IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Table 54.7
Topic Summary: Minimum CSA of CPC
Created On: 23 February 2010 01:57 PM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 23 February 2010 01:57 PM
User is offline View Users Profile Print this message



gullanedave

Posts: 102
Joined: 02 December 2005

Hi,

I might be missing something here, but is this table telling me that final circuits of a CSA less than 16mm MUST have a CPC the same size as the current carrying conductor?

I usually specify 2.5mm XLPE/LSF cable with 1.5mm CPC, or 4mm with 2.5mm CPC (for lighting and ring mains respectively).

Do I need to review how I have been specifying the CPC sizes?

Thanks.
 23 February 2010 02:01 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11553
Joined: 13 August 2003

You're missing reg 543.1.1 - which says that CPCs can either be selected (table 54.7 via 543.1.4) or calculated via the adiabatic (543.1.3) - which usually permits a much smaller CSA.
- Andy.
 23 February 2010 02:12 PM
User is offline View Users Profile Print this message



gullanedave

Posts: 102
Joined: 02 December 2005

Originally posted by: AJJewsbury

You're missing reg 543.1.1 - which says that CPCs can either be selected (table 54.7 via 543.1.4) or calculated via the adiabatic (543.1.3) - which usually permits a much smaller CSA.

- Andy.


Cap doff'd kind sir.
 23 February 2010 02:15 PM
User is offline View Users Profile Print this message


Avatar for spinlondon.
spinlondon

Posts: 4439
Joined: 10 December 2004

I seem to remember something about smaller CSA being allowed if the CPC is incorporated in a cable, or in an enclosure formed by a wiring system.
I wouldn't specify 2.5mm² CPC for 4mm² T&E, probably be cheaper to run in a supplementary conductor than get the special order T&E.
 23 February 2010 02:20 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11553
Joined: 13 August 2003

I seem to remember something about smaller CSA being allowed if the CPC is incorporated in a cable, or in an enclosure formed by a wiring system.

543.1.1 again - 2.5mm2 if mechanically protected, 4mm2 otherwise - applies to all protective conductors I believe, not just CPCs, so often the limiting value for supplementary bonding (as reflected in 544.2).
- Andy.
 23 February 2010 02:23 PM
User is offline View Users Profile Print this message


Avatar for spinlondon.
spinlondon

Posts: 4439
Joined: 10 December 2004

Yes 2.5mm² for mechanically protected, and smaller for incorporated in a cable. I believe 1mm² for fixed wiring, even smaller for flex.
 23 February 2010 02:33 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11553
Joined: 13 August 2003

Yup -down to the calculated size, subject to table 52.3 (are CPCs a conductor in a circuit? - you'd have that limit for the live conductors anyway so it'd be hard to find a G/Y core that didn't comply).
- Andy.
 23 February 2010 04:39 PM
User is offline View Users Profile Print this message



jcm256

Posts: 1895
Joined: 01 April 2006

Could I draw your attention to the following points (1) "The Electricity at Work Regulations" (Reg.8) (4) As regards adequate earthing, the use of a conductor with a small cross-sectional area, which is not capable of carrying a heavy current for the duration of the fault, is not acceptable. This would be more likely to apply to the clearance times of your mains supply at the consumer unit (Domestic) but could also override BS7671 if push came to shove. Point (2) BS7671 543.7.1.3 one of the methods to comply states A single protective conductor having a cross sectional area of not less than 10mm should be used.
Regards
jcm
 23 February 2010 05:12 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11553
Joined: 13 August 2003

543.7.1.3 one of the methods to comply states A single protective conductor having a cross sectional area of not less than 10mm should be used.

A bit out of context - section 543.7 is about providing high integrity earthing for situations with large (as in >10mA) protective conductor currents - the 10mm2 in that context is for mechanical robustness, not electrical withstand.

Could I draw your attention to the following points (1) "The Electricity at Work Regulations" (Reg.8) (4) As regards adequate earthing, the use of a conductor with a small cross-sectional area, which is not capable of carrying a heavy current for the duration of the fault, is not acceptable. This would be more likely to apply to the clearance times of your mains supply at the consumer unit (Domestic) but could also override BS7671

Are you suggesting that calculations according to BS 7671 could prove inadequate??

- Andy.
 23 February 2010 05:54 PM
User is offline View Users Profile Print this message



jcm256

Posts: 1895
Joined: 01 April 2006

AJJewsbury question

Q]Are you suggesting that calculations according to BS 7671 could prove inadequate??

Possibly, (The original post did not say if it was a Domestic Environment or otherwise) (BS7671 can be used as a guide for installations (and PIR) in office blocks, shops, supermarkets etc, these are places of work and EWA could apply.
Regards
jcm
 23 February 2010 06:14 PM
User is offline View Users Profile Print this message


Avatar for spinlondon.
spinlondon

Posts: 4439
Joined: 10 December 2004

I thought that Reg. 8 of EAWR refered to earthing and bonding conductors, and that table 54.7 refered to protective conductors, other than protective bonding conductors?
 23 February 2010 09:02 PM
User is offline View Users Profile Print this message



jcm256

Posts: 1895
Joined: 01 April 2006

Regulation 8 of the EAW Regulations applies to all conductors, unlike reg 21 of 1908 which only applied to exposed metalwork.

The EAW Regulations will apply to electrical work in domestic premises. Such work will fall to HSE to enforce.

Please Note: The comment above from the EAW Regulations is it now Part P and LABC instead of HSE that enforce electrical work in domestic premises. Who is out of date that's the big question I will have to leave it at that. Thanks for making me jump to further investigations.

http://www.hse.gov.uk/lau/lacs/19-3.htm

http://www.hse.gov.uk/lau/

Regards
jcm
 23 February 2010 09:19 PM
User is offline View Users Profile Print this message


Avatar for spinlondon.
spinlondon

Posts: 4439
Joined: 10 December 2004

Perhaps if you were to quote the heading, it might make more sense:
"13. Earthing and other suitable precautions (Reg 8)
This regulation applies to any conductor and not just to metal. It also allows other suitable means of preventing danger as an alternative to earthing."
As you can see, this indicates that it is refering to Earthing conductors, not Circuit Protective Conductors, which is what table 54.7 refers to.
Here is the EAWR8:
Earthing or other suitable precautions
8. Precautions shall be taken, either by earthing or by other suitable means, to prevent danger arising when any conductor (other than a circuit conductor) which may reasonably foreseeably become charged as a result of either the use of a system, or a fault in a system, becomes so charged; and, for the purposes of ensuring compliance with this regulation, a conductor shall be regarded as earthed when it is connected to the general mass of earth by conductors of sufficient strength and current-carrying capability to discharge electrical energy to earth.
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.