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

[Michael J]

why is this not the case if the final circuits on a TN-S or TN-C-S system are protected by a RCD?
Funny what you think of when you can’t sleep

 

[telectrix]

because with TN earthing, the Zs should be < Zs(max.) for the OCPD, whereas with TT this is not usually possible, so you are relyingon the RCD for fault protection rather than the OCPD.

 

[Bobster]

RCD's are additional protection.

 

[telectrix]

RCD's are additional protection.

agreed, but with a TT system, due to the higher value of Ze, we rely on the RCD for fault protection as well as.

 

[Bobster]

agreed, but with a TT system, due to the higher value of Ze, we rely on the RCD for fault protection as well as.

We shouldn't really, but it's uneconomical an impractical at a domestic level not to rely on the RCD.

 

[telectrix]

strange that before the advent of RCDs, when the main earth was from the incoming lead water supply, we had Ze values way below what was achievable with a rod. now this is banned.?????

 

[Ian1981]

Yes but they installed a voelcb for TT installations before rcd’s but I’ve never witnessed a low enough Ze off a water pipe personally.
Maybe because they were all changed to plastic at some point we can’t rely upon water pipes unless a private supply and care is taken against its alteration and removal

 

[Bobster]

strange that before the advent of RCDs, when the main earth was from the incoming lead water supply, we had Ze values way below what was achievable with a rod. now this is banned.?????

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.

 

[Ian1981]

RCD's are additional protection.

Only upto 30mA.
There are other reasons to install rcds such as fire and fault protection

 

[Pretty Mouth]

Morning all. Have I misunderstood this thread? RCDs are fine for fault protection:

411.4 TN system
411.4.5 The following types of protective device may be used for fault protection:
(i) An overcurrent protective device
(ii) An RCD.