rcd to protect tails?

[jstephens251]

Hi all,

I have been to a job today to have a look. The customer is moving some stud walls in a ground floor flat and wants me to move the electrics within them and fix them temporary while other walls are built. My only concern is the meter box is outside and the 25mm tails and 16mm cpc are fed from a 100dp switch next to meter, about 10 metres to consumer unit all surface fixed in 50x50 trunking along the top of one of these stud walls. Customer wants to chase these cables in now, so do these require rcd protection?

many thanks

 

[Guest123]

If 1, no additional mechanical protection or 2, earthed metallic covering or 3, buried at a depth of less than 50mm then yes they will need additional protection via a 30mA RCD.

522.6.6 refers.

Also they should have additional overcurrent protection via a switchfuse for example, as they are over 4M in length the DNO will not guarantee their service fuse.

 

[tonys]

You shouldn't really be playing about with the tails if your not sure?

 

[baldsparkies]

If 1, no additional mechanical protection or 2, earthed metallic covering or 3, buried at a depth of less than 50mm then yes they will need additional protection via a 30mA RCD.

522.6.6 refers.

Also they should have additional overcurrent protection via a switchfuse for example, as they are over 4M in length the DNO will not guarantee their service fuse.

Lenny, How would you stand regarding Section 314 ie, To comply with these requirements the circuits of an installation should not be connected to a single RCD, as this could lead to loss of supply to the entire installation in the event of a fault on one circuit, clearly inconvenient for the user of the building.
Its just a thought, but the tails will be feeding the entire install. Or is this a case of the big red book contradicting itself ??

 

[jstephens251]

thanks Lenny. Thanks for reg number ill have a look.

is 4mts the maximum? ill see if theres a shorter route as the current route is a little odd. 30ma is a pain for nuisance tripping is a 100ma time delay rcd suitable or does it have to be 30ma?

 

[Guest123]

Lenny, How would you stand regarding Section 314 ie, To comply with these requirements the circuits of an installation should not be connected to a single RCD, as this could lead to loss of supply to the entire installation in the event of a fault on one circuit, clearly inconvenient for the user of the building.
Its just a thought, but the tails will be feeding the entire install. Or is this a case of the big red book contradicting itself ??

I would try and avoid this type of situation in any way possible for exactly for the reason you give. I would try and offer the required mechanical protection and keep the cable with the prescribed zone as per 522.6.6 (v).

 

[jstephens251]

You shouldn't really be playing about with the tails if your not sure?

i havent touched the tails yet ive came here first to double check

 

[Guest123]

thanks Lenny. Thanks for reg number ill have a look.

is 4mts the maximum? ill see if theres a shorter route as the current route is a little odd. 30ma is a pain for nuisance tripping is a 100ma time delay rcd suitable or does it have to be 30ma?

4M is the max my DNO permit, others may differ.

522.6.7 refers to the use of additional RCD protection where 522.6.6 (i) - (v) cannot be met and states that.....an RCD having the characteristics specified in 415.1.1 should be used.

415.1.1 states that.....'the use of an RCD with a rated residual operating current not exceeding 30mA' etc etc.

 

[Des 56]

You are only just assessing the job,so explain the requirements regarding the way they are installed. and what action you have to take for protection of those tails. and let him chose a method to comply

Try not to let the customer tell you what installation method is going to be used without you pre warning them of requirements
In the end, he can have whatever method he choses,the only restriction should be that, as long as your installation complies

 

[baldsparkies]

Never have been a fan of burrying the main tails. I go along with Lenny. Dont think an RCD would be wise on tails. Try to convince the customer that for practical purposes the tails would be better trunked in say, MT4. Plus the switchfuse would be needed as said.

 

[jstephens251]

many thanks guys you have been very helpful