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Topic Title: 18th EDITION DPC
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Created On: 02 June 2017 10:55 PM
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 17 July 2017 09:48 AM
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kenelmh

Posts: 83
Joined: 17 February 2012

Originally posted by: sparkingchip

18th edition blog from PTT



Arc fault breakers, earth electrodes to supplement the DNO earth terminals and metal clips everywhere, not just in escape routes.



Any comments?



Andy B.


Metal clips in all areas - makes sense as fire fighters would not confine themselves to escape routes, or those that are trying to escape.

Earth electrodes - makes sense as long as in duplicate to DNO earth in TNC-S supplies. Although I don't think the 18th states a performance for the electrode in ohms.

AFDDs - well, where are they to be applied? Or is it designer discretion as this will mean very low uptake with only the most onerous installations receiving them - or is that the point?

That's my 6p for now...
 17 July 2017 09:56 AM
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kenelmh

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Originally posted by: Zoomup

Originally posted by: dustydazzler


Things are indeed a lot more complicated. It would appear that we are being encouraged to TT installations and rely upon a not 100 per cent reliable plastic R.C.D. Failure rates of 4 percent have been reported.



Z.


How so? The 'new' earth electrode requirement is in addition to the DNO earth, so is still very much PME TNC-S.

I'd not like to put any RCD (functioning or not) at the intake to a multi-storey office block..

K

....okay I lied about the 6p thing...
 17 July 2017 11:18 AM
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AJJewsbury

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Earth electrodes - makes sense as long as in duplicate to DNO earth in TNC-S supplies. Although I don't think the 18th states a performance for the electrode in ohms.

Not as a single universal figure no, but it does suggest how to calculate it...
to prevent the appearance of a dangerous touch voltage in the event of the loss of the main connection to Earth.

presuming the same logic as they've been using for years for additional electrodes for PME supplied lampposts, then it's just Ohm's law based on the max demand of your installation and an acceptable touch voltage. Say 100A and 50V, then you'd need an electrode with a reliable resistance to earth of less than about 0.5 Ohms.

(That makes all kinds of assumptions of course - like you're only worried about breaks in the supply CNE feeding your installation alone - if the break was further upstream and a neighbour on the same phase didn't (yet) have their own supplementary electrode then the available current might be much higher so the touch voltage would increase, or that the installation won't have a large load added to it in the foreseeable future (say an EV charge point of a heat pump).

In the other direction you could argue that a domestic installation usually draws much less - say below 10A - so maybe you could size your electrode on that basis - accepting that it wouldn't be quite as safe during times of higher demand. Or rather than looking for an absolute voltage, you used something like a foundation trench electrode and allowed the voltage of the ground around the building to rise up to much higher levels - so reducing the voltage difference between inside and immediately outside, and hope that voltage decays gradually with distance from the building. Presuming that there isn't anything metallic radiating out from the building though - like tubular steel handrails or metallic fencing...

I'd not like to put any RCD (functioning or not) at the intake to a multi-storey office block

Actually, it shouldn't be that scary - much of the world uses TT so will be in just that situation. You wouldn't put in a 30mA RCCB of course - more like an adjustable earth trip attached to an MCCB set at perhaps a residual current of several amps and a delay of perhaps a second.

- Andy.
 17 July 2017 12:38 PM
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kenelmh

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Originally posted by: AJJewsbury

I'd not like to put any RCD (functioning or not) at the intake to a multi-storey office block


Actually, it shouldn't be that scary - much of the world uses TT so will be in just that situation. You wouldn't put in a 30mA RCCB of course - more like an adjustable earth trip attached to an MCCB set at perhaps a residual current of several amps and a delay of perhaps a second.

- Andy.


And all pole RCCBs downstream to isolate any N-E faults that would otherwise trip the main RCCB, delay or not? I still don't like it!

Okay - I now see what the fuss is about on the earth electrodes... the larger the intake (and in theory the load drawn), the lower the earth electrode resistance needs to be and in large commercial systems (fed at LV) this is going to be tricky. Hence the foundation electrodes I guess.. But then you need a way to verify the impedance in years to come, so are we left with testing the electrode in a similar manner as for LPS via a reference electrode or via the tail coming to the MET from the foundation bonds and accept that the reading might include lots of other paths to earth that aren't the 'electrode'?
 17 July 2017 01:25 PM
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sparkingchip

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So you do a calculation and install a earth electrode to the DNO earth terminal to the house you are replacing the consumer unit in, however the PEN conductor fails further along the street and the other three neighbouring houses don't have a electrode as they have not been upgraded.

