steel conduit-separate earth

M cebee

pdiddy wrote: »

in most buildings with steel trunking conduit and tray they are still used as cpc's because most back boxs are earthed and also trunking is normally earthed where it joins into the main panel but a seperate earth wire is also used therefore you actually have two earth paths one through the conduit etc and the other through the earth cable,thus reducing the risk of one or the other being faulty due to loose bushings or loose connections

ya they're parallel-fault paths and resistance/loop-impedance readings will a combination of the two ,with the trunking taking majority of fault current.

imo it wouldn't be correct to say one is the 'cpc' and the other is an earthed enclosure for cables.

i would work to the industry norm anyway which is separate earth

M cebee

Stoner wrote: »

you also have the issue with people breaking the links with some conduits and trunking, also on top of that most jobs with a decent bit of metal work will have to follow a particular and general specification of a consultant engineering company, most of these specs will insist on a dedicated earth for each circuit.

It's just a no no these days really, shows up in dilapidation reports etc as a failure of existing installation to meet the current regs.
Regardless of what the current regs say, clients pay money to correct it, contractors have their payments held up they use the tray and conduit as an earth on contracts.
If I saw it on a job I hold up up there with an unmarked switchboard as a sign of a potentially poor job, just as an indicator if you know where I'm coming from.

With respect to earthing I've never seen a backward step, it's always more bonding, larger CPCs etc.

tray is not allowed as protective conductor

conduit /trunking is allowed

but i take your point regarding specs.

Bruthal

M cebee wrote: »

ya they're parallel-fault paths and resistance/loop-impedance readings will a combination of the two ,with the trunking taking majority of fault current.

imo it wouldn't be correct to say one is the 'cpc' and the other is an earthed enclosure for cables.

i would work to the industry norm anyway which is separate earth

well its not rocket science, but the seperate earth is more reliable in my opinion, i think a 10sq earth to a 63 amp isolator is more reliable than 3 or 4 conduit couplings, if the bushing is not tight where conduit meets trunking then its a bad earth path.
I have never been on a job where seperate earths were not required. And i dont think its incorrect to call the earth wire the protective conductor as opposed to the trunking. Tray is earthed just the same as trunking, and conduits are often bushed onto bottom of tray, but i think you have said its not allowed as the earth, yet it probably is earthed as much as a trunking, so is it not also now acting as the protective conductor? But yet its not allowed be the protective conductor?

If all conduits are good and tight into trunkings etc then it might be a good earth path, but conduits are often installed with loose bushings, couplers etc, and i dont think a coupler to trunking is that great a connection anyway, and as has already been said the earth wire can of course be also not tightened, but if you went to look for bad earth connections on a circuit, would it be easy to find the loose bushing causing it? and then tighten it.

Anyway the topic was about not requiring seperate earths, so my simple opinion is they should be required, and in time will be by reg`s im sure. Then all of a sudden everyone will say of course they are required becauss the regs say so. Not because its the proper method.

And if one is not the cpc and the other is not the `earthed enclosure for the cables` then im taking up carpentry

M cebee

you would need a separate earth with tray even though it may function as a protective conductor(due to parallel fault path)

whereas 'technically' conduit/trunking can be utilised as cpc on it's own

but i take the general point about separate earth as 'best practice'

i certainly never saw a separate cpc in training or work on steel(early eighties to early nineties).

Bruthal

M cebee wrote: »

you would need a separate earth with tray even though it may function as a protective conductor(due to parallel fault path)

whereas 'technically' conduit/trunking can be utilised as cpc on it's own

but i take the general point about separate earth as 'best practice'

i certainly never saw a separate cpc in training or work on steel(early eighties to early nineties).

A thats the problem, i only started in the late 80`s, although this is now the 2010`s. As someone already mentioned its forwards we want to go not backwards. Showers were ok then without RCD`s, are they now? yet the electricity in them does`t do anything different now than then.

Anyway no one is correct or incorrect here i think. Whats correct today might be incorrect tomorrow even though the item they are talking about is still installed and did`t change in any way.

pdiddy

just a Hypothetical situation lets say i was to wire an installation using trunking and conduit as the cpc all connections tight and perfect earth path,then ten years down the line some of the steel conduits get damaged and is replaced by cheaper plastic conduit therefore now the earth path is broke and will require the extra expense of pulling in extra cables.

yes trunking and conduit are ok as a cpc but think that now a days this is an outdated practice and 99% of electricians will use an earth cable as the cpc