Dado 3 compartment or Maxi trunking - home office - use singles?

[Iggifer]

And to answer one of the original question that seems to have been missed.

You will need to use 3 compartment dado trunking, or 2 sets of maxi (or one with a divider) to segregate your mains voltage and extra low voltage cabling. You cannot run data/video and 230v cables in the same containment system (not that that ever stops anybody, but that's a whole different kettle of fish)

 

[flameport]

ref earthing, thanks for the link, there is no industrial equipment, all domestic and portable, so nothing designed with 3.5mA leakage or higher.

The point is that additional earthing arrangements are required if the total protective conductor current for the circuit exceeds 10mA - the type and portability of the individual items doesn't matter.

Given you will have 10 double outlets and potentially 20 items plugged into them, it is very likely that the total will exceed 10mA. Plus the socket outlets for the desk/floor lighting, and whatever else is already connected to the circuit.

 

[333rocky333]

I would not be surprised if switching that amount of loads trips the rcbo anyway

 

[JohnW2]

I would not be surprised if switching that amount of loads trips the rcbo anyway

We don't know any details of the loads, but we do know that the proposal is that all three branches, each with a switch, would be fed from a single 20A circuit, so the loads switched by each of the switches are presumably pretty low. RCDOs/RCBOs tripping when things are switched on or off is a rare, difficult-to-understand and much discussed phenomenon at the best of times, and I personally would very much doubt that it would be an issue with what the OP proposed.

Kind Regards, John

 

[JohnW2]

The RCBO / RCD might not trip, but there is also the issue of the protective conductor current in the circuit: http://electrical.theiet.org/wiring-matters/26/5437-earthing.cfm?type=pdf

That's true, but I never have really understood the requirements of 543.7 as regards 'high integrity earthing'.

The main ways (certainly the main ways likely to be implemented in a domestic setting) of satisfying the requirement for 'high integrity earthing' is for the circuit to have a CPC of ≥10mm², two CPCs totalling ≥10mm² or a mechanically-protected ≥4mm² CPC. That really makes little sense to me. The resistance/impedance of the CPC is essentially irrelevant - even 30mA leakage through a CPC with a ('ridiculously high') resistance of 10Ω would only raise the potential of the CPC and any connected exposed-c-ps by 0.3V. Even if the circuit were not RCD protected and had a (ridiculous, in normal functioning) leakage of 1A, the potential would only rise by 10V !

The actual real risk is that the CPC might somehow become disconnected from earth, in which case the 'leakage currents' could represent a real risk of electric shock (if one could find something at earth potential to touch simultaneously). However, I do no see that a 10mm² CPC, or a mechanically-protected 4mm² CPC, is significantly less likely to become 'disconnected from earth' than is a 1.5mm² one, can you? Am I missing something?

Furthermore, if there were a "disconnection from earth" between the earth bar of the CU/DB and 'earth', then the same problem would exist in all circuits in the installation, yet requirements for 'high integrity earthing' would only apply to circuits with a known/suspected high leakage.

Hence, whilst I understand the potential risks being addressed, I really don't see that the regs address them significantly or satisfactorily - do you? Again, am I missing something?

In the domestic setting, the most important thing is that the circuit is RCD/RCBO-protected. Such a device would at least operate if there were a leakage current (even if through a human being, rather than CPC) of, at most, 30mA (in practice usually apprecially less).

Kind Regards, John

 

[ban-all-sheds]

I was planning to use 3 compartment Dado trunking from screwfix, but then spotted that I could use Maxi trunking, any recommendations?

I recommend sticking with trunking which is properly compartmented, rather than messing about with separate dividers.

The trunking will be running at skirting level for most of the room, with a tee and riser to just above desk height and the grid switches.

There's a reason why most offices do not have the sockets used by people sat at desks at floor level.

I was thinking of running the various circuits in 2.5mm T&E radials from the CU which is about a 10m run.

I thought you said they were existing circuits?

 

[ban-all-sheds]

That's true, but I never have really understood the requirements of 543.7 as regards 'high integrity earthing'.

Indeed, and I agree.

But I do hope that this is not another instance of you deciding that you can just ignore any requirements which you do not understand, simply because you do not understand them.

 

[JohnW2]

That's true, but I never have really understood the requirements of 543.7 as regards 'high integrity earthing'.

Indeed, and I agree.

But I do hope that this is not another instance of you deciding that you can just ignore any requirements which you do not understand, simply because you do not understand them.

Not consciously or deliberately. However, I've never installed a 'high integrity earthing system' in my life, so if I have dealt with any circuits which have come to have a total leakage >10mA (which I probably have), then I probably have, in some senses, 'ignored' the regulation. In any event, in this case it's not that I "don't understand the requirements" - it is, as I said that I do not believe that the requirements in any way adequately or satisfactorily address (or significantly diminish) the hazards which that apparently seek to address/diminish. As a result, I don't think I'd lose too much sleep if I discovered that I had ('non-deliberately') 'ignored' the requirements of the regulation.

