Exporting to sub main

[jpadie]

Long story which I will try to shorted 

 

Main CU with a TT earth 

 

Garden room with mains exported from main CU via SWA.  

 

Garden room circuits are all RCBO.

 

Local authority didn't like the RCD that was in the main cu protecting the swa because it used 15mm cable that was not manufacturer specific.  Said I could change to use manufacturer cable or an MCB with a bus bar.

 

Wylex don't sell separate links so I went the mcv route.  Inspector now says he made a mistake and because it's a TT the SWA will need RCD protection. 

 

Choices as I see them are

 

1. Complete replacement of the main cu. Expensive, challenging and will need separate notification 

 

2. Find a 63amp rcbo that can fit.  Feels possible.  But then both ends will trip.  Do I then need to change all the sub main breakers to mcb's?

 

3. Henley block to split the tails and add a separate cu in the main house.  Challenging as there is no space. 

 

4. Find a reason and evidence to challenge the inspector.  I e.  

 

A. Does the 18th edition really require manufacturer links?

B.  Is it a requirement to have RCD protection on SWA with a TT?  

 

 

Am feeling aggrieved as it took ages to find a solution to the original non-compliance finding only to be told that it wasn't the whole story! 

[Mattg4321]

No offence intended, but sounds like the blind leading the blind!

 

No requirement to have RCD protection on a TT system, you just have to ensure the circuit can reliably meet max zs values. In reality, on a domestic job this isn't reliably possible, so you need to provide additional protection. ie an RCD. The only proper way to do it is to fit an S Type RCD on the sub main and double pole/single pole switched neutral RCBO's at the garden room. This means a separate switch fuse at the origin and probably a new consumer unit/RCBO's at the garden room as your RCBO's are probably single pole. Probably not what you wanted to hear.

 

You could fit a 50A RCBO on the sub main and end up with no selectivity, but it would be safe, just not right imo. I doubt you will find a 63A RCBO.

 

15mm cable? Not a size that exists.

 

Could you post a picture of inside both house and garden room consumer units? Then it can be decided if it can be easily sorted or if I feel like you should be seeking help from a local electrician. Apologies for bluntness.

[jpadie]

Local electrician said to install an s type RCD in the main CU.  Which is what I did. But didn't use manufacturer links (I borrowed conductors from a length of swa to make sure they were adequately sized). 

 

Inspector said to change for an MCB or use manufacturer links.  He's tonight changed his mind on the mcb. 

 

The earth impedance values on all circuits in the garden room have been tested as ok.  

 

I will post photos in the morning.  The garden room rcbos are all single pole (I have spent the last 20 years in France so was really surprised that things are/were single pole here!). I don't object to the expense of substituting with dual pole rcbos.  Not sure whether BG fortress have them in the range but I'd hope so.

 

I guess at worst I can replace the s type RCD and buy a cheap wylex cu to harvest the links.  

 

Am typing on a mobile phone. SWA is 16mm2 three core.  One core used for exporting the earth (also connected to the armour). 

Edited February 18 by jpadie

[jpadie]

@Mattg4321, per request for photos here is a Google album with both boards in it

 

https://photos.app.goo.gl/wWMgCdv2KCVN3ABL7

 

The gap from the top of the cupboard to the cu in the main house is 220mm. I'd need to remove the metal adaptable box to fit anything else in.  That just houses the SWA termination for the garden room.  

The house is listed so there's a general admonition not to remove/damage fabric. Thus most of the wiring is surface mounted.  

Edited February 19 by jpadie