TT fuseboard question

[m4tty]

Hi,

Went to do an estimate tonight and as soon as I pulled up outside the whole street went out. Powercut lol so had to do quote with a torch

Customer wants to change his current fuseboard to a split load. Checked Earthing and it's earthed through water pipe so need to TT it. He also wants a 6mm 3C SWA connected to new fuseboard and also wants to put a board in the garage (5 way). The armored is already run and is deep enough so I'll test it first then connect.

As it's going to be a TT all circuits require rcd protection but I didn't really want rcd the swa as it'll be a pain when it trips to come up house end to reset. Plus if he's using tools in workshop and gets plunged into darkness could be dangerous.

What other options are there apart from Henley block to switchfuse then swa to garage with 30mA front end in workshop.

Have I missed anything? As been a long day so feel free to flame me

 

[Robin Spark]

Why dont you use high integrity board at the house, RCBO for submain to garage/workshop and then emergency bulkhead in workshop. That way any faults ie he does something wrong in the workshop wont plunge the house (or whole street lol) into darkness and he will have emergency light back up.

Another option is the garage integral/ attached or separate as if separate could you make that a separate TT and put lights and sockets on RCBO's in there?

 

[NozSpark]

Matty,

All circuits (even distribution circuits) require RCD protection in a TT installation.

You could use the "old" type of configuration from the 16th ed.. 100mA 'S' type main switch then MCB for sub main and 30mA RCD protection for all final circuits (both in house and garage)

You could do it with a high integrity 2+5+5 and change the main switch for a 100mA 'S' type RCD or go with a std cu, change the main switch (again) and use RCBOs for all final circuits

 

[M107]

Time delay the submain

 

[Andy™]

SWA needs RCD, no way around it

doesnt need to be 30mA though, could be 100mA time delay

 

[steptoe]

100mA 'S' front end,

ISCOs

tails to main CU and mc adaptable box

SWA from MC adaptable box to garage,

that way you have protected your SWA against fire, and any tripping in the garage will not cause any havoc to the house.

remember, as its TT you are perfectly allowed to have rods at both the incomer and garage end as well without any issues,

 

[Robin Spark]

Time delay the submain

Think my brain is on a time delay tonight (must be the german wine I have delivered tonight, yes I know its only wednesday as well, bad me!)

Yes Matty go for timedelay on the submain as Noz and M107 said.

 

[steptoe]

putting you SWA into ISCOs from the mc box,

cut a slot in one side of the box and grommet strip it.

 

[M107]

Very tidy Steps

 

[steptoe]

AH,

I have spotted an error in my method though, :|

you will need to provide some sort of OCPD for your submain,

100mA 'S' RCD in a 4way enclosure with a 32A MCB,

from RCD to main CU,and link RCD to 32A MCB

from 32A MCB to ISCO

from N side of RCD to ISCO > then split to CU and SWA

 

[Robin Spark]

100mA 'S' front end,ISCOs

tails to main CU and mc adaptable box

SWA from MC adaptable box to garage,

that way you have protected your SWA against fire, and any tripping in the garage will not cause any havoc to the house.

remember, as its TT you are perfectly allowed to have rods at both the incomer and garage end as well without any issues,

Steps,

the swa submain is 6mm 3core, switchfuse also needed at front end?

 

[steptoe]

Very tidy Steps

thanks, the rough tails were a temp to the old 3036 board while all the circuits were moved to the new CU that was about 3m away, - staged re-wire, and CU moved from porch to inside house.

---------- Post Auto-Merged at 22:31 ---------- Previous post was made at 22:29 ----------

Steps,the swa submain is 6mm 3core, switchfuse also needed at front end?

as post #12

I did notice on re-reading OP the SWA was only a 6mm,

well spotted though,

 

[Robin Spark]

Steps,

in the interest of cost and ease of job especially as 100ma time delay rcds are expensive. what do you think about my original idea

ie

high integrity board in the house

30ma RCBO to protect rcd submain,

then dont connect earth at sub board (as when its TNCS but converting to TT)

but install new earth rod at workshop and then just put in a 2 way garage consumer unit with 30ma rcd main switch.

 

[steptoe]

why not connect earth from submain at garage?

its all TT so there is no 'exporting' going on, drive another rod in too, its all good.

my thoughts on protecting TT with SP RCBOs are well documented, and I dont find it acceptable, but thats only my opinion, other (probably more) esteemed members find SP protection on TT more than adequate.

Im still at a loss as to why 2X 30mA devices on the submain/garage, not connecting the earth at the garage wont make any difference as to which RCD goes first, its just detracting from your earthing effectiveness,

 

[NozSpark]

Steps,in the interest of cost and ease of job especially as 100ma time delay rcds are expensive. what do you think about my original idea

ie

high integrity board in the house

30ma RCBO to protect rcd submain,

then dont connect earth at sub board (as when its TNCS but converting to TT)

but install new earth rod at workshop and then just put in a 2 way garage consumer unit with 30ma rcd main switch.

But that won't offer any discrimination between the RCDs

 

[Robin Spark]

Was thinking that by not connecting the armour/ cpc at garage end, then any earth faults in the garage would just open the garage rcd main switch and not affect the house and the garage cu has its own rod

 

[Sidewinder]

Ahh, no it does not, but at least you have backup protection, as we know RCD's are unreliable and we also know that consumers always test them every 3 months don't we!

 

[NozSpark]

Was thinking that by not connecting the armour/ cpc at garage end, then any earth faults in the garage would just open the garage rcd main switch and not affect the house and the garage cu has its own rod

If you had a VOELCB at the house then it would, but as you know RCD's monitor any imbalance between L&N

 

[Robin Spark]

Erm, actually now i have thought about it for a bit longer I can see why discrimination wont be achieved.

 

[m4tty]

So would you just use mainswitch in garage board or have 2 rcds which won't discriminate?