IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Earthing Conductor Sizing
Topic Summary: PME - calculation
Created On: 14 February 2012 02:37 PM
Status: Post and Reply
Linear : Threading : Single : Branch
1 2 Next Last unread
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 14 February 2012 02:37 PM
User is offline View Users Profile Print this message


Avatar for Dale76uk.
Dale76uk

Posts: 412
Joined: 04 February 2010

Hey guys,

I have an interesting question on the sizing of Earthing Conductors.

Scenario:-

1 Phase - 230v PME (Ze 0.22) Protective device is an 80amp BS88.
16mm Tails, 6mm EC and 10mm MPBC's (Gas/Water)

is it possible to retain the 6mm EC or will I have to upsize it to a 16mm???

I was looking at using the adiabatic equation from P160 to calculate the sizing. 6mm seems to be enough.

I have contacted the NICEIC help desk and they're telling me to use table 54.8???? this is for MPBC's and not EC's. He then told me that it had something to do with ESCQR 2002 as it was a PME. blah blah blah.

Furthermore, reg 543 says i can calculate it or choose it from 54.7. your thoughts please.

dale
 14 February 2012 02:48 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11768
Joined: 13 August 2003

I have contacted the NICEIC help desk and they're telling me to use table 54.8???? this is for MPBC's and not EC's.

Table 54.8 does need to be considered as a minimum too - reg 542.3.1 directs you to 544.1.1 where PME conditions apply (hence to table 54.8).

- Andy.
 14 February 2012 02:55 PM
User is online View Users Profile Print this message



John Peckham

Posts: 7612
Joined: 23 April 2005

What is the size of the suppliers neutral? That will determine the size of the bonds and earthing conductor in accordance with 54.8.

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

http://www.astutetechnicalservices.co.uk/
 14 February 2012 03:01 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

Regulations aside, ask yourself why is bonding in a PME system "large" - when you can answer that, then decide if the earthing conductor also needs to be "large" - at least as large as the bonding conductors to address the same potential risk

That should clarify the regs Andy pointed you to above

Regards

OMS

-------------------------
Failure is always an option
 14 February 2012 03:06 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11768
Joined: 13 August 2003

Although this question did make me think - if an installation needed no main bonding (e.g. all plastic supply pipework), the earthing conductor wouldn't be acting as a bonding conductor (hence couldn't carry any diverted N currents) - so wouldn't it be reasonable to size it as a c.p.c. alone (and ignore 54.8)?
- Andy.
 14 February 2012 04:11 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

It would Andy - so could be designed based on an adiabatic check alone.

Regards

OMS

-------------------------
Failure is always an option
 14 February 2012 04:25 PM
User is offline View Users Profile Print this message


Avatar for spinlondon.
spinlondon

Posts: 4573
Joined: 10 December 2004

542.3 Earthing conductors
542.3.1: "Every earthing conductor shall comply with Section 543, and where PME conditions apply, shall meet the requirements of Regulation 544.1.1 for the cross-sectional area of a main protective bonding conductor."
Regulation 544.1.1 refers you to Table 54.8.
As such the NICEIC are quite correct in their advice.
 14 February 2012 05:08 PM
User is offline View Users Profile Print this message


Avatar for Legh.
Legh

Posts: 3538
Joined: 17 December 2004

Ok,
So lets say that the upstream supply neutral 'failed', then the earthing conductor would take no part in any diverted neutral currents?
your only protection would be fault currents finding their way through the property via the main protective bonding conductors to extraneous services and possibly an earth electrode, where RCDs are fitted then they would trip automatically when something was turned on. or more likely, just trip with so much kit already on.
A downstream supply fault or imbalance of phases elsewhere might well be a different matter. So an application of the largest suitable sized earthing conductor would be to your advantage.

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 14 February 2012 05:15 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

So lets say that the upstream supply neutral 'failed', then the earthing conductor would take no part in any diverted neutral currents?


It would carry it all - the earthing conductor connects the MET to the suppliers neutral - anything that flows in the bonding conductors flows in the EC

Regards

OMS

-------------------------
Failure is always an option
 14 February 2012 05:17 PM
User is offline View Users Profile Print this message


Avatar for Legh.
Legh

Posts: 3538
Joined: 17 December 2004

Originally posted by: OMS

So lets say that the upstream supply neutral 'failed', then the earthing conductor would take no part in any diverted neutral currents?