In any event, it's the usual problem of the impossibility of properly designing sockets circuits, since, particularly in a domestic setting, one usually has no idea as to what subsequently is going to be, or might be, plugged into what socket. Indeed, I would imagine that a fairly high proportion of domestic sockets circuits out there were designed and installed before domestic premises started filling up with devices that produced L-N 'leaks' (such that a total >10mA became quite possible).

I wonder how often, if ever, the average electrician installs a high-integrity earthing system in any domestic property?

But what about my questions? One can satisfy 543.7 merely by incresing the CSA of the CPC - and I really don't see that that significantly helps anything, or significantly diminishes the hazards. Am I missing something, or is the reg just daft?

Kind Regards, John

 

[flameport]

I wonder how often, if ever, the average electrician installs a high-integrity earthing system in any domestic property?

Probably fairly often, as many properties have socket outlets on a ring final circuit.

But what about my questions? One can satisfy 543.7 merely by incresing the CSA of the CPC - and I really don't see that that significantly helps anything, or significantly diminishes the hazards. Am I missing something, or is the reg just daft?

Larger sizes (10mm) are far less likely to be physically damaged and broken, same with 4mm which is mechanically protected.
In the absence of such sizes, two smaller conductors are acceptable as it's unlikely that both would simultaneously be damaged as they would be separate items.

 

[endecotp]

I never have really understood the requirements of 543.7 as regards 'high integrity earthing'.

The main ways (certainly the main ways likely to be implemented in a domestic setting) of satisfying the requirement for 'high integrity earthing' is for the circuit to have a CPC of ≥10mm², two CPCs totalling ≥10mm² or a mechanically-protected ≥4mm² CPC.


I do no see that a 10mm² CPC, or a mechanically-protected 4mm² CPC, is significantly less likely to become 'disconnected from earth' than is a 1.5mm² one, can you? Am I missing something?

My assumption was that they think you would be unlikely to sever one of those thicker or protected CPCs without noticing what you'd done. Thinner wires could be severed while e.g. drilling into a wall without noticing.

 

[JohnW2]

I wonder how often, if ever, the average electrician installs a high-integrity earthing system in any domestic property?

Probably fairly often, as many properties have socket outlets on a ring final circuit.

I asked for that one - I should have added "for a radial circuit" . I was talking specifically of 543.7.1.203 ('General') which also applies to all sockets circuits, not 543.7.2.201.

But what about my questions? One can satisfy 543.7 merely by incresing the CSA of the CPC - and I really don't see that that significantly helps anything, or significantly diminishes the hazards. Am I missing something, or is the reg just daft?

Larger sizes (10mm) are far less likely to be physically damaged and broken, same with 4mm which is mechanically protected. In the absence of such sizes, two smaller conductors are acceptable as it's unlikely that both would simultaneously be damaged as they would be separate items.

Hmmm - that might perhaps be an argument with singles (although they would be protected in conduit) but is it not almost "vanishingly improbable" that the CPC within a T+E cable would break (without something far more catastrophic also happening) - surely far, far less likely than that the terminations of the CPC would be become disconnected (or poorly connected) (in which case a higher CSA would be of no assistance).

Kind Regards, John

 

[SimonH2]

I wonder how often, if ever, the average electrician installs a high-integrity earthing system in any domestic property?

Probably fairly often, as many properties have socket outlets on a ring final circuit.

Yes, but just being a ring doesn't actually meet the requirements for a high-integrity earthing system.

Even if they connect the two CPCs to the two separate terminals in the sockets, they then usually cock it up by putting both ends into one terminal in the CU.

However, I recall this coming up at work some years ago. The electricians refused to fit double sockets as this was apparently "not allowed". They couldn't really explain it and suggested I ring the local NIC chap who could. I then went and measured a representative selection of our IT gear and couldn't find a single item with a "high" leakage current - told the contractors who then fitted the number of (double) sockets we needed.

 

[ban-all-sheds]

Yes, but just being a ring doesn't actually meet the requirements for a high-integrity earthing system.

No, you need 2 cpc rings.

 

[a546345]

Hi all, fascinating discussion as always on this forum. Thank you.

Just to give an update on what I'm going to do.

1. Marco 3 compartment skirting trunking used as skirting board around the room at floor level. The room has been re-plasterboarded so it needs skirting anyway. I can't bring the trunking higher as there will be a radiator in the way.
2. Marco 3 compartment dado trunking just above the desk and extend round to where the light switch should be near the door.
3. Extend the existing power and lighting radials to Varilight grid switch housings within the trunking, and then distribute from there within the trunking using T&E.
4. Test as much as I can myself, then have my friendly electrician complete the testing when I have some other work done in a couple of months. I'll find out the earth leakage current!

The total loads are fairly low. Less than 500w. The biggest load will be if I plug a hoover in, but that wouldn't happen very often!

 

[JohnW2]

Yes, but just being a ring doesn't actually meet the requirements for a high-integrity earthing system.

No, you need 2 cpc rings.

Where does that come from? It would seem that a 'standard' ring final circuit (with a single, usually 1.5mm², 'CPC ring') would satisfy 543.7, by satisfying 543.7.1.203(iii) and 543.7.2.201(i), so long as it has the 'separate terminations' required by 543.7.1.204.

What, if anything, am I missing?

Kind Regards, John