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Topic Title: CSA of the CSCs
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Created On: 27 December 2012 06:55 PM
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 27 December 2012 06:55 PM
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Apostolos1983

Posts: 21
Joined: 03 December 2012

Good afternoon to all of you.
I am very new here and this is really my first atempt to communicate with you through the IET forum.
First of all, I hope the best for you and your families.
To my question now:
I have been reading the BS7671, the on-site guide, the ECA guide to the BS7671 and despite all my efforts I have not cleared out the minimum CSA of the Circuit Protective Conductors. I suppose that the base on which every discussion is held is the table 54.7 where it states that:
1. Up to 16mm2 line conductor, the CPC is of the same size
2. From 16 to 35mm2 line conductor, the CPC is constantly 16mm2
3. Above 35mm2 the CPC is halh the size of the line conductor.

The questions are:
1. Does this apply to ALL the systems, for example TN-S, TN-C-S, TT?

2. When it says in 543.1.1 that

"...If a protective conductor is not an integral part of a cable and is not a conduit or duct of trunk and is not contained in an enclosure formed by a wiring system, then CSA shall not be less than 2.5mm in case of mechanical protection or 4mm if there is not such protection..."

this means that if my CPC is actually part of a 5-core cable, or a 4-core cable then this thing here does not apply and I put a CPC of the same size as the line conductor?

3. What is "..an enclosure formed by a wiring system.."? Does it mean a CPC surrounded by line conductors?

4. What is a mechanical protection for a protective conductor (see question 2)?
 27 December 2012 09:59 PM
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Zs

Posts: 2966
Joined: 20 July 2006

Welcome to the forum Apostolos.

The big guns are all on their holidays at the moment so I'll have a go at helping you out.

The minimum Cross sectional Area of a circuit protective conductor depends on several factors and includes the obvious such as what the circuit is going to be serving in terms of current, how quickly you need it to disconnect in the event of a fault, the length of it and the Ze, and more.

First I would suggest you allow yourself some time and bury yourself in the adiabatic equation. The adiabatic equation will guide you to calculating the most economical CSA of your CPC. The adiabatic is featured in section 543.1.3.

What you have stumbled upoin in table 54.7 is, IMO, the soft option. Most often, if you actually take the time to calculate you will find a smaller CSA will do the job for you. Table 54.7 is for the wealthy installer .

With regard to your two questions;
1. About the type of earthing arrangement; I'm pretty sure 543.1.1 applies to all. It'll be worth checking this thread after the real brains are back at work (OMS = 8th Jan...) There may be some nuance I'm not using in my own calculations and I shall be looking in too.

2. However, if the CPC is an integral part of the cable then it will probably work as the same size as your other conductors but you musn't assume that. If you think about twin and earth with 2.5mm conductors, the CPC is only 1.5mm and it serves just as well in most cases.

Watch out for short length circuits. That is often where assumptions fall down.

3. I don't really know the answer to that one.

4. Mechanical protection is a way of minimising damage to cables from things like being hit, scraped, compressed etc. and is usually something like the armour of a cable or a metal surround such as conduit or trunking. I'm sure if you have a look in the index of the green book you will find a better definition. But worth remembering that you can use the protection as a part of your circuit protection and that it will be a part of your calculation.

My wholesaler does not sell two core armoured cable, because most of us like to use a real core as our CPC, in my design (desk) job I am not allowed to use a) a separate CPC running alongside a cable or b) a separate core inside an armoured. I must use the armour as CPC. Now those little gems can be a challenge.

Zs

Edited: 27 December 2012 at 10:18 PM by Zs
 27 December 2012 10:17 PM
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Apostolos1983

Posts: 21
Joined: 03 December 2012

Hi.
Thanks for the trouble answering during Christmas.
Well, I started with CPCs beacause it is the easy subject to deal with. After I get some answers on this and solve my questions out I will post as well a subject about the meaning and the sizing of the earthing conductors as they are presented in BS7671. But this can wait.
I am spending a very considerable amount of time trying to solve out the Earthing and Bonding matter and I am sure that people much better than me in here can provide me with a little insight to this very complicated in my opinion chapter of BS7671.
I know that the adiabatic equation is the most scientific and cost effective solution, but I am not a very sophisticated engineer and I do a lot better with tables. That is why I insist on BS7671 tables.

