Terminating 50mm tails into rccb?

[wozzit99]

Hi

Due to the long length of a distribution circuit I'm installing, the cable csa is 50mm sq.
The earthing arrangement at the origin is TT.
25mm tails at the service are to be connected to a 300ma 100amp time delayed rcd then into a 100amp s/fuse ( fuses rated at 80 amp )
From the s/fuse a 50mm sq 4 core cable is going to be ran to a out building some distance away which is to supply a 4 way 3 phase dis board.
The NICEIC advised me to install a separate TT earthing arrangement at the db end.
The problem is I have to terminate 50mm sq 4 core armoured cable into an rccb. I know I have to keep the armoured insulated from the db earthing system but how am I going to connect the tails?
Are there any rccb's which will take this cable size?
It would be a hell of a lot easier to export the origin TT that way I could terminate to a isolator at the db end.

Cheers

 

[Rockingit]

Most main switches will accept 50mm, but I think you'll struggle with a domestic RCBO. One option might be to crimp the ends with a size-reducing bootlace ferrule, so 50>25 just at the last few mill. But you might have a bit of a hunt to find some, not commonly stocked by wholesalers.

 

[johnboy6083]

Its probably to stop earth faullts travelling through the system at tyhe supply end of the submain. I dont see a problem with installing more rods at the DB end, as long as you have taken a logical path to that conclusion, and deem tthere to be no risks. to rely on the 1 rod is IMO, asking for trouble.
If the job is worth a few grands worth of cable, then a few rods, and some earth cable wont hurt. In fact it will make the installation a lot safer.

 

[Marvo]

I've never seen reducing ferrules, or a domestic RCD that will accept a 50mm wire. Also with a 50mm cable you'll struggle to get the required bend radius in the dis board unless it's a big one.

Maybe make the cable off in an adaptable box nearby with some din mounted terminal blocks. Come out again in 25mm as Rockingit suggested.

 

[jaresquire]

If the origin is TT why separate the earthing?

 

[WDMDL]

Why not just provide a suitable enclosure and terminal arrangement (similar to where you may install a spreading chamber?) at the far end with 25mmsq flexible tails from there to the RCCB?
Your distribution circuit is fused 80A at its origin so well within the rating of the 25mmsq tails and no need to fuse down again.

On some old installs I've seen the armouring gapped a foot or so below the board, the sheath made good with an adhesive lined heatshrink and a warning notice about separate earthing fixed to the wall adjacent.
Is that still allowed in the 17th?

Simon.

 

[spark 68]

The problem is I have to terminate 50mm sq 4 core armoured cable into an rccb

Are you talking about at the remote end ?

Because if you are, do you need to terminate it into an RCBO, won't you just connect to the main switch at this DB ?, because you already have rcd protection (albeit 100A @ 300mA) at the supply end, and you could use normal sized (individual) RCBO's fitted at the out going ways of the DB, of the lower mA rating of whatever is required.

 

[Engineer54]

Exactly, ...how far away from the source supply will this sub-main 3 phase DB be??

 

[wozzit99]

Cheers for the pointers guys.

Change of plan then.

What about terminating into a bill switch isolator with spreader box at the db end using 50mm lugs then out of there to the 3 phase db with 25mm tails. Still a bit confused with the earthing tho. Sepereate the the TT systems or not?

The set up in the db is going to be 4 100ma rccb's + 4, 3 phase mcbs

 

[wozzit99]

Engineer 54

Aprox 40-50m

 

[spark 68]

Why can't you terminate directly into the DB ?, using the main switch as your Isolator at this (remote) end ?

The armoured cable should be earthed at the supply end, using the supply's earthing, but if you require seperate earthing at the remote end you will have to insulate the gland at this end.

 

[wozzit99]

Spark 68

The problem is terminating 50mm tails, I think the max cable size you can get in the main switch is 35mm. The only other way round it is to use the bootlace ferral reducers Rockigit suggested

ROUND COPPER PIN TERMINAL - Copper Pin Terminals - Copper Tube Terminals - Cablecraft - Cable Identification and Cabling Accessories

 

[Engineer54]

IS that all?? What kind of load are you expecting to have on the end of this sub-main????

No, there is no need to separate a TT system earth, ...for what reason did this NIC person give you?? The more rods you have on a TT system the better the Ra is gonna be!! ... You don't separate a TN-S system earth do you, so why separate a true earth TT system??


The project i'm involved with at the moment is based on an overall TT system, and that connects to every building on the project. Your only talking about one outbuilding around 50 metres away from the source TT system. Connect them altogether and improve the overall TT system, using the SWA of your 50mm 4 core cable...

 

[spark 68]

On Merlin Gerin, ABB and similar DB's they do termination kits that expand the capablities of the isolator, that will happily take 50mm^ cables, these are like tinned copper 'stabs' that you can terminate rings onto.

 

[netblindpaul]

E54, A "TRUE" PME system...

 

[Guest123]

If you do still need to find a suitable termination, pin lugs......

 

[spark 68]

@ E54,

Connect them altogether and improve the overall TT system, using the SWA of your 50mm 4 core cable...

That is what I would have done, for the same reason, but I did not want to contradict the NIC, in case it was for a specific reason they wanted it done this way.

 

[netblindpaul]

You could use an SWA 50UP to go from 50mm sq core quite easily and really acceptably down to a 35mm sq pin.
Try here:
http://www.s-w-a.co.uk/resources/Copper_Tube_terminals/un_insulated_copper_reducing_pin.pdf

 

[netblindpaul]

Lenny,
You beat me to it a little, though I gave a part no & maker for a 50>35 reducing pin!

 

[Engineer54]

Sod the NIC, ....They do not make the rules, even though they like to THINK they can!! Ring them and ask for specific reasons Why they want to separate a TT system into 2 separate earthing systems, ....i for one would like to know that one!!!

 

[wozzit99]

Hi Engineer 54

The load is a 50kw micro generator 70amps/ phase max load, the cable is this size to keep the vault drop within the less than 1% range
There will be four inverters supplied from the sub db.


As for the Nic ,the guy did not give a clear reason about seperating the earths just said do it.

 

[Engineer54]

E54, A "TRUE" PME system...

Not with you, have i missed something somewhere??

 

[netblindpaul]

Think what PME stands for exactly...

 

[Engineer54]

Hi Engineer 54

The load is a 50kw micro generator 70amps/ phase max load, the cable is this size to keep the vault drop within the less than 1% range
There will be four inverters supplied from the sub db.


As for the Nic ,the guy did not give a clear reason about seperating the earths just said do it.

That's because there isn't a clear reason!! Different if the source supply was PME, but it isn't so don't try separating the TT system, you'll be defeating the very nature of a TT system...

 

[spark 68]

Think what PME stands for exactly...​

Protective Multiple Earthing​

 

[Engineer54]

Think what PME stands for exactly...

Protective Multiple Earthing , i know, and yes that is exactly my point!! But by just stating PME on this thread it will confuse the young or inexperienced into thinking along the lines of PME/TNC-S supplies, but that was not really what you (or i) were talking about is it??...lol!!!