Testing garage CU for assesment!!!

[swaley]

Blimey. Had to go to work guys sorry if not replied. Basically I've run a 6mm SWA from main DB to a 2 way garage DB. Didnt opt to use PME from house, instead earth staked the outside building. Obviously a big no no from the sparks on here. All I asked was advice about testing. Thanks

 

[swaley]

How does it connect at the house end.?
What is the configuration of the outhouse CU?

Into a 32a mcb - rcd protected board. 2way board in outhouse. Earth staked.

 

[Octopus]

Into a 32a mcb - rcd protected board. 2way board in outhouse. Earth staked.

Hum .... so if the cabin trips the RCD it WILL affect the house.

Sounds like you have 2 x 30 mA RCD's in series.

 

[swaley]

32a mcb in main board.

What is the sub main connected to at the house?

 

[Octopus]

32a mcb in main board.

What configuration CU do you have in the cabin?

 

[Midwest]

Ahhhhh I was an apprentice at a site in Burnt Oak when on the 15th of February one of the sparks turned up.

He was asked how his Valentines date went with his missus and said not well as he had bought her some stockings and suspenders, she told him to F off and called him a perv and to wear them himself, which he did to work under his jeans and showed us all......... I was 18 he reminded me of Kenny Everett.......

They must of chaffed somewhat

 

[swaley]

Hum .... so if the cabin trips the RCD it WILL affect the house.

Sounds like you have 2 x 30 mA RCD's in series.

Not necessarily not connected earth in main DB only in log cabin end. I agree now I should of created a main side in house DB and used PME for earthing, would of been a better solution.

 

[Octopus]

Not necessarily not connected earth in main DB only in log cabin end. I agree now I should of created a main side in house DB and used PME for earthing, would of been a better solution.

On the basis you MUST have an RCD in the cabin ...

You clearly don't understand RCD's .......... they detect the imbalance of the line and neutral ......... the earthing is irrelevant.

You need to make "improvements" before your assessment IMHO

 

[swaley]

Hence creating a main side in main DB

On the basis you MUST have an RCD in the cabin ...

You clearly don't understand RCD's .......... they detect the imbalance of the line and neutral ......... the earthing is irrelevant.

You need to make "improvements" before your assessment IMHO

 

[Octopus]

Hence creating a main side in main DB

What do you mean by main side?

 

[swaley]

Not protected by RCD

What do you mean by main side?

 

[davesparks]

Didnt opt to use PME from house, instead earth staked the outside building. Obviously a big no no from the sparks on here. Thanks

It’s not a big no no, it is sometimes necessary to do this, however in your installation it sounds like there were better options.

Unless your earth electrode has a stable Ra somewhere below 1ohm then you’d be better off having used the TNCS earth in my opinion.
How have you isolated the earth of the SWA at the load end of the cable? Which earthing system have you connected the armour of the SWA to?

Merely putting in an earth electrode at the shed end won’t prevent the RCD in the house from tripping. The neutral is still referenced to true earth at the substation so the RCD will see the imbalance from any earth fault or leakage. To achieve what you want you would need to install an isolating transformer and set up a TNS supply to the shed by referencing one leg of the output to earth.

This hasnt even touched on the subject of discrimination and why feeding a submain from an mcb is rarely the best practice.

I realise you only asked about testing, but all of the points raised should also come up during your NIC assessment

 

[davesparks]

Didnt opt to use PME from house, instead earth staked the outside building. Obviously a big no no from the sparks on here. Thanks

It’s not a big no no, it is sometimes necessary to do this, however in your installation it sounds like there were better options.

Unless your earth electrode has a stable Ra somewhere below 1ohm then you’d be better off having used the TNCS earth in my opinion.
How have you isolated the earth of the SWA at the load end of the cable? Which earthing system have you connected the armour of the SWA to?

Merely putting in an earth electrode at the shed end won’t prevent the RCD in the house from tripping. The neutral is still referenced to true earth at the substation so the RCD will see the imbalance from any earth fault or leakage. To achieve what you want you would need to install an isolating transformer and set up a TNS supply to the shed by referencing one leg of the output to earth.

This hasnt even touched on the subject of discrimination and why feeding a submain from an mcb is rarely the best practice.

I realise you only asked about testing, but all of the points raised should also come up during your NIC assessment

 

[swaley]

Thanks for the constructive reply. And yes I realise it's all relevant. Been googling all morning and picked up a bit more knowledge that I didnt have. Gonna alter my design get rid of earth stake & export existing earth. Thanks

It’s not a big no no, it is sometimes necessary to do this, however in your installation it sounds like there were better options.

Unless your earth electrode has a stable Ra somewhere below 1ohm then you’d be better off having used the TNCS earth in my opinion.
How have you isolated the earth of the SWA at the load end of the cable? Which earthing system have you connected the armour of the SWA to?

Merely putting in an earth electrode at the shed end won’t prevent the RCD in the house from tripping. The neutral is still referenced to true earth at the substation so the RCD will see the imbalance from any earth fault or leakage. To achieve what you want you would need to install an isolating transformer and set up a TNS supply to the shed by referencing one leg of the output to earth.

This hasnt even touched on the subject of discrimination and why feeding a submain from an mcb is rarely the best practice.

I realise you only asked about testing, but all of the points raised should also come up during your NIC assessment

 

[Pete999]

What do you mean by main side?

What he said^^^^^^^^^^^^^^^

 

[Octopus]

All I asked was advice about testing. Thanks

I suppose we could have told you how to test it and then ignored ALL the inherent issues with your design & installation.

So, you've taken some flack, quite rightly for a daft design and now you won't have to answer awkward questions from a NICEIC assessor!

Surely you should be greatful.

My only other comment is to use 3 core in the future ... and a switch fuse.

 

[Taylortwocities]

My only other comment is to use 3 core in the future ... and a switch fuse.

Absolutely

It is very difficult to get discrimination between two MCBs. Just look at the discrim tables, and you’ll see why.
Hopefully your assessor will understand that you are just starting out and will be kind.
Buy good coffee and a big pack of chocolate hobnobs.

Do let us know how it goes.

 

[Midwest]

Can I ask why the OP is being recommended to not use a not RCD protected way in the house CU, with RCD CU in log cabin, presuming he uses the PME?

 

[Octopus]

Can I ask why the OP is being recommended to not use a not RCD protected way in the house CU, with RCD CU in log cabin, presuming he uses the PME?

You lost me with that ... but I simply questioned why he would purposely have 2 x RCD's in series...

 

[Midwest]

You lost me with that ... but I simply questioned why he would purposely have 2 x RCD's in series...

Sorry Murdoch, you've mentioned using a switch fuse (#36) and Davesparks said 'why feeding a submain from an mcb is rarely the best practice' (#33). I understand the use of a switch fuse feeding a distribution circuit for a domestic property CU, but for a small CU for a log cabin or garage etc?