Support & fixing of cables 3.9.3.3

Post Reply
WhyEr
Posts: 9
Joined: Fri Sep 04, 2020 9:31 am
Has thanked: 7 times
Been thanked: 0

Support & fixing of cables 3.9.3.3

Post by WhyEr »

Debating this rule with a co-worker today & looking at other forums it’s a grey area for some too...hopefully we can settle this once & for all!

Specifically looking at 3.9.3.3 wiring systems likely to be disturbed...copied as below.

Wiring systems installed in the following locations are deemed likely to be disturbed:
(a) On the surface of a wall or on the underside of a ceiling or roof.
(b) In a space between a floor and the ground to which a person may gain entry.
(c) In parts of a ceiling space where access is greater than 0.6 m in height.
(d) Within 2.0 m of any access to any space to which a person may gain entry.
(e) Below raised floors.


In this situation we were specifically debating the need to pin clip cables in an accessible ceiling space.
From my interpretation it sounds to me as long as they are supported at suitable intervals to prevent the undue sagging of cables then they are fine?

There is a mention of mechanical protection but I don’t think it’s applicable....thoughts anyone?

Any comments much appreciated!
JamieP
Posts: 176
Joined: Tue Apr 14, 2020 11:08 am
Has thanked: 26 times
Been thanked: 9 times

Re: Support & fixing of cables 3.9.3.3

Post by JamieP »

3.9.3.1 says "Wiring systems shall be supported by suitable means, in accordance with
Clause 3.3.2.8.
Wiring systems shall be fixed in position, in accordance with this Standard,
by suitable clips, saddles or clamps or by means that will not damage the
wiring system and that will not be affected by the wiring system material or
any external influences."

So every cable must be supported and fixed, but the level required varies depending on location and installation conditions so it's up to you to decide what you deem suitable to meet these requirements

3.9.3.3 gives you a list of situations where cables are likely to be distrubed as you've noted above, cables in these situations need to be "supported at suitable intervals to prevent the undue
sagging of cables"

Generally this indicates that cables in these locations need a higher level of support and fixing than those in other areas

Pins clipping to a supporting timber is one way to achieve support and fixing

None of this is to do with mechanical protection, 3.9.4 covers the requirements for mechanical protection and as stated in 3.9.4.1 "Wiring systems installed in positions where they may reasonably be
expected to be subject to mechanical damage shall be adequately
protected in accordance with Clause 3.3.2.6 and the applicable
requirements of Clauses 3.9.4.2 to 3.9.4.4", it's up to you to decide what locations are danger zones and yo protect accordingly for such location and dangers
These users thanked the author JamieP for the post:
Dan L (Thu Nov 26, 2020 9:24 pm)
Rating: 16.67%
AlecK
Posts: 286
Joined: Thu Apr 16, 2020 11:24 am
Answers: 1
Has thanked: 1 time
Been thanked: 91 times

Re: Support & fixing of cables 3.9.3.3

Post by AlecK »

Exactly.
And 3.3.2.8 sets the minimum fixing as "so as to minimise damage to cable insulation, sheathing and connections during installation, operation, and maintenance".
So for a ceiling space there's no absolute requirement to use any form of fixing at all.
eg for runs parallel to joists; laid across ceiling battens is perfectly OK, as the battens / ceiling provide support, and even in accessible areas generally no-one is going to damage the cables by walking / kneeling in them 'cos the ceiling structure isn't strong enough.
Laid across joists would also be OK for "support" in most cases (depending on spacing); so OK for non-accessible areas.
But in accessible parts of the ceiling, need to provide not just support but also mechanical protection. most common is by placing cable close to structure, or adding blocks; such that such crushing cannot occur - but then then need to clip / tie the cable so it can't move away from the protection and become liable to damage.
WhyEr
Posts: 9
Joined: Fri Sep 04, 2020 9:31 am
Has thanked: 7 times
Been thanked: 0

Re: Support & fixing of cables 3.9.3.3

Post by WhyEr »

Thanks & correct me if I’m wrong - 3.3.2.6 has an asterix next to it which means it’s a new amendment in version 2018 so therefore hasn’t come into force yet?

Also even if it has the wording of 3.3.2.6 is extremely vague & highly open to interpretation as per the paste & copy below....

