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Topic Title: Cable ladder
Topic Summary: Loading/recommendations of heavy duty cable ladder
Created On: 08 November 2017 12:40 AM
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 08 November 2017 12:40 AM
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basil.wallace

Posts: 251
Joined: 01 April 2006

I've been told by Legrand this afternoon that my choice of 150mm wide heavy duty cable ladder would normally be subject to loading/recommendations.

Would the loading/recommendations apply to using 4 core XLPE/SWA/LSF cable based on the size and number of cables that are mounted on the cable ladder?

Any suggestions would be greatly appreciated.

Basil Wallace PgDip MIET EngTech
 08 November 2017 12:46 AM
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mapj1

Posts: 9568
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The key parameters are really how much weight you are putting in the tray - this will determine the form of fixings, and the maximum unsupported span you can have between supports. Secondary but not by much are the cable loading and dimensions

-------------------------
regards Mike
 08 November 2017 02:25 AM
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basil.wallace

Posts: 251
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Do you really mean that this could also apply to cable ladder?

I'm not using cable tray.

Basil Wallace PgDip MIET EngTech
 08 November 2017 08:22 AM
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Jaymack

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Originally posted by: basil.wallace
I've been told by Legrand this afternoon that my choice of 150mm wide heavy duty cable ladder would normally be subject to loading/recommendations.

Manufacturers in my experience, Unistrut etc. have loading tables for their particular product, with supports/m, span lengths and permissable loadings/m. For dynamic loadings, the possibility of the installer/s, sitting, walking or standing on the ladder should also be designed in; this was 70kg/person again in my experience, when designing installations.

Regards
 08 November 2017 09:21 AM
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mapj1

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well, now we know what you weigh ... But you are quite right, depending on location the cable support, be it tray basket or open ladder often becomes a support for an arm or a foot, or a ladder is leant against the side to change a lamp or hang Christmas decorations. It may not be the designers intention, and it may well break the local H and S policy, but it does not stop nasty questions being asked if it all falls out of the sky, so the question about what if needs asking at the design stage.

-------------------------
regards Mike
 08 November 2017 12:15 PM
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basil.wallace

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Originally posted by: mapj1

well, now we know what you weigh ... But you are quite right, depending on location the cable support, be it tray basket or open ladder often becomes a support for an arm or a foot, or a ladder is leant against the side to change a lamp or hang Christmas decorations. It may not be the designers intention, and it may well break the local H and S policy, but it does not stop nasty questions being asked if it all falls out of the sky, so the question about what if needs asking at the design stage.
 08 November 2017 12:18 PM
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basil.wallace

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Many thanks for the information. Your feedback have provided a sound foundation on which to start building and expanding my working knowledge of cable ladder. This would also be useful for cable tray.
Do I need to refer to the site guide?

Basil Wallace PgDip MIET EngTech
 08 November 2017 07:09 PM
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OMS

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Typically the two considerations are

deflection between supports (usually in mm) - this needs you to know your loads per m run

From there, you need to know the total static load, imposed on the number of fixings for the agreed deflection - this sizes your support brackets/drop rods etc (usually with a x 3 factor of safety to allow for occasional additional loads

If you also need to contend with dynamic loads (eg where your containment needs a seismic withstand) then typically you won't need to increase the carrying capacity, but you will need to deal with deflections/swaying etc in all 3 axis of movement with additional restraints/support bracketry

As Jaymack mentioned, get some data tables for cable tray - the process is quite straightforward (until you need to specify the final fixing into the soffit or similar - then you usually need to interact with a structural engineer)

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 09 November 2017 12:34 PM
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basil.wallace

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OMS

I've had a look at the latest cable tray and cable ladder product technical guides from Legrand.

With regards to the use of loading graphs, both technical guides states that the cable support systems are not designed for use as walkways or to support personnel during cable installation.


Basil Wallace PgDip MIET EngTech
 09 November 2017 02:54 PM
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OMS

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Originally posted by: basil.wallace

OMS

I've had a look at the latest cable tray and cable ladder product technical guides from Legrand.

With regards to the use of loading graphs, both technical guides states that the cable support systems are not designed for use as walkways or to support personnel during cable installation.

Basil Wallace PgDip MIET EngTech


Sure - but you are the designer, not Legrand - it's up to you to ascertain if they will credibly be misused during installation, operation, maintenance, repair, alteration and eventual decommissioning - and if you have to design for that condition - CDM regs etc

regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 09 November 2017 03:39 PM
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mapj1

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My dining chairs are not designed to withstand being used as an adventitious support or working "platform of opportunity" during the replacement of failed lamps. However, its not unknown for it to happen.

Some imagination is needed to avoid responsibility when it comes to 'foreseeable' accidents, 'the maker said it was unsuitable' is not going to help you, it may actually make it worse if you knew it was unsuitable and yet in that situation, likely to be used like this, and fitted it anyway.

