Shed Installation Disconnection time?

[Just]

Hi I have a socket Radial off a 20A MCB and a lighting Radial off 6A MCB in a shed the board is fed by a 32amp MCB protected by an RCD on a TNCS syatem in the house. If I want the first point of fault ie the shed to trip first should I be using an RCBO for a TT system that trips under .2s? This would avoid both RCDs or the one in the house tripping first if I use an RCD 0.4s disconnection. I could change the type of RCD in the house but that protects a number of circuits. I have a 4 way CU so could either have a RCBO main switch, protect two MCBs with a different type RCD or fit two RCBOs. Too many options!
Thanks for your advice,

 

[ezzzekiel]

possibly better to get an isolator fitted by dno (most likely meter company) which will allow diconection in future average charge is around £80

 

[pushrod]

It doesn't matter whether it is a circuit the same as all the others.
Only circuits of special locations are required to be RCD protected.

Among other situations as well, any socket-outlet in a domestic situation requires rcd protection (special location or not) - apologies if i have missed the correction later.

 

[Deleted member 26818]

I did actually put DB instaed of CU, but then thought people might get confused.
There appears to be two meanings for Zdb.
Originaly Zdb was a Ze test carried out at the board with all the bonding conductors connected so that parallel paths would be included. This value would then be added to the R1+R2 values for a circuit to determine Zs.
It now seems to be used to identify a Ze test carrried out at a second board.

 

[IQ Electrical]

Zdb is a far more logical term, there can only be one Ze for an installation, the reading at the end of a distribution circuit actually just being Zs.

 

[Deleted member 26818]

Among other situations as well, any socket-outlet in a domestic situation requires rcd protection (special location or not) - apologies if i have missed the correction later.

There does appear to be quite some confusion about the requirements in BS7671 for RCD protection.
The requirement in BS7671 is that socket-outlets intended for general use by ordinary persons and those for use with mobile equipment outdoors, must have RCD protection; the requirement is for the socket-outlets to be protected not the circuit.
Of course generally the easiest thing to do is to place an RCD at the origin of the circuit thereby protecting all the socket-outlets in one go. Another option would be to use individual RCD socket-outlets, which would be quite costly. However it is the installer's or client's choice, there is no requirement to protect the whole circuit.
In BS7671 the only reference to RCD protection for circuits, is where they are of special locations.

 

[Just]

Crikey some of you will be suggesting a board change soon.. I was only suggesting an RCD shed side to save the walk but if I thought it would cause this much trouble... I have got a DNO isolator buy the way, but some of you chaps look set on me spending a fair wedge. Some nice ideas, Change main RCBO to main isolator and then add RCBOs for all other rcd circuits... bit pricey? How about plug top on lighting radial, plug top on socket radial into double socket extension lead into RCD house sockets. LOL. I'll go for a Henly blocked unprotected enclosure as it's SWA and RCD protected MCBs shed side, only if everyone's happy?

 

[Deleted member 26818]

You really should provide the cable with overload and fault protection.

 

[SimpleSimon]

Crikey some of you will be suggesting a board change soon.. I was only suggesting an RCD shed side to save the walk but if I thought it would cause this much trouble... I have got a DNO isolator buy the way, but some of you chaps look set on me spending a fair wedge. Some nice ideas, Change main RCBO to main isolator and then add RCBOs for all other rcd circuits... bit pricey? How about plug top on lighting radial, plug top on socket radial into double socket extension lead into RCD house sockets. LOL. I'll go for a Henly blocked unprotected enclosure as it's SWA and RCD protected MCBs shed side, only if everyone's happy?

Like Spin said maybe a protected enclosure MCB. TT at shed or export TNCS?
PS was it 4mm SWA youve got?

 

[IQ Electrical]

Crikey some of you will be suggesting a board change soon.. I was only suggesting an RCD shed side to save the walk but if I thought it would cause this much trouble... I have got a DNO isolator buy the way, but some of you chaps look set on me spending a fair wedge. Some nice ideas, Change main RCBO to main isolator and then add RCBOs for all other rcd circuits... bit pricey? How about plug top on lighting radial, plug top on socket radial into double socket extension lead into RCD house sockets. LOL. I'll go for a Henly blocked unprotected enclosure as it's SWA and RCD protected MCBs shed side, only if everyone's happy?

