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

[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.

 

[Pretty Mouth]

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

Haha, those are non compliances, I'm not that much of a cowboy. I appreciate that you old timers may not believe in RCDs, but I do. As we all agree, you'd never get anything close to 1667Ω in any healthy circuit meeting voltage drop demands etc, so disconnection times are met by default

 

[Toneyz]

The problem with RCBO's & RCD's is that they are not regularly tested and the mechanism sticks.

 

[Michael J]

That's because they're ripping these pipes out and using PVC instead. So they can no longer be relied on.

Me, I've only ever installed rods in quarries, and I had the blast shot drilling rig to bore me some nice 30m deep holes. A 30m rod, back filled with bentonite in a saturated limestone table. Sub 1 ohm was real easy to achieve. Especially for new substations where 4 to 6 of these rods were connected around the perimeter.

Not exactly feasible at someones terraced house.

Went through that experience on De Beers,Venetia Diamond Mine in South Africa.
The ground conditions were solid rock and climate was dry (lucky to see rain once a year)
From what I can remember it was very challenging and very expensive.
There again De Beers could afford it

 

[Pretty Mouth]

The problem with RCBO's & RCD's is that they are not regularly tested and the mechanism sticks.

Post #14 old chap.

Some questions for all you RCD-phobic types:

If you are so concerned about RCDs failing (which I grant you they do), what are you doing about cables installed <50mm deep? Are you using pyro or SWA or similar, or running them in earthed metallic containment? Or are you chasing out deeper than 50mm perhaps?

Same for steel stud walls?

Are you sup bonding everything in bathrooms?

How are you testing your MCBs to ensure that they are working as they should be?