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Posted By Topic: Earthing metal flushboxes

Dec 29 2019 15:58

What is the preferred method or easiest way to connect a PEC to metal flushboxes?

People I work with don't earth them but I thought you'd have to due to 5.4.6

Dec 29 2019 18:00

It's not a structural or building material so 5.4.6 doesn't apply.

It's also not an exposed conductive part as long as the fitting is properly installed.

So the current versions used with modern fittings don't require earthing as long as they aren't exposed or able to be touched.

Dec 29 2019 18:20

I totally agree with the not exposed metalwork

But I thought it would come under the classification of a building material but maybe I'm the one who is wrong

Dec 29 2019 18:21

*exposed conductive part

Dec 29 2019 20:16

They aren't a building material.

Dec 29 2019 22:22

Would you be kind enough to give a definition of building material? For this purpose

Or give some examples of what you consider a building material?

Dec 30 2019 06:55

Simply the flush box is not part of the building material, but following thru on the assumption that it is then every nail and screw hold *any* flush box in would also have to be earthed.

Yes it's metal, but they are not exposed or accessible under normal conditions so no requirement to earth. this doesn't stop you from earthing them if you want to as you are allowed to exceed the minimum standards set by AS/NZS300

Dec 30 2019 08:47

Well I'm so glad I was wrong about this one haha really didn't like the idea of having to earth them so it's music to my ears

Dec 30 2019 10:32

Actually I don't think you were wrong. And it's a very good question.

It comes down to whether a flush mount (box or open-back) for an electrical fitting falls within the term "building materials.
I believe the term covers all materials used not just for the structure of the building, but also the services installed in the structure - including plumbing, electrical, & other services. And on that basis, if the mount is conductive, then it has to be earthed.

Such mounts are a unique case, due to their location in close proximity to unsheathed conductors. Unlike, for example, a TPS crossing a gang-nail plate, double insulation is not maintained [ (b)] and there IS a risk of contact (of the mount) with insulated, unsheathed cable and even, under fault conditions - eg broken-off wire - with live parts [ (a)].

The rules about what does, and doesn't need to be earthed have changed quite a lot since I stated in the trade. But the current clause has not been changed substantially since it was introduced in 3000:2000. However it only became mandatory from 1/4/10; before that the Regs only mandated some particular clauses of the standard, and this wasn't one of them. So many practitioners just kept on doing what they were used to doing. Others followed the "deemed safe if complies with" route, and changed our methods.

The point that flush mounts are not generally accessible is true, but not relevant; because the rule applies to ALL building materials not only to materials that are accessible after completion.

They could also be regarded as "electrical equipment", and therefore need to be earthed under; though under that rule accessibility is an issue via the definition of "earthed situation".

Bottom line, whatever each of us decides, we need to be confident of being able to justify our decision if ever called to account, eg by EWRB.


Dec 30 2019 12:07

That's basically what I thought

But like you said, whether it's classes as a building material or not makes all the difference

But this leads me back to part of my first question that is, if they do need to be (which I'm not saying they are or aren't), what do you believe is the easiest way to achieve such an earth, for those of you that are doing so?

Dec 30 2019 13:59

IMO, that wasn't the intended meaning of the term "conductive building materials" as used in the clause.

Dec 30 2019 17:04

Sometimes intent is clear. Other times less so.
It's possible the writers didn't even think about flush mounts. Equally possible that they did, and decided that metal mounts should be earthed, just like any other conductive building material; and also just like any conductive enclosure for electrical connections. Probably the only thing we can be certain of on this one is that they didn't make a firm decision the conductive flush mounts don't need to be earthed; 'cos if the6y'd reached that sort of decision they'd have said so clearly.

Dec 30 2019 20:54

so there's another days labour and a drum of 6.0mm to bond all the structural steel in a house nowadays. Ridiculous.

Dec 30 2019 22:12

Earth, not bond

And 2.5mm2 earth would be sufficient for most situations, as cables would usually be 6mm2 or less

Provided that such earthing is required

Dec 31 2019 07:29

If it's a house, all structural metalwork that's part of the frame has to be earthed regardless of what type of flush mounts are used. Which - as I said above - raises the issue of whether steel ceiling battens are structural framework. I believe they are. And when they are used in an otherwise wood-framed house, yes it's a real pain to earth them all (but still far less work than clipping all cables as we used to).

Dec 31 2019 22:37

AlecK: I agree with you saying that all conductive building materials should be earthed.
But, in , in November, you argue against cable tray being earthed. Seems a little inconsistent to me.


Dec 31 2019 22:56

Kings well, I agree. You go to all that trouble replacing a cable and not replace the outlet at the same time.

Having said that though an observation of the light switches: some of them have outlasted the more modern equivalents (Clipsal and HPM) and are a 50 years old and have turned on and off tens of thousands if not hundreds of thousands of times over the years.

Dec 31 2019 22:57

Whoops, wrong thread.


Jan 01 2020 12:41

With cable tray, in most cases double insulation - between cable(s) & tray - is maintained, so no need to earth it.

My personal feeling is that non-accessible flush mounts shouldn't need to be earthed. But Wiring Rules aren't about my - or anyone else's - personal feelings; they're about what the words say.

I'm applying exactly the same rule for both cable tray and flush mounts, treating both as conductive building materials. The difference is that with cable tray double insulation is maintained, while for flush mounts it isn't.

Jan 01 2020 19:08

I'd be thinking a cable tray having x amount of cables on it would have a higher chance of becoming live than structural metal work which don't have any contact with cables (not in the houses i've done anyway) and are often nearly fully encased in timber. What's the thinking behind the structural metal work - specifically steel beams? I've always earthed rondo purely because its likely that someone will come into contact with the steel - cutting out down lights and looking for cables etc...

Jan 01 2020 20:01

Structural metalwork doesn't have to be earthed unless there's a risk of contact with single insulated cables. That includes rondo battens which don't have to be earthed if double insulation is maintained.

Other than than the frame only, of a domestic dwelling which must be earthed.

Jan 02 2020 11:10

The second part of the 'structural metalwork" earthing clause covers the deemed higher risk for domestic. And when earthing is required, the size of earth conductor is driven by the largest size of active that could make contact - so for flush boxes, earth them using the PEC(s) of the circuit(s) concerned.

There is another complication, which is that any metalwork in electrical contact with mass of earth will need to be bonded; to avoid differences in potential between the metal and parts that are earthed.

Jan 02 2020 14:40

I recall one brand of metal flushbox had a brass tunnel type earth terminal built into the box.

Jan 02 2020 20:36

The Voltex Fire/ Acoustic Rated Wall Box has an earth terminal.

Jan 26 2020 18:05

@Mazdaman I used some of those recently, what are your thoughts about the cable entry on them and the lack of holes for fixing to framing?


Jan 26 2020 18:37

Hi, I have not used any and will return them. As they cannot be screwed to stud etc the idea of making a neat fitting hole for over 50 boxes in gib up to 38mm thick while keeping cables out of the way did not appeal. The cable slots are too small as well. I asked about screw fastening but apparently that detracts from the performance.

Jan 26 2020 18:39

My thoughts too, I also thought that the edges on the cable entry holes were a little on the sharp side.