80A 100mA RCD in Domestic TT installation?

leascliffe · May 22, 2013

Hi,

I am starting out as a domestic installer and would really like some of your advice please??

I went to preview a job today to replace some pendant light fittings. The customer said the CU was replaced in 2010 together with a new earth rod being a TT supply. The overhead lines come into the distributor's fuse, tails out to the meter and just one pair of tails from the meter into an MK insulated C/U. The C/U consists of a 80A 100mA Time delayed RCD as main switch and 10 MCB's. There does not appear to be any other C/U at the property so I was wondering why the original installer went for a time delayed 100mA RCD?

I'm concerned there is no 30mA protection and believe the 100mA time delayed RCD should be swapped for a standard 30mA. Wondered if any of you have experienced the same and agree or disagree? Thanks!

Spudnik · May 23, 2013

I always thought that the CU had to be all insulated to avoid the 100ma rcd, as its only there to protect the tails.

driverman · May 23, 2013

In the past on TT installations.......light circuits with no cpc......100ma time delayed RCD's were used as a main switch / isolator.
But then usually 30ma RCD for skts etc giving discrimination.

Engineer54 · May 23, 2013

driverman said:
In the past on TT installations.......light circuits with no cpc......100ma time delayed RCD's were used as a main switch / isolator.
But then usually 30ma RCD for skts etc giving discrimination.

Not in 2010, TT installations have required overall 30mA protection for some years now, sockets on TT systems, have required 30mA protection even longer.

leascliffe · May 23, 2013

JASON said:
I always thought that the CU had to be all insulated to avoid the 100ma rcd, as its only there to protect the tails.

Seems the 100mA is normally used either upfront followed by 30mA RCD's in a C/U, or if it's in a distribution board powering another C/U with RCBO's elsewhere so that there is discrimination between them. All insulated on TT to avoid the live tails unlikely touching a metal case (before any protection from RCD's)

leascliffe · May 23, 2013

Sounds good, it seems the current installation is just wrong, even on the day it was installed in 2010...

..in reply to Engineer54 at #33

Octopus · May 23, 2013

leascliffe said:
Seems the 100mA is normally used either upfront followed by 30mA RCD's in a C/U, or if it's in a distribution board powering another C/U with RCBO's elsewhere so that there is discrimination between them. All insulated on TT to avoid the live tails unlikely touching a metal case (before any protection from RCD's)

Yes metal enclosure, not for a plastic one.

Engineer54 · May 23, 2013

JASON said:
I always thought that the CU had to be all insulated to avoid the 100ma rcd, as its only there to protect the tails.

I know what your saying, and agree in principle with your above comment. But how many follow the complete recommendations on TT based CU's, like all MCB's/RCBO's being of the DP variety, very few i'd wager...lol!!
Tails into metal-clad CU's used on a TT system, can be more than adequately protected entering the CU by use of a suitable gland or even a suitable grommet/blind grommet. Hell, the tails are insulated and sheathed to start with...

What an upfront 100/300mA S type will achieve on a TT system, is provide an extra/added level of protection to single RCD's covering multiple circuits. The alternative is totally relying on those single RCD's for your earth fault protection. And we all know that RCD's are not the most reliable of protective devices, be they from the high end manufactures, or from budget manufactures...