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,

 

[IQ Electrical]

The only guaranteed way of achieving discrimination between RCD devices is by the use of a time delayed 'S' type rcd upstream.

 

[Sintra]

Sorry but I'm lost on this one.


Sent from my iPhone using Tapatalk.

 

[ian.settle1]

Sorry but I'm lost on this one.


Is the answer 4?

 

[telectrix]

think you are getting confused here. when you say a RCD tripping in under 0.2 secs, you are referring to the fact that it must trip within 0.2 secs. on a TT system. you will find that 99% of RCDs trip well within this time, whatever the earthing arrangement.the only way to ensure no trip in house CU is to supply shed from RCBO in house CU,independent of the house circuits, so that it will only trip it's own circuit. you then will not require an RCD in the shed, just a 2 pole isolator/main switch.(i am assuming that the feed to the shed itself requires Rcd Protection)

 

[Just]

Thanks Telectrix, that sounds great but a little expensive (and still have to walk to the house) as the board is 16th RCD protected but agree separate RCBO in house would do it. An 'S' type RCD (IQ Electric) in the house would still need a separate unit as it needs to control only the the shed CU.If cost was an issue could I just use a 20ma RCBO and a 6ma RCBO in the shed or would this still trip the house RCD. If I chose to say sod it I'll use a main switch and two MCBs and walk to the house if I cut through my cable, do I still need to discriminate to meet regs from the RCD protecting the other house circuits on MCBs as one 32A MCB is feeding the board in the shed. Now I've written that down I'm even more confused!
Thanks all for bearing with me!

 

[ezzzekiel]

fit an rcd protected board from an unprotected way in house.

 

[telectrix]

fit an rcd protected board from an unprotected way in house.

as long as the cable feeding the shed doesn't need RCD protection itself. i.e. buried within 50mm........blah, blah

 

[Deleted member 26818]

The cable supplying the shed is a distribution circuit. As such there is no requirment in BS7671 for this to be RCD protected. Depending on the earthing system used for the shed, it may be that only the socket-outlets require RCD protection.
You mention that the house is TN-C-S, have you exported this earth to the shed, or isolated the shed from this earth and TT'd the shed?
If the shed is TT'd then both circuits in the shed will require RCD protection.
What cable have you used to supply the shed?

 

[telectrix]

The cable supplying the shed is a distribution circuit. As such there is no requirment in BS7671 for this to be RCD protected.


i would agree with this if the cable feeding the shed came off the supply side of the house CU, but as i understand the op, it is fed from the load side of a MCB within the house CU, so i would class it as a circuit same as all the others. i may be wrong here, and would love to find the correct answer.

 

[Deleted member 26818]

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.

 

[telectrix]

any circuit with cables buried in walls without earthed metallic conduit require RCD protection

 

[Deleted member 26818]

No not the circuit, just the cables.

 

[Just]

Hi Spinlondon Used 4mm SWA and yes earth from CU. SUre I could use an RCD board in shed from an unprotected way but house CU is 16th with a RCBO main switch protecting all ways. Could I just use a 20ma RCBO and a 6ma RCBO in the shed or would this still trip the house RCD? Also if I use an RCD in the shed won't that trip first as the fault path will be marginally quicker than to the house?

 

[Deleted member 26818]

Unless you use a time delayed RCD at the house, there is no saying which one will trip first, in fact they both trip.
It's a shame the Main switch has been changed to RCD.
There are ways to sort it, but it starts getting expensive or messy.