IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Determining an extraneous conductive-part by measurement
Topic Summary:
Created On: 12 April 2018 11:26 PM
Status: Read Only
Linear : Threading : Single : Branch
<< 1 2 3 4 Previous 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.
 17 April 2018 11:45 AM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 16513
Joined: 13 August 2003

I think that is fundamentally what I have been saying, isn't it?

Hopefully it's what you've both been saying, if using differing words!

However, where RCDs are fitted, isn't the Ia, the I delta n of the RCD making the value 1,666 Ohms instead of 0.3125?

If there are 30mA RCD protection on all circuits of the entire installation - then yes I'd agree. If however there are any distribution circuits or final circuits without 30mA RCD protection (still common on existing installations of course, even those with refurbished bathrooms) then much lower R values may need to be considered. RCDs on bathroom circuits limit the duration of faults within the bathroom, but can't limit the duration of faults imported from elsewhere in the installation via extraneous-conductive-parts.

- Andy.
 17 April 2018 05:04 PM
User is offline View Users Profile Print this message



Alcomax

Posts: 303
Joined: 12 November 2009

If however there are any distribution circuits or final circuits without 30mA RCD protection (still common on existing installations of course, even those with refurbished bathrooms) then much lower R values may need to be considered


Perhaps lower than we would expect if we were not relying on a final circuit OPD without RCD.

There is of course the TN system and metal consumer unit and so the supply fuse presents the worst case scenario, much lower than 0.3125 ohms.

Pre- 16th in existing installs with no, or difficult to achieve neatly, supplementary bonding, would tie myself in knots deciding what resistance value between metal bits in bathroom was "safe"....then would generally default to it is safe enough if main bonding was in place and there was a 30mA RCD upfront of all circuits and / or before metal DB , if there were one.

The value 0.05 ohms was / is still guidance to confirm supplementary bonding is "effective". That figure was I believe an arbritary value based on meter accuracy, but also seems to be in the region of what is required for a 100 amp supply fuse.
 17 April 2018 05:22 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 16513
Joined: 13 August 2003

There is of course the TN system and metal consumer unit

I did think about that case, but decided not to mention it! My thinking was that a fault at or very near to the MET would raise the potential of the installation's entire protective system pretty much equally (rather like a fault outside the installation) - so of itself shouldn't cause a significant p.d. within the bathroom. More of a worry, I think, are faults on long circuits with relatively large R2 vales - creating a higher voltage difference between the fault and the MET. But certainly a good long submain on a relatively large fuse or MCB could be even worse than my example and require significantly lower values.

- Andy.
 17 April 2018 05:34 PM
User is online View Users Profile Print this message



Weirdbeard2

Posts: 226
Joined: 29 November 2017

Originally posted by: gkenyon

Originally posted by: geoffsd



Originally posted by: gkenyon







If the CH pipes are metal, and they are an "extraneous-conductive-part" for the location (bathroom), then EITHER:







(a) Main bond them; OR







(b) Provide supplementary local protective bonding in the bathroom.




No, neither is necessary - if all the conditions of 701.415.2 are met.
But they can't be met if the CH pipes are extraneous-conductive-parts of the bathroom (even if they are not for the installation). Even if the pipes are NOT extraneous-conductive-parts of the installation, this can occur if the pipes are earthed, say by the protective earthing of a boiler which is located away from the bathroom.





Hi GK, if the pipes are earthed by a protective conductor they are exposed conductive parts as they will become live in the event of a fault, they cannot be both extraneous and exposed conductive parts at the same time?
 17 April 2018 05:51 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 16513
Joined: 13 August 2003

Hi GK, if the pipes are earthed by a protective conductor they are exposed conductive parts as they will become live in the event of a fault, they cannot be both extraneous and exposed conductive parts at the same time?

The pipes can't be exposed-conductive-parts as they're not part of electrical equipment (see the part 2 definitions) - where they can however introduce a potential due being in contact with an exposed-conductive-part (or true earth) they do qualify as extraneous-conductive-parts. (I would agree that the definitions in this area are far from clear though - especially where 'second hand' fault voltages are concerned.)

- Andy.
 17 April 2018 06:10 PM
User is offline View Users Profile Print this message



Zoomup

Posts: 3749
Joined: 20 February 2014

I was working in a 1989 built first floor flat today. It is the highest flat. The bathroom had been re-tiled and all pipes were boxed in. No pipes are visible or accessible. The airing cupboard is sited away from the bathroom in a back bedroom. The only metal water features in the bathroom were a basin mixer tap and a shower head over the bath. No pipes could be seen at all. No confirmation of bonding could be seen. The main water pipe into the flat could not be seen anywhere. Any water pipes in the kitchen were hidden by units and panelling. A 10mm2 green and yellow cable could be seen for a run of about 2 meters in the loft running towards the kitchen, but terminated where? I presume that the incoming water pipe is plastic, but can not confirm this as it can not be seen nor any main bonding earth clamp. The metal sink top is earthed but this could be via the immersion heater. The loft has deep, deep glass fibre insulation so may cover up anything. So it is very difficult to draw any conclusions about the main and supplementary bonding.

What do others think?

Z.
 17 April 2018 06:47 PM
User is offline View Users Profile Print this message



geoffsd

Posts: 1941
Joined: 15 June 2010

Originally posted by: Alcomax

The value 0.05 ohms was / is still guidance to confirm supplementary bonding is "effective". That figure was I believe an arbritary value based on meter accuracy, but also seems to be in the region of what is required for a 100 amp supply fuse.

0.05 Ohms is a value accepted as near negligible and is used to indicate a good connection between the conductor and the bonded part.

It is not a relevant value between parts - although, obviously, if only a short distance it will be the case.

The effectve value between parts is, as above, 50/Ia or 50/I delta n.
 17 April 2018 07:35 PM
User is online View Users Profile Print this message



Weirdbeard2

Posts: 226
Joined: 29 November 2017

Originally posted by: AJJewsbury

Hi GK, if the pipes are earthed by a protective conductor they are exposed conductive parts as they will become live in the event of a fault, they cannot be both extraneous and exposed conductive parts at the same time?


The pipes can't be exposed-conductive-parts as they're not part of electrical equipment (see the part 2 definitions)



- Andy.


Andy, thanks for the reply, but pipes are allowed as earth electrodes let alone protective conductors, which seems to me that they generally qualify as electical equipment especially where a fault will cause them to become live?
Statistics

New here?


See Also:



FuseTalk Standard Edition v3.2 - © 1999-2018 FuseTalk Inc. All rights reserved.

..