrong? Yes I am an electrician.

 

[Guest111]

And quite a few people I've noticed don't seem to understand that RCD is 'earth fault' and nothing to do with overload or short circuit !

some people think RCD's are the answer to everything,Saw it in the paper the other day "RCDto solve syria crisis and end world poverty" oh no wait that's gonna be in next week lol:smiley2:

 

[silverfox1]

This Q&A from ESC website

Q4.25
During a periodic inspection, a circuit having RCD protection is encountered and the design information regarding the provision of fault protection is not available. Which value of maximum permitted Zs applies – the value for the overcurrent device (circuit-breaker or fuse) or the value for the RCD (or the RCD element of an RCBO)?
Either value can be used. However, it is essential to confirm that the measured value of Z[SUB]s[/SUB] is not indicative of a defect, such as a loose connection or an inadequate means of earthing.
Regulation number(s)

• 411.4.4
• 411.4.5
• 411.4.9

 

[Engineer54]

This Q&A from ESC website

Q4.25
During a periodic inspection, a circuit having RCD protection is encountered and the design information regarding the provision of fault protection is not available. Which value of maximum permitted Zs applies – the value for the overcurrent device (circuit-breaker or fuse) or the value for the RCD (or the RCD element of an RCBO)?
Either value can be used. However, it is essential to confirm that the measured value of Z[SUB]s[/SUB] is not indicative of a defect, such as a loose connection or an inadequate means of earthing.
Regulation number(s)

• 411.4.4
• 411.4.5
• 411.4.9

What does this question prove then??

Depending on the test Zs value it will be clear that there is a potential problem within that circuit. Are you still going to say ...all's fine because i have a cover-all RCD in circuit, that is basically there to afford additional rather than the ONLY or SOLE means of earth fault protection on TN systems!!


Does nobody ever check these day's, as to WHY a circuits ADS Zs value no longer complies?? As RCD protection can never be the sole means of protection on a newly installed TN circuit, the offending circuit must have complied at one time... Or are we now actually installing circuits these day's, that rely on a RCD's Zs protection from the word Go??

 

[Dizzy_Maskell]

^^^^^^
It's weird he can remember all his regs details, standards codes of practice etc lads but ask the guy to remember the
T-bags when it's his turn and he fails to deliver.
Just a reminder any chance of a brew tomorrow Mr Trim

Lol I've brought them today

Yes no biscuits though

 

[silverfox1]

What does this question prove then??

Depending on the test Zs value it will be clear that there is a potential problem within that circuit. Are you still going to say ...all's fine because i have a cover-all RCD in circuit, that is basically there to afford additional rather than the ONLY or SOLE means of earth fault protection on TN systems!!


Does nobody ever check these day's, as to WHY a circuits ADS Zs value no longer complies?? As RCD protection can never be the sole means of protection on a newly installed TN circuit, the offending circuit must have complied at one time... Or are we now actually installing circuits these day's, that rely on a RCD's Zs protection from the word Go??

Depending on the test Zs value it will be clear that there is a potential problem within that circuit.

The answer says

Either value can be used. However, it is essential to confirm that the measured value of Z[SUB]s[/SUB] is not indicative of a defect, such as a loose connection or an inadequate means of earthing.

RCD protection can never be the sole means of protection on a newly installed TN circuit

The question and answer refers to a PIR now called EICR not a new install.


The Reg numbers are there its up to the individual electrician to interpret them as he will.

Totally agree should not be used as a get out for bad design on a new install

 

[Rampantchilli]

I was told during my studying for 2391, that if max Zs is found to be higher than the max permissible Zs, it is possible to cure in a number of ways, one of which was use of an RCD, the others include additional earthing, change of rating of MCB, change of type of MCB.

 

[Donkey_Dave]

Yes, that was at the back of my mind when I tested the circuits. As has been said its not ideal, but away of getting over the problem as long other criteria has been met. I will give the option to change the breakers. Thanks.

 

[Edd]

Find the Fault as to why Zs is so high, thats what should be done, if not then add earthing or change MCB,,, last resort fit RCD but this really should be a last resort.
Remember this install should have had a respectable Zs when the install was done! if it's now not satisfactory why not get paid to find out why.
If the install had an unsatisfactory Zs at time of install and was left like it then the chances are the rest of the install may have problems..

 

[C&H]

quite often find high zs on C and D rated circuits, not necessarily a fault present on the circuit, just not designed properly. sounds like the OP needs to get himself a B rated breaker and problem solved. 90% of large EICR's i do have high Zs on distribution circuits, it can be hard to meet the max value of a 630a mccb sometimes lol

 

[Paultrim123321]

People don't realise when a high Zs is measured there is a potential this is due to poor design which will result in a reduced PSC because of increased resistance in conductors for example due to length of run. This will effect the tripping time of the protective device in the event of a short circuit fault and therefore the RCD will make no difference.

 

[44ALLAN]

People don't realise when a high Zs is measured there is a potential this is due to poor design which will result in a reduced PSC because of increased resistance in conductors for example due to length of run. This will effect the tripping time of the protective device in the event of a short circuit fault and therefore the RCD will make no difference.

Zs has nowt to do with short cct fault though

 

[Paultrim123321]

I know that Al but not sure some people do!

 

[44ALLAN]

I know that Al but not sure some people do!

Read your previous post Paul and do a bit of editing then

 

[Deleted member 9648]

The point is with this is that it is an EICR....as long as the circuit complies with earth fault disconnection times via either the OCPD or the RCD...and there are no other issues causing the Zs to be higher than the max for the OCPD,no code is applied.
An EICR is simply verification of compliance with 7671...not a take on the original design.

 

[Deleted member 9648]

What does this question prove then??

Depending on the test Zs value it will be clear that there is a potential problem within that circuit.The only problem here appears to be in the initial selection of the OCPD,and as it is a 3871 device chances are it is an old install and complied at the time. Are you still going to say ...all's fine because i have a cover-all RCD in circuit, that is basically there to afford additional rather than the ONLY or SOLE means of earth fault protection on TN systems!! It isnt the only means of earth fault protection,the mcb will still trip but not in the required 0.4s,which the RCD will take care of......on many circuits an MCB is the ONLY or SOLE means of earth fault protection,never hear you knocking that.


Does nobody ever check these day's, as to WHY a circuits ADS Zs value no longer complies?? As RCD protection can never be the sole means of protection on a newly installed TN circuit, the offending circuit must have complied at one time... Or are we now actually installing circuits these day's, that rely on a RCD's Zs protection from the word Go??It's an EICR on an older installation....likely there is nothing wrong with the circuit,just the initial choice of OCPD,which I say again,likely complied at the time

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