The maximum value for Zs on a TT system is 50/I delta. So for a 30mA RCd is 1667 ohms

[telectrix]

on a TN installation, the Zs values should be compliant with OCPD disconnection times, if you've not made a cock-up of the design. using RCDs as a "get out of jail" card is, IMO, sloppy,unprofessional, and just lazy. leave that shyte to the septics with there GFD crap .

 

[Pretty Mouth]

Telectrix, what you've just said directly contradicts the reg I quoted above. Can you back it up with another regulation please?

Bear in mind disconnection times only apply to earth faults, not short circuits

 

[telectrix]

Telectrix, what you've just said directly contradicts the reg I quoted above. Can you back it up with another regulation please?

Bear in mind disconnection times only apply to earth faults, not short circuits

a RCD should only be used for fault protection where the Zs values exceed those in tables for each OCPD. e.g. in a TT system where the Ze is too high to achieve compliant figures. RCDs that are not regularly tested have been known to stick.' sometimes an unnoticed N-E fault will prevent the RCD from tripping. i would not rely on one for fault protection unless absolutely necessary.

 

[Pretty Mouth]

That's true, RCDs can stick, however, they can and should be tested, and replaced if faulty. The same cannot be said for MCBs - we only have blind faith that they will do their duty when the times comes. I understand that MCBs are manufactured to be more reliable, but you see my point?

The IET are fine with us using RCDs as fault protection - the max length tables for standard circuits in the OSG have columns of longer values specifically for RCD protected circuits on TN systems

 

[telectrix]

maybe old school, but been brought up before RDCs were even invented. back in the 70's , local DNOs were converting to PME so as to ensure disconnections as used to be called eebads, now ads. before then a fault could take a fortnight to diss supply. not good if you got a live -earth fault.

 

[Pretty Mouth]

In answer to the OP, it seems a max Zs of 1667 for a 30mA RCD does in fact apply to TN systems, see reg 411.4.204

 

[Wilko]

Hi - my pre coffee thoughts are :
If TN circuit had anywhere near 1667 Ohms earth loop then either it’s way way way too long (so reg fail on volt drop) or needs Further Investigation for high resistance in a circuit conductor (so EICR fail).

 

[static zap]

In answer to the OP, it seems a max Zs of 1667 for a 30mA RCD does in fact apply to TN systems, see reg 411.4.204

Surely the REF should RED card this at any higher than 1/8 of 1667 Ohms on stability grounds for TT .
(I know not TN , but 1k6-Ohms is more theory-than practice)

 

[Pretty Mouth]

Agreed, it would imply that something is seriously wrong somewhere. However, it does mean, all other things being correct, that lazy types like me can ignore max disconnection times calculations

 

[telectrix]

Agreed, it would imply that something is seriously wrong somewhere. However, it does mean, all other things being correct, that lazy types like me can ignore max disconnection times calculations

slippery slope. do you then ignore the lack of cpc's to metal fittings, sockets close to a bath, andonandonandon.