I always thought that a CPC would/should be a core inside a multicore cable. For example a 4-core cable to feed a motor should contain L1, L2, L3 and PE. A distribution board feeder should be a 5-core cable containing L1, L2, L3, N and PE. That is what I am used to, to be honest. I am not used to using a separate single core cable for PE running in parallel with my live cable. Even more I am not used to use the armour of a cable.
All these things are new for me here in UK and I try to famaliarize myself with it. Back to my country the common practice was that one core of the multicore cable was PE. Not a separate conductor out of the main cable.

Thank you very much for your answer.

Edited: 27 December 2012 at 10:25 PM by Apostolos1983
 28 December 2012 09:23 AM
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peteTLM

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Some other bits:

2/ this is saying that if you have an external cpc not protected by the conduit or trunking of the wiring system, 4mm is the smallest size if it is not mechanically protected, 2.5mm is the smallest if it is protected.
This is similar again to the reg for TT electrode connections.



3/ enclosure formed by the wiring system, is the conduit, trunking, adaptable box etc of the wiring system.

-------------------------
----------------------------------------
Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 28 December 2012 10:07 AM
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perspicacious

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Guidance Notes 1 to 8 will be helpful.

Regards

BOD

http://electrical.theiet.org/b...idance-notes/index.cfm
 28 December 2012 12:39 PM
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aargeitakis

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Apostolos1983

Welcome to the forum

regards

Apostolos (Paul) Argeitakis
 28 December 2012 03:17 PM
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Apostolos1983

Posts: 21
Joined: 03 December 2012

perspicacious I have been in UK for a month now and I have most of them (too much money for a complete and full purchase of the whole packet).
In deed I have already:
1. The BS7671:2008 with the 1st ammendment
2. On-site guide BS 7671:2008 (2011)
3. Guide for wiring regulations form ECA
4. Earthing and Bonding: Snag and solutions from NICEIC
and I am trying using all these to get a thorough insight into Earthing and Bonding chapter of BS7671. Not very easy thing I have to say, but finally I have reached to several conclusions that I believe are correct.
aargeitakis I believe from your name that you are also Greek or Cyprian.
Thank you for the welcome. I will be a very active member in the forum for the next months until I familiarize myself with the UK regulations. So, I am gonna ask for your patience.
Kind Regards
 28 December 2012 04:44 PM
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Zs

Posts: 2966
Joined: 20 July 2006

Apostolos, so you chose the bad weather to welcome you? Tell us what you are planning on doing as you establish yourself. Will you be a domestic electrician, a commercial, an inspector ans so on? We'll be able to help you out much better as we get a picture of what you are doing. And of course, where in the UK you are will be relevant as you start to build your business, just in case you have pitched yourself against aargeitakis and we need to take sides . I bet you 50p that you two are already on the phone.

Zs
 28 December 2012 04:57 PM
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Apostolos1983

Posts: 21
Joined: 03 December 2012

Ha ha!!
Correct about the weather!!
My name is Kasinalis Apostolos and I am an Electrical Engineer from Greece. I was born in 1983 (as my nickname implies), which means that I have a lot to learn....
I have been working for the past 4 years in Greece as an Electrical Design Engineer in the design of Waste Water Treatment Plants and sewage networks.
One month ago, me and my wife (engineer as well) moved from Greece to Bristol. I have started working in an Engineering Consultant company which also deals with water and waste water treatment.
So, my work is an office job to be honest.
I am willing to learn everything needed to produce a good quality elctrical design work so that I become a really productive engineer. So far (this month I have been in UK), I have been trying to familiarize myself with the UK regulations.
I have already become member of the IET and I am very fond of using electrical engineering forums (I am used to using them also back in my country).
That is why I am asking for your patience for the first months, because my questions sometimes may seem a bit naive, or very simple to answer.
Until now, I have paid my ass off in books and regulations, and I have started studying them from the beginning.
Unfortunately, in the company I work there is only one regular electrical engineer who is every day in the office, whiich means that I do not have acess to a lot of similar engineers for help.
This is why I count on your help in here.
Kind Regards.
 29 December 2012 12:56 PM
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aargeitakis

Posts: 141
Joined: 14 July 2005

Hi Apostole,

I am Greek and I live in Staffordshire,

my email is : ap_argeitakis@hotmail.com

Regards

Paul
 29 December 2012 11:11 PM
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AJJewsbury

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I think Zs has covered most of the points, just to add:

In the UK it is normal NOT to use table 54.7, but to use the calculation approach (as Zs said - reg 543.1.4 - often called the adiabatic, as it calculates how much energy the c.p.c. can absorb without overheating on the assumption that no heat is lost from the cable during the fault). Many UK fixed wiring cables have a c.p.c. smaller than the live conductors, so if you look at 54.7 alone, you'll be seeing problems where they don't exist.

