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Topic Title: BS6396 - Earthing Cable Basket for Office Desk
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Created On: 22 January 2019 09:26 AM
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 22 January 2019 09:26 AM
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Jallyman

Posts: 3
Joined: 20 July 2014

Hi Everyone,

I'm currently working on a specialised desk which has a cable basket holding the standard office desk cables, desk power unit/PDU (6 socket power strip) and an energy monitoring device known as the Sonoff Pow R2.

I would like to know if it is necessary to earth the cable basket using the earthing lug from the power unit.

The part in BS6396 I would like clarified is section 5.1.2. Which says that earthing is not required if equipment has double or reinforced insulation. How do I tell if the Sonoff or Power Unit is double insulated if there is not an identifiable symbol? Should I just assume the cable basket requires earthing?

Links:

Sonoff Device Image

Example Power Unit Image

Any help would be appreciated!

Thanks,

Jasper Gillespie

Edit:
The Sonoff case is plastic but the cables are held in by push pin terminals which I'm worried will allow the cables to slip out!
 22 January 2019 10:54 AM
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mapj1

Posts: 12039
Joined: 22 July 2004

Is it possible for someone to come along and plug in another item to this desk that is class I, that is earthed rather than doube insulated ? (computers of certain types, most desk lamps are class II these days.)
It it credible that a mains cable may be pulled and snagged in the cable tray, and the insulation on the cable damaged ? this will depend on the design of any smooth or sharp edges, and how cable grip is organised.
If the cable tray came live, would it be accessibe to the user ?
The fall back option to these is that earthing the tray is the safer option.

More serilously, there needs to be a proper cabpe grip somewhere to transfer any tugging force to the jacket of the cable and not the contacts - either a saddle clip like you see in a mains plug,


or a stuffing gland,

or perhaps if there is a suitable hole in metal, then a strain relief bush



-------------------------
regards Mike
 22 January 2019 11:19 AM
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

I'm not sure of the context of the BS 6396 quote, but the basic principle is that only parts that are liable to be made live by a single fault (e.g. breakdown of basic insulation) need earthing. In general the insulation and sheath of flexible cables count as double/reinforced insulation and any other equipment should be encased either in an insulating enclosure (class II) or earthed metal (class I) - both of which normally prevent anything coming into contact with them being made hazardous live (be it metallic basket or human finger or a metal barrelled pen that's rolled off the back of the desk) . In general you'd only need to earth things like the tubular steel frames of desks where they contained unsheathed conductors (i.e. they were also acting as earthed steel conduit) (although it's possible that BS 6396 has additional requirements).

To reinforce Mike's point - if you think it's plausible that the basket could be made live by a loose wire or other fault, you've probably got a bigger problem (i.e. the hazard of being able to touch a hazardous live part directly) - solve that and the need to earth the basket probably goes away.

- Andy.
 22 January 2019 11:53 AM
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Jallyman

Posts: 3
Joined: 20 July 2014

Mike & Andy,

Thank you for the replies. The Sonoff device does have a proper cable grip it is just not shown in the linked image, so then any loose wires should be mitigated by that.

My next question does a piece of equipment with an insulating enclosure automatically assume it is a class II piece of equipment? I have read on multiple websites that a class I piece of equipment can actually have a plastic casing. I am finding it hard to find the exact definitions of class I and II pieces of equipment.

It might seem like a stupid question, but I am genuinely struggling to find an answer I fully trust.

Thanks,

Jasper
 22 January 2019 12:17 PM
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broadgage

Posts: 3170
Joined: 07 August 2007

MOST class 2 appliances will have a case of plastic or other insulating material.
It is possible to manufacture class 2 appliances with a metallic case, this requires that internal live parts be protected by double or reinforced insulation. Some power tools and some light fittings are of metal yet class 2 construction.

There is no simple way to verify if such appliances ACTUALLY have double or reinforced insulation, or if the double insulated marking has been fraudulently applied.
 22 January 2019 12:32 PM
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

My next question does a piece of equipment with an insulating enclosure automatically assume it is a class II piece of equipment? I have read on multiple websites that a class I piece of equipment can actually have a plastic casing. I am finding it hard to find the exact definitions of class I and II pieces of equipment.

Generally if it needs an earth connection it's class I, if it's class II it should have a 'square inside a square' (double insulated) symbol on it somewhere. (But care is needed as some double insulated equipment provides an earth terminal for convenience of installation or connecting downstream class I equipment, even though it doesn't need an earth connection itself to be safe). There's also Class 0 equipment which is neither earthed nor double insulated - which is normally not permitted, but still occasionally found, and Class III where extra low voltage (SELV/PELV) is used.

As broadgage says appearances can be deceptive - Class II equipment can have metallic outer cases provided there's still double/reinforced insulation around live parts internally and Class I equipment can have an insulating outer case; or a single item of equipment might use a mix of methods in different parts (e.g. a metal box with a plastic lid) - but then the lower classification normally applies.

- Andy.
 22 January 2019 01:52 PM
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gkenyon

Posts: 5354
Joined: 06 May 2002

Just wondering why Basket was selected in the first instance, and not slotted trunking or similar?

Data cables (Cat 5, Cat 6, etc.) generally require a mat to be used with basket, to prevent the cables distorting over time (and degrading their performance).

You might see similar issues with flexible mains cables unless they are tiewrapped above an axial basket wire.

As BS 6396 is being used, and not BS 7671, provided the basket is below the desks (so no risk of entanglement), then potentially lidded low smoke zero halogen plastic slotted trunking is perhaps the best bet for this kind of job?

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 22 January 2019 02:37 PM
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chrispearson

Posts: 1095
Joined: 15 February 2018

Originally posted by: Jallyman

I am finding it hard to find the exact definitions of class I and II pieces of equipment.


You need look no further than Part 2 of BS 7671.
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