So the electrode you installed is now providing the return path for all four houses, will the voltage be limited to a "safe" level? Without a maximum Ra being stated in BS7671 electricians will find themselves up S*** Creek without a paddle.

Andy B
 17 July 2017 01:26 PM
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OMS

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But then you need a way to verify the impedance in years to come, so are we left with testing the electrode in a similar manner as for LPS via a reference electrode or via the tail coming to the MET from the foundation bonds and accept that the reading might include lots of other paths to earth that aren't the 'electrode'?


Does it matter ?

We routinely design to "split" the earth nests at substations and many buildings have a Type B electrode system that is tested as an "entity" from a remote position (as you say)

We aren't actually relying on it as the means of earthing (other than collectively in conjunction with other systems)

Regards

OMS

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Let the wind blow you, across a big floor.
 17 July 2017 01:26 PM
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sparkingchip

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In the aftermath of Grenfell Towers there won't be any arguing accepted about metal clips everywhere.

Andy B.
 17 July 2017 01:31 PM
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OMS

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How so Andy - early doors I appreciate, but I've not seen anything that indicates that people couldn't escape nor FRS activity continue because of any "fallen cables"

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 17 July 2017 01:42 PM
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sparkingchip

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Originally posted by: OMS

But then you need a way to verify the impedance in years to come, so are we left with testing the electrode in a similar manner as for LPS via a reference electrode or via the tail coming to the MET from the foundation bonds and accept that the reading might include lots of other paths to earth that aren't the 'electrode'?




Does it matter ?



We routinely design to "split" the earth nests at substations and many buildings have a Type B electrode system that is tested as an "entity" from a remote position (as you say)



We aren't actually relying on it as the means of earthing (other than collectively in conjunction with other systems)



Regards



OMS


Only if someone is likely to disconnect the gratuitous earths, I am eating a sandwich and drinking a coffee leaning against a steel shed that has a TT installation within it. As well as the electrode there are eight steel stanchions bolted to concrete pads all bonded to the MET, I would not like to rely on them as earth electrodes as they are not buried deep, but they do help to reduce the measured Ra of the proper electrode. Though if the RCDs fail I cannot guarantee someone won't get a belt off the shed steel frame or cladding. So the local 30 mA devices are backed up by a time delayed 100 mA at the intake.

Andy B
 17 July 2017 01:57 PM
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weirdbeard

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I'm holding out to get the book, just in case there are major changes between the DPC and the new edition as usually happens.

-------------------------
:beer)
 17 July 2017 02:02 PM
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sparkingchip

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Originally posted by: OMS

How so Andy - early doors I appreciate, but I've not seen anything that indicates that people couldn't escape nor FRS activity continue because of any "fallen cables"



Regards



OMS


I won't like to be someone to been seen trying to stop the implementation of a regulation that will be easy enough to comply with and improves fire safety. Anyone doing so may well become tabloid and internet fodder with fire safety being under discussion rather a lot now and for some years to come.

Generally it is a few metal clips inside the uPVC mini trunking, though the old Saren buckle clips could reappear as well.

The only things I don't particulary agree with in the proposed revisions to BS7671 are the proposals for the supplementary earth electrodes, if we are to adopt MEN in the UK it needs to be done in a carefully planned manner after proper industry and government discussion taking existing legislation regarding PME and ESCQR into consideration with appropriate guidance on compliance being issued. Not just a few words tucked away in BS7671 revolving around the undefined use of the word "dangerous"; and also proposals for any requirements to install arc fault devices, which I don't think many people really think are going to solve any real issues.

As an aside I did hear mention of problems on TV and radio that were noted in the Shirley Towers fire report that were also present at Grenfell Towers, such as locked dry risers, but we will have to wait quite some time for the new report to come out with it being reported today that the Police investigation likely to last another five months

Andy B.

Edited: 17 July 2017 at 02:08 PM by sparkingchip
 17 July 2017 02:04 PM
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sparkingchip

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Originally posted by: weirdbeard

I'm holding out to get the book, just in case there are major changes between the DPC and the new edition as usually happens.


I'm on a promise of a lift to the local Elex show, so I might ask the live forum about how to install the electrodes.

