metal conduit

[Casterbridge]

Not used metal conduit before -

http://www.tradingdepot.co.uk/DEF/p... Accessories/20mm Galvanised Tee Junction Box

with these Tees do you simply just use a termianl strip?

http://www.screwfix.com/p/30a-termi...tracking url&gclid=CIKJ49Pa2rsCFUzHtAodvmAAfQ

??

 

[plugwash]

You can use a terminal strip if you have to but afaict normal practice would be to pull the cables through unbroken.

 

[Spark123]

Yep, there is no need to break the live/neutral conductors in a conduit tee box, the wires usually run unbroken all the way between accessories etc.
The earths on the other hand need to go into 4mm dia ring crimps and use an M4 screw to terminate into the tee box.

 

[plugwash]

The earths on the other hand need to go into 4mm dia ring crimps and use an M4 screw to terminate into the tee box.

Hmm, I was under the impression that strictly speaking you didn't have to run earth cores in metal conduit at all, the conduit itself was considered an acceptable earth.

 

[Spark123]

I haven't seen any recent installs without an earth wire, I wouldn't like to rely on metal conduit as the sole CPC - too much in my eyes to go wrong - top hats, corrosion, mechanical damage etc.

 

[mfarrow]

Using conduit as a cpc is perfectly acceptable by BS7671. GN8 clarifies this and states:

Metal conduit (and also steel trunking and ducting) has traditionally been used as CPC for many years, but more recently it has fallen out of favour, with many designers adopting a 'belt and braces' approach to using a separate CPC contained within the wiring system. It has to be said that in many, if not most, cases the additional separate CPC is wholly unnecessary. Additionally, it has to be recognised that a steel containment system is required to be effectively earthed where the containment cables are not sheathed. Thus, it is important that joints in the containment system be mechanically and electrically sound even where a separate CPC is employed.

A metal conduit often has a large enough csa to allow it to be used as a CPC for the circuits whose wiring it contains.

So that's clear. If you're not choosing to employ "Double or Reinforced Insulation" when designing your containment system (i.e. insulated & sheathed cables or insulated cables in PVC whatever) then you're probably employing "Automatic Disconnection of Supply" and the method of executing that choice of fault protection (i.e. steel conduit) needs to be effective earthed. And if you're effectively earthing that system, it may as well form your CPC for the contained circuits.

 

[mfarrow]

I wouldn't like to rely on metal conduit as the sole CPC - too much in my eyes to go wrong - top hats, corrosion, mechanical damage etc.

When running singles in it though, it needs to be designed to remain effectively earthed in case of a fault of the basic insulation. If it isn't, it has the potential to become an exposed live part, with no fault protection. Before that happens therefore, you need to redesign the containment system so it's suitable for its environment.

 

[Lectrician]

You only need to earth the conduit at one location, not all.

It is common practice to run a separate CPC these days, especially if not using serrated washers at accessories.

 

[RF Lighting]

I've found plenty of accessories without an effective earth supplied by steel conduit. Usually due to slack bush connections or corroded joints.

I always run in a seperate CPC as well as earthing the conduit. Belt and braces, just for my own piece of mind, and given the quality of bushes these days, I don't fell like it's a truly reliable connection any more.

 

[Adam_151]

I've found plenty of accessories without an effective earth supplied by steel conduit. Usually due to slack bush connections or corroded joints.

Or lack of removal of paint on accessories, had one in a school one supplying a few sockets in a classroom and one in a corridor, after I'd scraped the paint from under the bushes and put it back together the site manager then told me that the cleaner had been complaining about shocks from her (presumably class 1) vaccumm cleaner when cleaning that corridor

 

[maltaron]

Surely the "rusty bolt syndrome" is relevant here. The threads on conduit are not galvanised!

 

[securespark]

In my day (when I were a wee laddie apprentice) we connected the conduit to earth at source (the DB!) and screwed all joints together well. Before wiring the conduit, we tested it with a high current tester to ensure everything was soundly screwed up.

If there were any poor joints, you could hear a fizzing noise and sometimes see sparks flying as well!

 

[Casterbridge]

Thanks - I ask because I am thinking about running parallel lights from an existing ceiling rose and didn't want to run the cable above the ceiling - instead I was going to use these below the ceiling??

 

[sparkwright]

Have seen some worrying effects on badly coupled metal conduit. Also seen galv conduit corroded away to buggery. Always run a separate cpc on all new conduit work, and where reasonably possible on remedial work.

 

[securespark]

Like to see some pix of that SW...

Are you saying the badly corroded conduit was not due to adverse conditions?

What are the worrying effects you have seen?