It's not causing trouble, it's good debate!

This is what happens when you turn to a forum for design ideas, if you have 20 replies, you'll generally get 20 scenarios.

The good thing is it's free and you can cherry pick the bits that you like best as long as they are compatible.

Now get busy on the 'thanks button'

 

[Just]

Only joking IQ enjoying the banter. It's 4mm SWA Simple Simon. if I am to protect the cable even though it's SWA and I don't want the expense of time delays and separate enclosures can I come from a RCBO main isolator protected MCB or must I provide discrimination between circuits. I know that blows my original post of not wanting control for the shed in the house but it's looking the cheapest and straightforward option?

 

[Deleted member 26818]

We're not talking about RCD protection use a fuse or a MCB in an enclosure separate from the CU.
This will provide the cable with overload and fault protection.

 

[SimpleSimon]

Not that we can answer the question because we don't know the layout of your shed or size but will 4mm take all your requirements?
You don't want to overload your SWA than you really will have to keep walking back into the house

 

[pushrod]

There does appear to be quite some confusion about the requirements in BS7671 for RCD protection.
The requirement in BS7671 is that socket-outlets intended for general use by ordinary persons and those for use with mobile equipment outdoors, must have RCD protection; the requirement is for the socket-outlets to be protected not the circuit.
Of course generally the easiest thing to do is to place an RCD at the origin of the circuit thereby protecting all the socket-outlets in one go. Another option would be to use individual RCD socket-outlets, which would be quite costly. However it is the installer's or client's choice, there is no requirement to protect the whole circuit.
In BS7671 the only reference to RCD protection for circuits, is where they are of special locations.

TBH this would be so ridiculously expensive as to not make it any sort of option at all!

A circuit is an assembly of equipment supplied from and protected from overcurrent by one protective device. This would include cables and sockets. Ordinary twin and earth (which must make up 95%+ of domestic cabling) that is either not surface mounted or deeper than 50mm or mechanically protected/protected by earthed metallic covering
will require rcd protection regardless of location. Reg 411.3.3 specifically talks about rcds for additional protection.

To persist with the notion that

Only circuits of special locations are required to be RCD protected

is at best a case of semantics, but is, quite frankly, a statement that will be misleading and will lead to confusion in others. And since you talk about "confusion" statement like yours will not help clarify matters. References to the OSG 3.6.1 which clearly mention the need for rcds for socket outlets in domestic situations, 'special locations' and for cables unless they meet particular requirements is a much more useful direction to go in.

 

[nickblake]

What was the question .... its a great thread watching the sparks fly .. pardon the pun lol

 

[Deleted member 26818]

TBH this would be so ridiculously expensive as to not make it any sort of option at all!

A circuit is an assembly of equipment supplied from and protected from overcurrent by one protective device. This would include cables and sockets. Ordinary twin and earth (which must make up 95%+ of domestic cabling) that is either not surface mounted or deeper than 50mm or mechanically protected/protected by earthed metallic covering
will require rcd protection regardless of location. Reg 411.3.3 specifically talks about rcds for additional protection.

To persist with the notion that is at best a case of semantics, but is, quite frankly, a statement that will be misleading and will lead to confusion in others. And since you talk about "confusion" statement like yours will not help clarify matters. References to the OSG 3.6.1 which clearly mention the need for rcds for socket outlets in domestic situations, 'special locations' and for cables unless they meet particular requirements is a much more useful direction to go in.

There is already confusion.
If we were to alter an existing non-RCD protected RFC, by simply changing an existing single socket to a double, would there be any requirement to provide RCD protection, and where would that protection be required to be placed?
As it stands if the socket is intended to be used for a specific item or items of equipment then there is no requirement for RCD protection. However if the socket was intended for general use there would be a requirement for RCD protection. This could be achieved by either the use of an RCD socket or by placing the RCD at the origin of the circuit.
If however you believe that the circuit requires protection, then the RCD would have to be at the origin of the circuit.
Simple enough if the CU can accept an RCBO, not so simple if the CU can't.
Before you know it, you are looking at a board change and all the attendant inspecting and testing; completely unesseccary as at most all that is required is the installation of an RCD socket.