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Topic Title: Horizontal clearance and grouping factor
Topic Summary: Extract from a book
Created On: 07 November 2017 11:01 AM
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 07 November 2017 11:01 AM
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Apostolos1983

Posts: 120
Joined: 03 December 2012

Good morning gents.
I have in front of me a good book (a bit older):
ECA Guide To The Wiring Regulations 17th Edition IEE Wiring Regulations (BS7671:2008)
On page 39 there is a table of grouping factors.
There are some Notes and I found something very useful there:
...
''Where horizontal clearances between adjacent cables exceeds twice their overall diameter, NO RATING FACTOR need to be applied''
...
How do you interpret that?
Example:
I have two cables, A and B.
Cable A has an overall outside diameter of 1cm
Cable B has an overall outside diameter of 2cm
In order to apply NO FACTOR the spacing between the two cables should be
2 x the bigger diameter?
i.e. 2 x 2cm = 4cm?
2 x the sum of the two diameters?
i.e. 2 x (1+2) = 6cm?
Thank you
 07 November 2017 11:51 AM
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OMS

Posts: 22394
Joined: 23 March 2004

The factors (or lack of in this case) are based on cables of the same size

In your example, for single layer cables, then if you provide 2 x 20mm (2cm) = 40mm spacing to the next cable of 10mm, you can ignore grouping between those two cables - this would exceed 2 x 10mm grouping required for the smaller cable

So, as a rule of thumb, use 2 x diameter of the bigger cable in the group for that cable spacing to partners

Basically what we are saying is cables are touching, or spaced (when at least 1 x cable diameter spacing is used) or not grouped when at least 2 x cable diameter is used.

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 07 November 2017 02:46 PM
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Apostolos1983

Posts: 120
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Thanks OMS.
Case...solved
 07 November 2017 03:07 PM
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Apostolos1983

Posts: 120
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OMS, a colleague asked the following question:
If instead of two cables we have two bunches of cables or one bunch of cables and a single cable on the other side,
is there anything we can say about the distance we can leave in between to consider that the two will not interact thermally???
 07 November 2017 05:13 PM
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kenelmh

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Joined: 17 February 2012

I would say it is still 2x the biggest cable if you are looking to use Cg=1 on the single cable (no grouping factor applied). The bunch is just a group of cables, same as if there were 'x' no. laid flat and touching with the single cable then spaced.

Same for the two bunches, space by 2x biggest conductor diameter and the groups are sufficiently separated - in my opinion, assuming they are not boxed in - which is another story.

I'm sure OMS will be along to correct or confirm!
 07 November 2017 05:20 PM
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OMS

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I'd go along with that

As I said, once you get to at least twice the major axis dimension for spacing, then you can consider it not to form part of the group

Before considering boxing in (ie in trenches or similar) you need to consider where you may have containment run horizontally but several layers deep - ie stacked containment - the thermal effects on the lower trays or ladders influences the upper trays or ladders in terms of their ability to lose heat - basically the ambient temperature surrounding the cables has increased beyond 30C

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 08 November 2017 08:37 AM
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Apostolos1983

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Ok.
I will apologize in advance for not understanding.
So lets assume two bunches of cables (5 and 5) laid flat in a cable tray.
The cables of each bunch are not spaced. They are one next to the other.
How do I evaluate the distance needed between the two groups of cables so that these two groups do not affect each other?
I am really sorry for not understanding the previous posts.
Regards
 08 November 2017 08:44 AM
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Jaymack

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Originally posted by: Apostolos1983
How do I evaluate the distance needed between the two groups of cables so that these two groups do not affect each other?

Thermal or interference?

Regards
 08 November 2017 08:58 AM
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Apostolos1983

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Thermal.
We discuss the minimum distance between two groups of cables so that they are considered thermally independent.
:-)
 08 November 2017 12:11 PM
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OMS

Posts: 22394
Joined: 23 March 2004

Originally posted by: Apostolos1983

Ok.

I will apologize in advance for not understanding.

So lets assume two bunches of cables (5 and 5) laid flat in a cable tray.

The cables of each bunch are not spaced. They are one next to the other.

How do I evaluate the distance needed between the two groups of cables so that these two groups do not affect each other?

I am really sorry for not understanding the previous posts.

Regards


Which bit don't you follow ?

You have group of 5 cables all touching - so you derate that group for 5 cables - ditto for the second group of 5 cables

You then space each group of 5 cables by at least 2 x the diameter of the biggest cable in the group and you then need take no further action for grouping effects

Once you have a reasonable gap between cables (in air) usually defined as 2 x De where De is the major axis diameter of the cable, then they are no longer grouped

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 08 November 2017 12:14 PM
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Apostolos1983

Posts: 120
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Thanks a lot OMS.
So if I get it correctly:

From the 10 cables (2 groups of 5) we have, I select the one (from the 10...) with the bigger overall diameter, and then I space the two groups by 2 times this overall diameter.
Yes?
 08 November 2017 12:31 PM
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OMS

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Yes - simple as that

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 08 November 2017 12:36 PM
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Apostolos1983

Posts: 120
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Wow!!!
This means we can get away with quite small spacing distance. Something like 50-60 mm would be enough in most of my cases :-)
 08 November 2017 01:36 PM
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OMS

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Well 2 x De of the biggest cable in a group - I've no idea what size or type of cable you are specifying so no idea if 50 or 60mm is reasonable

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 08 November 2017 02:34 PM
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Apostolos1983

Posts: 120
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Hahaha
Yes you are right.
Sorry.
Most of the cables in my example are multicore feeders 4x6mm2 XLPE/.SWA/PVC
Overall diameter 18.7mm, so the two lots of cables will be spaced 2 x 20mm=40mm apart.
Well, I may increase this to 50mm or more just to be on the safe side, however this is a lot (an i mean A LOT) less than what I had in mind before the discussion.
P.S. I will post on a separate thread a question about what a circuit is in the grouping factor tables and what we do with single core cables. Please feel free to help me there as well!!!! :-) :-) :-)

Thanks OMS.
Really appreciate!
 08 November 2017 04:53 PM
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mapj1

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You need to visualise the air flow around the cables - basically the cable only heats the air around it for perhaps a cable radius all round, and when this 'jacket' of hot air is free to rise, and draw cold up from below, that is fine, the cable is essentially cooling itself by convection all on its own. If cables are in each others rising draught, or obstructing each others cold air intake then this air flow is not working, and they are starting to affect each other.
When they touch, and heat is conducted from one to the other, they are tightly grouped, and more like one big cable.

-------------------------
regards Mike
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