Split con submain

[jakew009]

Ok I'm not a practising sparky but I'm not a clueless diy'er either. Please don't start dissecting me..

We have got a couple of barn conversions to holiday lets happening at the moment. One is a single barn that has been designed so it can be let out as one single house or two mini house. Two kitchens etc but no major electrical loads.

I won't bore you with the details but for simplicity rather than being fed from the existing 3 phase distribution at the farm, the house is on it's own supply from dno with a 83a cutout.

The design was each house will have a CU. One CU is fed from the cutout, the other CU will be fed from the first. Conduits have been laid to allow a second dno supply to be brought in in the future if it is ever sold off.

The cable run between the two buildings is about 8m and is embedded in the wooden studwork, certainly got no mechanical protection.

The plan in my head was a largish CU would go in the house with the meter, 100a isolator, 50a mcb for the submain, and then standard split rcd for the house. Secondary house would just have a normal twin rcd board. Submain would be three core swa. All fine up until this point.. bear with me.

The sparky doing the work (subcontracted by builder) has bunged a 16mm split concentric in to join the two buildings and he's not exactly routed it in a way where it would be protected (runs up the stairwell wall just behind the plaster and then back down the other side).

The builder has asked him what the thinking behind the split con was and why he used it compared to swa. He's being pretty illusive with the answer, but says he does not intend on putting it on the RCD side of the board (for obvious reasons) and swears split con is fine and "is in the regs" and "I use it all the time".

I'm very uncomfortable with it given it's routing.

So am I overreacting to be deeply suspicious of the whole thing. Over to you...

 

[freddo]

No, which is why there are few installations where it can be properly used.

 

[freddo]

No, which is why there are few installations where it can be properly used

Split con also has the advantage of having the hurty wire covered by conductors that should be at 0V

 

[DPG]

Split con also has the advantage of having the hurty wire covered by conductors that should be at 0V

'Hurty wires' should be in the list of definitions at the start of the regs.

 

[jakew009]

'Hurty wires' should be in the list of definitions at the start of the regs.

The non-hurty wires can still make a big bang if they touch the hurty wire in the middle, especially if all that is protecting them is big fuse but I guess the same is true for swa...

 

[freddo]

I know, my previous boss drilled through one, took out power to the whole road.

 

[pc1966]

I think the consensus is that even though it’s not technically in the regs, there nothing unsafe about using it.
However, it’s got no more protection than a twin and earth. Would you run a 16mm twin and earth underneath plaster outside of a safe zone with just the dno fuse for protection?

In terms of stopping a drill then no, copper is not as tough as steel. But really would SWA armour stop a battery drill with HSS bit? I doubt it.

What SWA has is an earthed shield that makes sure if you do penetrate the cable then it shorts it out, so you don't have the risk of a live screw, walls support, etc, going unnoticed until it causes a fatal shock some day. That is the reason for the 30mA RCD with cable not buried more than 50mm behind walls, etc. Same idea for the likes of Flexishield.

Split concentric though is an odd one to consider. The construction means you can't reach the line conductor without going through either (or both) the CPC or neutral. So in that sense it is as safe as SWA in terms of ADS under such an accident condition.

But...you might be in contact with neutral after a minor accident, and technically that is "live" (as in current carrying), so you should have it disconnected under fault conditions. But RCBOs are (in the majority) only disconnecting the line, and don't switch neutral. So if split-con cable is unsafe due to the risk of exposed neutral under failure/fault conditions, how can SP RCBOs be considered safe?

Ladies and gentlemen, discuss!

 

[westward10]

Split concentric during the 16th Ed reign was deemed to be one of the cables which could be buried in a wall without additional rcd protection, this was later removed.