Andy B
 17 July 2017 02:13 PM
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weirdbeard

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Originally posted by: sparkingchip


Generally it is a few metal clips inside the uPVC mini trunking, though the old Saren buckle clips could reappear as well.





Easy enough for new installs but how about EICRs? Will you be removing the lids of mini trunking to inspect the cables?

-------------------------
:beer)
 17 July 2017 04:05 PM
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kenelmh

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Originally posted by: sparkingchip

Originally posted by: OMS



How so Andy - early doors I appreciate, but I've not seen anything that indicates that people couldn't escape nor FRS activity continue because of any "fallen cables"







Regards







OMS




I won't like to be someone to been seen trying to stop the implementation of a regulation that will be easy enough to comply with and improves fire safety. Anyone doing so may well become tabloid and internet fodder with fire safety being under discussion rather a lot now and for some years to come.

Andy B.


Fair points - why would we get in the way of improving standards, when the solution is cheap/quick/easy, no matter of what current tragedy has occurred? (We should not need that as a motivation) That's a positive step.
 17 July 2017 04:46 PM
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OMS

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LoL - I'm probably one of few designers who has been banging the drum about metallic containment for about the last 30 years chaps

In my world we put cables in metallic containment systems, and more importantly we don't put it in escape routes (Approved Document B)

I'm not trying to stop the implementation, just that the regulation should be properly considered and well written - rather than the crap that comes out at the moment from JPEL 64

Personally speaking, I'm not convinced that a blanket of metallic fixings is required - as they are simply not necessary in may areas - and trust me, a proper fire rated fixing isn't that easy to achieve - the metal band is easy - securing it is the drama - not during an escape phase - but during a fire fighting phase - which could well be quite a long time after the fire start

Regards

OMS

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Let the wind blow you, across a big floor.
 17 July 2017 10:16 PM
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sparkingchip

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There isn't any compulsion attached to the use of arc fault devices, the reg simply says "may".

421.1.7 Arc fault detection devices conforming to BS EN 62606 may be used to provide additional protection against fire caused by arc faults in AC final circuits.

I picked up the current Professional Electrician magazine at a wholesalers today. Again there is a article about the the revision of BS7671 that totally ignores the requirement for a earth electrode to supplement the DNO earth terminal.

Has this revision already been dropped or will it simply slip through without discussion?

Andy B
 17 July 2017 10:34 PM
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sparkingchip

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Originally posted by: OMS

I'm not trying to stop the implementation, just that the regulation should be properly considered and well written - rather than the crap that comes out at the moment from JPEL.

OMS


No doubt you are right, however with electricians and other tying themselves in knots deciding what constitutes a escape route this regulation removes the need for them to make a decision on where to use metal restraints.

This reg is down to electricians not applying the original regulation correctly rather than an absolute need.

Many years ago extensions to existing homes were zero rated for VAT, however the Chancellor announced in a budget that builders seemed to find it difficult to decide what was a repair and what was new work, so he was going to make it easy for them by putting VAT on all work to existing homes. Same principle.

Andy B.
 18 July 2017 12:07 AM
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mapj1

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Well human behaviour and definitions are very fluid, usually flowing in the direction of most reward or least effort. As I'm sure that post part P there are likely to be more work placed in the category of "extension to existing circuit" than creating new ones, as the bill is the same, but every 15 mins saved adds up to either an early home or starting the next job earlier.
So I'm sure in terms of clips, when no one is looking very had, a lot of wiring is right now described as not in an escape route, when perhaps it maybe.

-------------------------
regards Mike
 18 July 2017 12:41 AM
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geoffsd

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Do ordinary dwellings have "escape routes"?
 18 July 2017 07:09 AM
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sparkingchip

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Originally posted by: geoffsd

Do ordinary dwellings have "escape routes"?


The regulation now refers to access or egress routes rather than escape routes, but is written in such a way that they apply everywhere.

521.10.202 Wiring systems shall be supported such that they will not be liable to premature collapse in the
event of a fire.
NOTE 1: Wiring systems hanging across access or egress routes may hinder evacuation and firefighting activities.
NOTE 2: This regulation precludes, for example, the use of non-metallic cable clips or cable ties as the sole means of support where cables are clipped direct to exposed surfaces or suspended under cable tray, and the use of non-metallic cable
trunking as the sole means of support of the cables therein.
NOTE 3: Suitably spaced steel or copper clips, saddles or ties are examples that will meet the requirements of this regulation.

Andy B
IET » Wiring and the regulations » 18th EDITION DPC

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