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Posted By Topic: Cable tray/ladder tray earthing

Oct 31 2019 17:59

I've been told by almost everyone I've know to work with the stuff that it needs earthing and many also saying bonding (which I don't know if they literally think it needs bonding or if they are just misusing the term for earthing)

I can find anything to back this up, it definitely doesn't require bonding and I don't see why it would need an earth when everything run on it is double insulated

Is there something I'm missing or possibly was this an old requirement that's now dead? Please point me in the right direction so I can find some info to back this up

Oct 31 2019 18:12

You are correct. Simply installations it doesn't require earthing or bonding if it's only supporting double insulated conductors.

But in some high current situations (from what I've heard), it may be earthed because of induced currents or eddy currents (I don't know which, or why exactly).

Or maybe some engineer just mistakenly wrote on a spec that it must be earthed.

Oct 31 2019 18:27

Lots of sharp edges and cables being pulled past it... I'd want it earthed even if it wasn't a literal requirement.

And engineers are (sometimes unfortunately) entirely entitled to demand things that aren't legally required provided they put it in the spec. You sign a contract to perform the work in accordance with the spec.

Oct 31 2019 18:32

The cable should be protected from the sharp edges. And if you damage a cable pulling it in, if should be repaired or replaced.

Oct 31 2019 18:41

To me it sounds like the classic situation of people still demanding the bonding or bench tops

And the same argument of what if the double insulation breaks down etcetc

But like I said I thought I could be missing something

The induced current and Eddy currents is an interesting one, hadn't considered things like that

Nov 01 2019 12:28

I don’t think you should be comparing two different situations with regard to bonding (cable tray and domestic stainless steel bench tops).

This is a question about electrical safety.

There are many different scenarios where cable tray is needed. From a small shop with a dozen cables to a large industrial situation with many hundreds of cables of all types and varieties.

It only takes one slice through the outer sheath to liven a cable tray.

I wouldn’t be happy to sign off on a COC/ESC when electrical safety is compromised.

Nov 01 2019 13:52

"It only takes one slice through the outer sheath to liven a cable tray."

to liven the tray / ladder requires damage to not only sheath but also conductor insulation.

The rules are clear; if double insulation is maintained there is NO need to earth the tray / ladder.


Nov 01 2019 15:46

I stand corrected.

Cable tray edges are sharp and will slice through sheath and conductor insulation easily (particularly TPS).

Err on the side of caution.

Nov 01 2019 15:59

If cables are installed across sharp edges, then the installation has not been done in a compliant manner [3.1.2 (f)].
The idea is to avoid damage (fence at top of cliff), not to use earthing as means of covering for poor workmanship (ambulance-at-bottom).

Nov 02 2019 20:50

Maybe earthing/bonding of cable trays is not a requirement of the NZ regs, but it is normal practice in most industrial installations in all the countries I worked and is written into the construction contracts. General requirement is solid earthing at one point and bonding straps across bolted joints. All the trays on a cable route are also bonded together.

Nov 02 2019 21:27

I find that interesting.

AlecK's reasoning for tray not to be earthed/bonded is astonishing.


Nov 03 2019 09:00

You both keep saying earthed/bonding

Are you arguing that you need earthing of the tray or are you saying equipotential bonding it because as you know they are both very different things and the conductor sizing for the 2 tasks can be significantly different

I'm just curious to understand your argument fully

Nov 03 2019 21:16

The NZ regs are based around domestic and small-scale commercial users. In the oil/gas, mining, and smelting industries there is the possibility of a small fire or equipment failure causing large numbers of power cables to short to trays and whatever is nearby. In those industries stray voltages are a major hazard due to flammable gases or products.
So, everything is thoroughly bonded together, and earthed. The cost of bonding is a significant part of the cable tray cost, but that is what the customer specifies.

Nov 04 2019 09:32

Firstly, this is not a matter of what "NZ regs" require. The Regs don't set any of these rules.
And they are not based around domestic and small commercial either.

Nor are the installation Standards, which is what we are talking about - with the general requirements being in AS/NZS 3000, where earthing is NOT required provided double insulation is maintained.
That is based on IEC Standards, so is internationally acceptable practice.

True in hazardous areas, more is required. And engineers often believe they know better, and insert additional requirements into specifications. Sometimes they have a point; but more often they just don't know as much as they think they do. In which cases yes, work to the specification or relevant Standard.

The fact is that when installing double insulated cables on conductive tray or ladder in normal situations (eg non-hazardous); there is NO good reason to earth the cable support. Which is why Clause 5.4.6 does not require it in non-domestic installations.
Different in domestic, where structural metalwork must be earthed if it's part of the building's frame.