I agree, c.p.c.s are sized in same way for all common earthing systems (although the actual numbers may vary).

An "an enclosure formed by a wiring system" to me is whatever contains and protects the live conductors - so could be a composite (i.e. sheathed) multi-core cable, a collection of individually sheathed single-core cables, single conductors within a conduit or trunking or bus-bars within an enclosure. Occasionally (especially with multi-core cables) the in-built c.p.c. is inadequate for the particular circumstances (perhaps its also acting as a bonding conductor), so an extra c.p.c. is added.

- Andy.
 30 December 2012 01:13 AM
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Apostolos1983

Posts: 21
Joined: 03 December 2012

So it is common practice to use the adiabatic equation? That is most interesting. I will spend some time on it.
One question:
"Cable seath", "Cable armour" and "Insulation" mean the same thing???
 30 December 2012 08:53 AM
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maltrefor

Posts: 121
Joined: 01 November 2009

Originally posted by: Apostolos1983

So it is common practice to use the adiabatic equation? That is most interesting. I will spend some time on it.

One question:

"Cable seath", "Cable armour" and "Insulation" mean the same thing???


No these do not mean the same thing !

Cable sheath is the external layer of insulation of a cable.


Cable armour is the protection against damage layer of a cable, ie :- SWA = steel wire armoured, which is the most common but can also comprise of copper or aluminium and also be of tape or braid instead of wire.


Insulation is "suitable non conductive material enclosing, surrounding or supporting a conductor".

Basic insulation - "insulation applied to live parts to provide basic protection and which does not necessarily include insulation used exclusively for functional purposes".

Double insulation - "insulation comprising both basic insulation and supplementary insulation".

Edited: 30 December 2012 at 11:09 AM by maltrefor
 30 December 2012 10:16 AM
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Apostolos1983

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Joined: 03 December 2012

Ok. So insulation is the very very basic protection against touching a nacked conductor. I imagine that the green-yellow layer of a single-core copper cable is just plain insulation.
What I do not understand is the difference between sheath and armour.
Armour is consisted of steel or other conductive material and is conductive while seath is not metallic?
 30 December 2012 10:38 AM
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John Peckham

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Apostolos

The armour is the steel wire protection for the cable that is covered with a PVC or XLPE plastic sheath.

I would recommend you get a copy of IET Guidance Note 8 which is an excellent book on earthing and bonding.


If you PM me your email address I will send you something to read you may find useful.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 30 December 2012 12:05 PM
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maltrefor

Posts: 121
Joined: 01 November 2009

Originally posted by: Apostolos1983

Ok. So insulation is the very very basic protection against touching a nacked conductor. I imagine that the green-yellow layer of a single-core copper cable is just plain insulation.

What I do not understand is the difference between sheath and armour.

Armour is consisted of steel or other conductive material and is conductive while seath is not metallic?


Armour is protection, originally a leather and later a metal suit, to protect somebody in combat fighting with weapons.

An armour plated veihicle has protection against penetration from bullets, mortars, explosives etc.



Sheath is a cover for something.

A sheath knife is a knife that is carried in a leather sheath or holder normally attatched to a trouser belt.

A condom is also referred to as a sheath as it is a covering!
 30 December 2012 01:05 PM
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Zs

Posts: 2966
Joined: 20 July 2006

Originally posted by: Apostolos1983

Unfortunately, in the company I work there is only one regular electrical engineer who is every day in the office, whiich means that I do not have acess to a lot of similar engineers for help.

This is why I count on your help in here.

Kind Regards.


Aztec West by any chance? Methinks there is a huge coincidence brewing.

Zs
 30 December 2012 03:13 PM
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Fm

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Joined: 24 August 2011

Download a copy of wimes 3.02 and the others regarding water industry specifications.
 02 January 2013 02:05 PM
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AJJewsbury

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Sheath can be metallic - as is mineral insulated copper sheathed (bare pyro) - or the old lead sheathed rubber cables (pre cab-tyre).
- Andy.
 02 January 2013 03:13 PM
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maltrefor

Posts: 121
Joined: 01 November 2009

Originally posted by: AJJewsbury

Sheath can be metallic - as is mineral insulated copper sheathed (bare pyro) - or the old lead sheathed rubber cables (pre cab-tyre).

- Andy.


Or if you want to go back that far even PILS (Paper Insulated Lead Sheathed) cable.
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