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.

 

[Engineer54]

One has to ask, why are these socket circuits being protected by a Type 3 MCB's??

I'm the wrong person to ask, as far as making use of 30mA RCD to circumvent a fixable circuit Zs failure. I'm definitely in the ''only as a last resort'' camp!! ..lol!!

 

[Donkey_Dave]

It's one of those don't know why, but they are type jobs. Who knows what they were thinking in the 70's! I have five circuits that exceed the max Zs, all on the socket circuits and all have the additional protection.

 

[Edd]

The 1667 Ohms should only be used on TT systems. Not on PME or TNS As in these cases the RCD is used as additional protection

 

[nickblake]

I Don't like the Idea of RCD's in use for high Zs readings ,and RCD is there for additional protection , there is way to much reliance on RCD's, if the circuit Zs is within that laid out by the regs then the disconnection time should be met ,without the use of the RCD, what im really getting at is if the RCD fails and the Zs is high then the circuit my well not disconnect but within the max Zs then the required time should be met , see what I mean

 

[Paultrim123321]

Without telling us the Zs you recorded its hard to give an accurate reply. The RCD could have been selected due to the environment the circuits are in, for example chemistry lab with wet areas and lots of portable equipment with long trailing leads.
I personally would consider installing a type 2 or 'B' type as this is what is recommend for socket circuits unless demand requires otherwise for example heavy inductive loads etc creating high inrush currents.
Using RCDs to 'beat' the system when high Zs are found is all to common, especially without proper further investigation to determine the causes of the original high Zs.
Find out length of run etc, see if you can select 'B' type MCB.

 

[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

 

[yellowvanman]

411.4.9

Whilst some don't like it, its valid to use an RCD if the Zs for the overcurrent device is too high. Although you have to make sure the impedance for a L-N fault will trip the overcurrent protection!

 

[Dizzy_Maskell]

411.4.9

Whilst some don't like it, its valid to use an RCD if the Zs for the overcurrent device is too high. Although you have to make sure the impedance for a L-N fault will trip the overcurrent protection!

Good man

 

[44ALLAN]

411.4.9

Whilst some don't like it, its valid to use an RCD if the Zs for the overcurrent device is too high. Although you have to make sure the impedance for a L-N fault will trip the overcurrent protection!

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 !

 

[Engineer54]

Originally Posted by yellowvanman
411.4.9

Whilst some don't like it, its valid to use an RCD if the Zs for the overcurrent device is too high. Although you have to make sure the impedance for a L-N fault will trip the overcurrent protection!

Good man

Wow!! These sort of comments really makes one wonder, what's going on out there??

 

[GLENNSPARK]

Wow!! These sort of comments really makes one wonder, what's going on out there??​

yep...​

 

[GLENNSPARK]

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 !

yep....
heard it in here a few times....as well as elseware n all...

total lack of understanding...

 

[Donkey_Dave]

Points taken, thank you for the replies. The Zs recorded were above the max zs ,1.01 being the greatest reading of one of the circuits. I have flagged it in the report and will try and source the B type breakers. I was just curious as to whether the additional RCD protection would be acceptable in this case. My gut instinct was that it was wrong, but in a TT system you rely on the RCD where the disconnection times cannot be met. Thanks again.

 

[Paultrim123321]

^^^^^^
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