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.

 

[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.