You could say that in your opinion, as the installer, that the location (b) (even though likely to be disturbed) you deem to provide enough ‘mechanical protection’ - no need for pin clipping - support only.

& using (a) you could also say in my interpretation that the double insulated characteristics of the wiring system (tps cable) also provide sufficient mechanical protection.

Copy of the proposed rule below.

3.3.2.6 Mechanical damage
Wiring systems shall be selected and installed so as to minimize the risk of
mechanical damage.
Protection against mechanical damage shall be provided by one or any combination of the following:
(a) Mechanical characteristics of the wiring system.
(b) Location selected.
(c) Provision of additional local or general mechanical protection.
NOTE: Guide to adequacy and WS classification is provided in Appendix H.
User avatar
DougP
Posts: 145
Joined: Sun Apr 12, 2020 10:11 pm
Answers: 2
Has thanked: 8 times
Been thanked: 21 times

Re: Support & fixing of cables 3.9.3.3

Post by DougP »

3.3.2.6 in the 2018 version is the same as the 2007 version apart from the title of the clause.
The asterix is because the name of the clause changed from "impact" to "mechanical damage.
Appendix H was already in the 2017 version.
These users thanked the author DougP for the post:
WhyEr (Wed Nov 25, 2020 8:39 pm)
Rating: 16.67%
AlecK
Posts: 286
Joined: Thu Apr 16, 2020 11:24 am
Answers: 1
Has thanked: 1 time
Been thanked: 91 times

Re: Support & fixing of cables 3.9.3.3

Post by AlecK »

Yes there's a degree of interpretation needed.
It's a high-level statement; and can't deal with the specifics of every possible situation.
(the book's long enough now!)

But there's no way the double insulation of TPS can be taken as enough, by itself, to cope with being stood on against a hard object like the structure of the building.
N/s would be better, but still liable to be damaged by crushing - especially against a corner.
Conduit would be the minimum sort of wiring system that doesn't need additional protection.

Basically protection against external influences, including mechanical damage,
works on the basis that if something damages the wiring, and that circumstance was likely to happen, then the protection simply wasn't good enough.
So it's up to us to install cables so that they can't be damaged by any likely event.

In an accessible ceiling space; we know it's likely that people will go in, for various reasons.
We know they will inevitably stand on the joists / trusses, 'cos that's the only thing that they can safely stand on.
They're also likely to store stuff in the space.
Even if they can see cables, they(mostly) won't realise that the cables aren't up to being stood on or having stiff piled on top.
And these days the cables are often hidden under a layer of BTI.
These users thanked the author AlecK for the post:
WhyEr (Wed Nov 25, 2020 8:39 pm)
Rating: 16.67%
WhyEr
Posts: 9
Joined: Fri Sep 04, 2020 9:31 am
Has thanked: 7 times
Been thanked: 0

Re: Support & fixing of cables 3.9.3.3

Post by WhyEr »

I agree that double insulation of cables isn’t going to protect anyone from standing on it - But I was merely trying to poke holes in the wording “mechanical characteristics of the wiring system” is deemed an appropriate measure to minimise mechanical damage.

Location was the other one which you can basically interpret how you will - meaning in my opinion the original argument of having to pin clip or not is about as open as a $2 Hooker’s legs.

Anyway in future I’ll prob pin clip around the man hole to be safe (I alreadyrun under joists etc normally) - but I still think the waters pretty muddy on this one.
AlecK
Posts: 286
Joined: Thu Apr 16, 2020 11:24 am
Answers: 1
Has thanked: 1 time
Been thanked: 91 times

Re: Support & fixing of cables 3.9.3.3

Post by AlecK »

Poking holes is not a good approach
Better to try to understand the principles - and that's exactly what the words give you.
The mechanical characteristics of the wiring system must be taken into account when determining whether, in a particular situation, additional mechanical protection is required.
So must location.

Clearly some wiring systems offer more protection than others against any particular type of damage (effect of external influence) that may be expected in any particular location.
And equally clearly the same wiring system may require varying degrees 7 types of protection in different locations.

The wiring rules cannot be specific about every possible combination of wiring systems type locations.
So they just provide the principles and we have to apply them
And if we get it wrong by under=protecting, we can be held to account.
Post Reply