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.

 

[nickblake]

I have just seen a quote to install RCD's for a small guest house 3 phase QOE consumer unit , this quote was after one of the worst EICR's ive ever seen ,3 DP RCD's to be installed one on each phase , all I want to do is sit and watch what happens when they switch it on should be fun

 

[Engineer54]

Originally Posted by Engineer54
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.

That will very much depend if or not on any fault that is causing the Zs value to be way out of compliance...
I don't, ...that's because are they are a well tried and proved reliable device, which is unfortunately more than can be said for any RCD device out there!!!

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,

Maybe, but then again maybe not, which is why i stated, ...does anybody ever bother to check these day's.
Most non-complying circuit Zs values are easily fixable, but it seems rarely if ever are, when a cover-all RCD is in place!!

 

[GLENNSPARK]

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.

but on a properly designed circuit...it shouldn`t be......should it.
and thats the nut of it here...

 

[Silly Sausage]

but on a properly designed circuit...it shouldn`t be......should it.
and thats the nut of it here...

Glenn, See that brick wall over there, run headlong...

 

[Deleted member 9648]

but on a properly designed circuit...it shouldn`t be......should it.
and thats the nut of it here...

But circuits dont always stay as they were designed,they are altered/extended and change use.Most Zs issues arise only because the wrong OCPD has been selected because that was all that was available,or because it has been changed because of high inrush equipment being installed etc without checking Zs.
That said in the last few weeks I completed a EICR in a 5 year old school where every single sockets circuit was a 4.0/2.5mm radial on 32a/C rcbo's.Nearly all the Zs readings on those circuits came out at 0.9-1.2 ohms,poor design indeed. But while a note was included to that effect no code was applicable because it meets the required disconnection time and therefore complies.

 

[GLENNSPARK]

But circuits dont always stay as they were designed,they are altered/extended and change use.Most Zs issues arise only because the wrong OCPD has been selected because that was all that was available,or because it has been changed because of high inrush equipment being installed etc without checking Zs.
That said in the last few weeks I completed a EICR in a 5 year old school where every single sockets circuit was a 4.0/2.5mm radial on 32a/C rcbo's.Nearly all the Zs readings on those circuits came out at 0.9-1.2 ohms,poor design indeed. But while a note was included to that effect no code was applicable because it meets the required disconnection time and therefore complies.

had it myself on an ECR at a local football ground..
all c curve....all high.

and the groundsmans hut had socket outlets with no additional protection as well..