Testing garage CU for assesment!!!

[swaley]

Hi guys I have my assesment in little over a month, and looking for some advice when testing my log cabin.
Do I need to fill out 2 test sheets. As my log cabin has it's own TT system. How would I measure Zs at my cabin? Im aware I Would only have Ze at origin DB.
Sorry been out the game a while and I'm aware I need to really brush up on my testing. Thanks

 

[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?

 

[Octopus]

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 log cabin CU?

Why not.

If you read the OP, he thinks that making the cabin TT, any tripping will be internal to the cabin and not the house despite the submain being on a "shared" RCD.....

Its very difficult to get proper discrimination between MCB's so when a switch fuse can be used, its better all round.

Most people avoid this option because its far more expensive.

 

[Pete999]

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 can ask.

 

[davesparks]

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?

For a small CU in a log cabin discrimination is less of an issue I agree, and the implications of a lack of discrimination are unlikely to be much.
However it is better, in my opinion, to design and install to the highest possible standard.

Also this particular installation appears to be being done especially to please an NIC assessor so following the book to the letter may be sensible.

 

[Midwest]

Why not.

Its very difficult to get proper discrimination between MCB's so when a switch fuse can be used, its better all round.

Most people avoid this option because its far more expensive.

Thanks for that. I get the discrimination thing. But in most cases, I've seen garage CU's supplied from an mcb in main CU. Its recommended by various members on here as a default way of supply, when someone seeks advice on supplying garage, shed, office CU's etc. In reality, I can't see a customer being happy, with a switch fuse put along side their CU in the hallway, not withstanding it would be easier to achieve that sort after discrimination.

 

[Midwest]

For a small CU in a log cabin discrimination is less of an issue I agree, and the implications of a lack of discrimination are unlikely to be much.
However it is better, in my opinion, to design and install to the highest possible standard.

Also this particular installation appears to be being done especially to please an NIC assessor so following the book to the letter may be sensible.

Just that most domestic designs would follow the MCB approach, I don't think the assessor would be too critical of such a design?