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?

 

[Leesparkykent]

Have you done an earth loop to see what reading your getting from the existing rod?

Are you planning installing a dual RCD board or full RCBO board?

No need to dig a pit just use one of these Earth Rod Terminal Cover - https://www.tlc-direct.co.uk/Products/MTETC.html?source=adwords&ad_position=1o1&ad_id=45425533757&placement=&kw=&network=g&matchtype=&ad_type=pla&product_id=MTETC&product_partition_id=174056326627&test=finalurl_v2&gclid=EAIaIQobChMIqOzshbDf1wIVSrvtCh0fPg8SEAQYASABEgIm6fD_BwE

 

[chris day]

RCBOs within board will be installed

Existing Rod gives a healthy reading 0.30

Ahh mate that cover looks to be the perfect solution thanks, looks quite sturdy i guess you just bury the little stub into the ground?

 

[Leesparkykent]

That's an extremely good reading for a rod and would be mad to install a new rod that will probably not achieve any where near that reading.

No need for an upfront time delayed RCD, just make sure the tails are adequately supported and taken through a nylon tail gland .

You could just screw the cover to the floor and conduit down to it or clip down to it and finish it off with a stub of conduit, female adaptor and a stuffing gland.

 

[Pete999]

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?

 

[Des 56]

RCBOs within board will be installed

Existing Rod gives a healthy reading 0.30

Ahh mate that cover looks to be the perfect solution thanks, looks quite sturdy i guess you just bury the little stub into the ground?

That is one heck of a low reading for a TT which I assume is in a domestic setting
Was the reading taken with the earth conductor removed from the met ?

 

[Leesparkykent]

Try performing that test in a basement

 

[Ian1981]

I would say that if it’s buried in concrete then an earth electrode test wouldn’t be very practical and an earth fault loop impedance tester will be more than adequate

 

[Devonchris]

"That's an extremely good reading for a rod and would be mad to install a new rod that will probably not achieve anywhere near that reading."

Probably hitting the roof of the Central Line.

 

[chris day]

That's an extremely good reading for a rod and would be mad to install a new rod that will probably not achieve any where near that reading.

Thank you lee, the sub station is one road away so that's possibly the answer, I agree and it's a very good idea to add some surface inspection port to protect it.

I read up on the RCD on the rails and I agree it would just be a waste of time passing the problem further back.

 

[chris day]

That is one heck of a low reading for a TT which I assume is in a domestic setting
Was the reading taken with the earth conductor removed from the met ?

The reading was taken with the main Earth still connected i should have disconnected.

 

[Deleted member 26818]

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

 

[Leesparkykent]

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

But if fitting a dual RCD board it would be prudent to do so. Your scheme might have something to say about it as the flexible links within the board, feeding the RCDs have no earth fault protection without having the upfront time delayed RCD. This is also mentioned in the OSG.

 

[Deleted member 26818]

Does the OSG reference any Regulations from BS7671 when it mentions this?

 

[Leesparkykent]

Does the OSG reference any Regulations from BS7671 when it mentions this?

No of course it doesn't...Does that mean it's not prudent to do so because it doesn't reference a particular regulation?

 

[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?