AMD 3 DB on a TT system

[chris day]

Hello Guys

being a Londoner i have not had a great deal of experience with TT systems in a domestic setting.

The existing installation is in the basement, the existing earth rod is by the DB and sticking out of the ground, not inspection pit is that quite bad?
I was thinking of changing the rod and installing a pit, how long should the rod normally be to make sure i get a good reading.. on average?

all the final circuits will be 30ma RCD protected however with the DB being metal would i need to install a time delayed RCD before the DB?

 

[Ian1981]

It would be extremely wise to provide fault protection to the flexible tail links within a dual rcd consumer unit as we all know if a live link was to come into contact with the metallic case then in practically every case with a TT system the cut out fuse will not blow and some poor sod will cop 230 v when touching exposed parts

 

[Ian1981]

There is no need to install a time delayed upfront RCD if all circuits are to be provided with 30mA RCD protection.

I would agree if the tails where installed using a cable gland or an internal clamp to prevent movement of the supply tails and the board was fitted with rcbos only with no tail links.
I would still however install a separate enclosure with a 100mA time delay rcd before the consumer unit.
Bs7671 is the minimum requirement for safety requirements but it doesn’t preclude adding a bit of extra safety if you will.
That would be just my preference anyway

 

[Deleted member 26818]

The question asked was “however with the DB being metal would i need to install a time delayed RCD before the DB?”
The answer is no.
It may well be prudent, it may well be mentioned in the OSG, it may be some people’s preference, who knows?
What I do know, is that it is not a requirement of BS7671.

 

[Ian1981]

It maybe a requirement of the manufacturer as wylex have released an installation video regarding this very subject

 

[Deleted member 26818]

Is that the same one which recommends semi-blind grommets?

 

[Ian1981]

Is that the same one which recommends semi-blind grommets?

Where do they use semi blind grommets on that video?
If they do I can’t recall unless it’s when trunking is installed over them.
Besides what’s that got to do with a time delay rcd being installed?

 

[Deleted member 26818]

One of the videos recommends many things.
One is the use of semi-blind grommets to maintain fire rating (not required by BS7671).
Another is the use of ‘tail’ grommets for incoming Line and Neutral (does not comply with BS7671 Reg. 521.51).
Finally the use of an upfront time delayed RCD (not required by BS7671).

 

[Deleted member 9648]

I would agree if the tails where installed using a cable gland or an internal clamp to prevent movement of the supply tails and the board was fitted with rcbos only with no tail links.
I would still however install a separate enclosure with a 100mA time delay rcd before the consumer unit.
Bs7671 is the minimum requirement for safety requirements but it doesn’t preclude adding a bit of extra safety if you will.
That would be just my preference anyway

And your RCD in a separate enclosure will need to be in.......A metal enclosure!!
Better install an RCD before the RCD.....Oh, hang on though...
Kind of back to square one.

 

[Ian1981]

And your RCD in a separate enclosure will need to be in.......A metal enclosure!!
Better install an RCD before the RCD.....Oh, hang on though...
Kind of back to square one.

Plastic enclosure for the rcd as it wouldn’t be similar switch gear assemblies used for the same fundamental application as a consumer unit, as defined in bs 7671

 

[Deleted member 9648]

I disagree with that, it would be similar switchgear as a consumer unit, in the same location and equally likely to cause a fire if poorly installed.

 

[Ian1981]

How the hell is a stand alone rcd enclosure like a consumer unit?
Anything installed has the potential to cause fire if poorly installed.
Does that mean when we fit a rcd fused spur it has to be metallic?
part 2 definitions of a consumer unit isn’t the same as a stand-alone rcd unit.

 

[Deleted member 9648]

It would be utterly pointless to install a metal CU with the intention of containing a fire alongside a high current main switch of any description in a non fire containing plastic enclosure. The difference between a main switch and an RCD fused spur is that an RCD fused spur will not be installed within a CU so it is not similar switchgear, a main switch,whether RCD or otherwise could and will be installed within a CU therefore it is similar switchgear.

 

[Ian1981]

Similar switchgear assemblies
The phrase ‘similar switchgear assemblies’ in Regulation 421.1.201 means those assemblies used for the same fundamental application as a consumer unit. A consumer unit is defined in Part 2 of BS 7671:

‘Consumer unit (may also be known as a consumer control unit or electricity control unit). A particular type of distribution board comprising a type-tested co-ordinated assembly for the control and distribution of electrical energy, principally in domestic premises, incorporating manual means of double-pole isolation on the incoming circuit(s) and an assembly of one or more fuses, circuit-breakers, residual current operated devices or signalling and other devices proven during the type-test of the assembly as suitable for such use.’

An example of a similar switchgear assembly is a three phase distribution board that is intended to be operated by ordinary persons. This would have to have isolation that interrupts the three incoming line conductors and the neutral, rather than just double-pole isolation as mentioned in the above definition.

 

[Ian1981]

It would be utterly pointless to install a metal CU with the intention of containing a fire alongside a high current main switch of any description in a non fire containing plastic enclosure. The difference between a main switch and an RCD fused spur is that an RCD fused spur will not be installed within a CU so it is not similar switchgear, a main switch,whether RCD or otherwise could and will be installed within a CU therefore it is similar switchgear.

I’m talking about stand alone rcd not enclosed in a consumer unit as would be the case of a rcd fused spur
This topics been done anyway and there’s no argument at the moment to sway me otherwise

 

[Deleted member 9648]

If you think a non fire containing 100a RCD main switch alongside the fire containing CU that it serves makes any sense whatsoever then crack on.
But IMO you are wrong.