For such reasons we may end up fitting extra anchor bolts or brackets that are only needed to make installation (or reasonably foreseeable maintenance) easier. Much nicer than later finding that to change the fitting some poor fella has to drill a line of holes across the concrete ceiling while suspended above a stair well.

-------------------------
regards Mike
 09 November 2017 04:01 PM
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pww235

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While I can see the point of adding a bit of bunse to account for potential misuse; surely if someone were to come a cropper due to relying on the containment system to support themselves while working at height, the problem lies squarely at their feet as they are not following safe working practices. CDM and H&S swings both ways, we can't always design out stupidity. Working at height regulations exist for a reason.
 10 November 2017 11:07 AM
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basil.wallace

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Gents,
All your comments regarding cable ladder are greatly valued and appreciated which I will certainly take on board.

At the end of the day, we can't always design out stupidity but as long as we stick to CDM 2015 Industry Guidance for Designers by CITB along with HSE Working at Height Regulations with good judge for each situation, a better installation solution can be achieved.

Basil Wallace PgDip MIET EngTech
 10 November 2017 05:01 PM
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leckie

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So is it a question of quantifying what is going on the ladder, getting the total weight per metre, adding a multiplication factor of x3 for safety, and then checking what the ladder manufacturers recommendations are?

How do you know what load the fixings can take? Say you were fixing ceiling anchors with say a 6mm bolt into a concrete ceiling, wouldn't it depend on the concrete type, mix, etc? How would you know that?

I have been looking through a specification on this very thing, and it gives some quite technical sounding information on the loads per metre, etc. But then it adds that any tray or ladder below 3m in height should be strong enough to be able to withstand a fully grown man swinging off it! Well it gave me a laugh anyway. How on earth are you supposed to sensibly assess that?
 10 November 2017 05:13 PM
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rocknroll

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Find a fully grown 'man' and test the theory, according to my other half men never grow up.

Regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 11 November 2017 09:56 AM
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basil.wallace

Posts: 251
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Originally posted by: leckie

So is it a question of quantifying what is going on the ladder, getting the total weight per metre, adding a multiplication factor of x3 for safety, and then checking what the ladder manufacturers recommendations are?


In this case, it would be sub-main cabling using multicore XLPE/SWA/LSF type cables The client has insisted that all sub-main cabling must be laid flat and cleated to the cable ladder. No other services will be allowed to be installed on cable ladder carrying sub-main cabling. Finally, 25% spare space must be left on the cable ladder rack.

For TPN, I normally use Batt Cables for the following 4-core XLPE/SWA/LSF cables:

25 sq.mm based on weight (kg/km) of 2100
35 sq.mm based on weight (kg/km) of 2580
50 sq.mm based on weight (kg/km) of 3000
70 sq.mm based on weight (kg/km) of 4300
95 sq.mm based on weight (kg/km) of 5510
120 sq.mm based on weight (kg/km) of 7150

For SPN, the 2-core XLPE/SWA/LSF cables would be based on the following:

25 sq.mm based on weight (kg/km) of 1060
35 sq.mm based on weight (kg/km) of 1480
50 sq.mm based on weight (kg/km) of 1800
70 sq.mm based on weight (kg/km) of 2300
95 sq.mm based on weight (kg/km) of 3170
120 sq.mm based on weight (kg/km) of 3800


Basil Wallace PgDip MIET EngTech

Edited: 11 November 2017 at 11:18 AM by basil.wallace
 11 November 2017 10:36 AM
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AJJewsbury

Posts: 16007
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25 sq.mm based on weight (kg/m) of 2100

etc.

Eh? Over two tonnes per metre? What's the conductor - depleted uranium?

- Andy.
 11 November 2017 10:46 AM
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basil.wallace

Posts: 251
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Plain annealed stranded copper. Please refer to link:

http://www.batt.co.uk/upload/f...-1000v_1507711736.pdf

Basil Wallace PgDip MIET EngTech
 11 November 2017 11:00 AM
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AJJewsbury

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I see - kg/km not kg/m !
- Andy.
 11 November 2017 11:06 AM
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GeoffBlackwell

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Sometime ago I designed the supply to a reasonably large car body press. The installation consisted of 2 * 120mm² 4 core SWAs, these were mounted on a new tray that we installed so that we could space the cables by 2 * D to improve current carrying capacity.

All went well - the contractors I was working with were long term associates and I had no doubts about their ability to carry out the work.

Not long after it was commissioned we received a phone call telling us that a section of tray had collapsed. When we got to site we found that a considerable number of additional cables had been installed on top of the cables that we had specifically spaced! So many cables that the tray fixings were overloaded (helped by any number 'grown men' stomping all over it).

This work was carried out by a major national contractor. After the usual slanging match they accepted responsibility and had to reinstate the installation and re-route all of their cables.

The best laid plans of mice and men .

Regards

Geoff Blackwell
IET » Wiring and the regulations » Cable ladder

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