It would carry it all - the earthing conductor connects the MET to the suppliers neutral - anything that flows in the bonding conductors flows in the EC

Regards

OMS


Upstream from the installation?
The neutral is disconnected so that path would be unavailable, surely.

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 14 February 2012 05:24 PM
User is offline View Users Profile Print this message



anastasis

Posts: 588
Joined: 01 September 2009

In the event of an upstream neutral break, current from the live used by the installation returns to the combined neutral/earth terminal in the PME cut-out. It then can't go down the supplier's neutral wire, so comes out of the cut-out via the earthing conductor, to the MET, down the main bonds to metallic service pipes and then hopefully to earth.

But I'm not sure that's considered a diverted neutral current, which I would regard as being a current coming out of the earth via the main bonds and the EC and into the supplier's neutral.

Edited: 14 February 2012 at 05:36 PM by anastasis
 14 February 2012 05:40 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

Upstream from the installation?
The neutral is disconnected so that path would be unavailable, surely


Yes - any building load current gets to teh cut out - can't go upstream (cos it's broke) so flows down the EC to the MET and down the bonding to "earth"

With an intact PEN, some of the current still does this as you have parallel paths effectively

Regards

OMS

-------------------------
Failure is always an option
 14 February 2012 05:42 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

But I'm not sure that's considered a diverted neutral current, which I would regard as being a current coming out of the earth via the main bonds and the EC and into the supplier's neutral.


Other direction by definition, isn't it ?

Regards

OMS

-------------------------
Failure is always an option
 14 February 2012 06:46 PM
User is offline View Users Profile Print this message


Avatar for Legh.
Legh

Posts: 3538
Joined: 17 December 2004

Ok,
I've got it..
Yes, that's now obvious, if the PEN is looped in at the neutral bar and not jointed outside, a fault upstream would mean your EC will carry the full current to your MET and out via the MPB conductors; a fault downstream will find the nearest extraneous services back to the transformer.

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 14 February 2012 07:35 PM
User is online View Users Profile Print this message


Avatar for UKPN.
UKPN

Posts: 548
Joined: 17 January 2012

--Sizing earthing conductors.

The minimum earthing conductor size allowed for connection into a
DNOs neutral is 16mm2.
It became 16mm2 when the board started to ask for marshaling blocks
to be fitted to accommodate 3 or 4 bonding conductors, too many for the
neutral block.
In our posters topic it is likely that an old 6mm2 has been left in when
a 10mm2 bonding upgrade took place. It is not a hanging offence but
best upgraded.


Diverted neutral currents? thats another topic.

Regards.
 14 February 2012 07:39 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

The minimum earthing conductor size allowed for connection into a
DNOs neutral is 16mm2


says who - even BS 7671 recognises that 6.0mm would be acceptable for say street lighting connections to PME

Given a minimum 10mm2 bonding conductor, why would you need an Earthing conductor any bigger than that - even with 25mm2 tails

Regards

OMS

-------------------------
Failure is always an option
 14 February 2012 07:57 PM
User is online View Users Profile Print this message


Avatar for UKPN.
UKPN

Posts: 548
Joined: 17 January 2012

--16mm2 earth lead--says who?

Darrell Locke/ECA-it must be correct!
 14 February 2012 08:02 PM
User is offline View Users Profile Print this message



perspicacious

Posts: 7267
Joined: 18 April 2006

Book and page no?

Regards

BOD
 14 February 2012 08:05 PM
User is offline View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

Really - so an ECA opinion on a standard arrangement - not a DNO policy then - or a requirement of BS 7671

Regards

OMS

-------------------------
Failure is always an option
 14 February 2012 08:14 PM
User is offline View Users Profile Print this message


Avatar for Dale76uk.
Dale76uk

Posts: 412
Joined: 04 February 2010

this seems to be a tricky one. if the adiabatic equation works okay then why not a 6mm EC and MPBC to 54.8?
IET » Wiring and the regulations » Earthing Conductor Sizing

1 2 Next Last unread
Topic Tools Topic Tools
Statistics

See Also:



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