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,

 

[Deleted member 26818]

What exactly is your problem ezzzekiel?
I have not argued (rationally or irrationally) against your suggestions, just pointed out that it will be messy or costly.
However I would point out that you have ommited any form of overload or fault current protection for the cable supplying the shed.
Are you suggesting that the main fuse be relied on to provide this?

 

[High Tower]

Sorry but I'm lost on this one.


Sent from my iPhone using Tapatalk.

Glad its not just me!

Why are people still trying to use multiple RCD's on circuits

 

[ezzzekiel]

However I would point out that you have ommited any form of overload or fault current protection for the cable supplying the shed.
Are you suggesting that the main fuse be relied on to provide this?

you are correct but in my opinion no protection for overload or short circuit is required as this would only result in a more costly install

 

[SimpleSimon]

I keep coming back to this thread and I think I understand...but than another post blows me away. Lol

 

[SimpleSimon]

you are correct but in my opinion no protection for overload or short circuit is required as this would only result in a more costly install

Maybe I misunderstood you Ezzzekiel but if your splitting tails to a suitably rated enclosure would this than not house a suitable protective device for the SWA? Or am I reading it wrong?

 

[Dave_]

hello everyone.

if the supply to the shed is a suitably rated CB protected by an RCD then whats wrong with this.....? if you dont wanna spend money? lot of hassle splitting tails, 2 RCDs is unnecessary complicaton... upstream will trip first (0.00000003s ha ha) then the other due to a shorter fault path... well yeah both basically.... The problem is the lazy bugger that cant be bothered to walk back into the house when the RCD trips...! haha just kidding mate... if the install is sound then the RCD shouldnt trip......

 

[kmcgold]

32 hrc fuse with 100ma time delay rcd at consumer board
swa to shed tt system

small two way consumer unit with 6amp rcbo and 16 amp rcbo

 

[ezzzekiel]

Maybe I misunderstood you Ezzzekiel but if your splitting tails to a suitably rated enclosure would this than not house a suitable protective device for the SWA? Or am I reading it wrong?

yes simon you are correct the enclosure would house a suitably rated circuit breaker to provide overload and short circuit protection with swa then running to shed (mech protected so no rcd required) terminating into a two way rcd protected consumer unit which would enclose circuit breakers rated for the circuits they protect.

Also a neat install can be made at little or no major additional costs.

also a big argument on exporting earths on tncs systems but thats another day

 

[SimpleSimon]

That's what I thought as this is the simplest method mentioned for this scenario.
Just got confused with the argument between you and Spin. (nothing personal Spin it just confused me.)

 

[Deleted member 26818]

The first thing that should be established is whether the Zs at the shed CU are low enough to allow the use of an MCB for earth fault protection.
If Zs are acceptable then we have two options.
We can go with ezzzekiel's first suggestion which would entail changing the Main switch at the house CU back to a non-RCD switch and install RCBOs for whichever house circuits require RCD protection. Install an RCBO at the shed CU for the socket-outlets. This will probablly entail using 8 or 9 RCBOs.
Or we could go with ezzzekiel's second suggestion involving splitting the tails, installing an enclosure, should be able to re-use the MCB from the CU and install an RCBO at the shed for the socket-outlets.
To my mind the first option would be the neatest, but costs could be prohibitive.
The second option would be messy, and from a legal view point may entail contacting the electricity supplier for a disconnection to install the Henly blocks.

 

[ian.settle1]

32 hrc fuse with 100ma time delay rcd at consumer board
swa to shed tt system

small two way consumer unit with 6amp rcbo and 16 amp rcbo

Haven't heard them called that for years

 

[kmcgold]

are you changing the main sw live....

 

[Deleted member 26818]

Good point, have to call for disconnection either way.
Perhaps we could uprate the tails and even install a switch fuse at the same time?

 

[kmcgold]

the are many ways to do a job
some times cost comes into it
but it must be safe and to the regs.

 

[SimpleSimon]

The first thing that should be established is whether the Zs at the shed CU are low enough to allow the use of an MCB for earth fault protection

You mean Zdb? (just so I'm clear)