TT RCD query

[SparkJ]

The job is to change a consumer unit from semi enclosed fuse jobbie to 17th edition twin RCD jobbie. Earthing is TT.

There is already a 30ma RCD in an enclosure between the meter and the original consumer unit.

Can I just trash the stand alone RCD?

Or is there some regulation that demands that it stays.

If it needs to stay, what can I do about discrimination between the RCDs?

 

[danny7299]

no its doesn't need to stay... as everything will be 30mA protected ...

 

[Evans Electric]

What he said.

 

[Andy™]

unless its a metalclad board your installing, or tails buried in plaster, then no reason for it to stay

 

[danny7299]

metalclad? ? go on...

 

[Lurch]

metalclad? ? go on...

If you are connecting tails on a TT supply into a metalclad board the fault current required to operate the main fuse is quite high compared to a TN based system so you either need to ensure the tails cannot come into contact with the board or fit an RCD in the tails before they get to the metalclad CU. Been about for years this has.

 

[danny7299]

so would this be more than your standard grommet? ... in all honesty i have never head of this as any TT systems that i have done with metal clad have all been SWA sub mains.

learn somthing new every day... Makes total sense though

 

[Andy™]

you could use a gland & DI tails (but pref RCD in plastic enclosure before)

SWA submain on TT wold require an RCD before it, metalclad or not

 

[ian932]

If the tails are surface and its not a metalclad cu as mentioned then get rid of it, you may find that on a ramp test it goes at a lower value than the ones in the cu so it could lead to a the whole board going out.

 

[wozz]

Mk do a special grommet for this purpose, the grommet itself is class II for the life of me i cant find it on their website.

 

[Lurch]

No, it's not about a grommet and the tails connecting with the hole you make in the CU. It's about the tails touching the CU if they were to come adrift from the main switch\RCD. All cabling from the main fuse to the RCD should be protected from being able to come into contact with the case of the metalclad CU.

 

[danny7299]

I think hager boards have a complet plastic sourround that coveres the main isolator ... Cheers for the info. ...,