If DI cables are used in trunking or on cable tray does it count as an exposed-conductive-part?
Trunking earthing
- Thread starter ban-all-sheds
- Start date
Assuming that just-in-case, you-never-know, future-proofing isn't required, is it an e-c-p?
H
holmslaw
..
Err... why do you say that?
Deleted as it was off topic and now re-created in a new thread.
Can of earth worms.
Can of earth worms.
Not sure how any of that relates to the Q of whether a metal item supporting DI cables needs earthing...
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Now does DI mean double insulated?
Assuming that it does most cables are not actually double insulated because the outer sheath is there for mechanical protection, not insulation.
That said, BS 7671 does recognize that the outer sheath does provide insulation and this is indicated by 412.2.4.1.
Next the trunking and tray question - leave trunking to one side for the moment and just consider tray.
GN 8 addresses the issue in 10.12 (page 151) - it considers tray and cable basket.
The basic argument is that wiring systems are either Class I - such as bare MI or class II such as any wiring system installed to chapter 52 that has PVC outer sheath.
Given this the tray or basket is not considered to be an exposed-conductive-part and does not require earthing (please note I am not talking about bonding). That is, for the class I cable its sheath is already earthed and that is all that is required, and for the class II case fault protection is provided by 'double insulation'.
The next question concerns bonding. Is the tray or basket an extraneous-conductive-part. The general answer is no because it would not normally introduce a potential that does not already exist within the installation. We may have an issue with very extensive systems as (just like "Other Installation Pipework and Ducting") they may pick up the earth potential from the fabric of the building. GN 5 and GN 8 provide a test that could be used for this - its just guidance but it is widely accepted. This is measuring the resistance between the suspected item and earth - if the value exceeds 22 k Ohms don't bother bonding (GN 8, page 56).
Now to trunking - you might apply the arguments above to trunking but this would not hold if single insulated cables were installed in it. Now given that this could occur at some time in the future most would probably want to earth it - that is treat it as an exposed-conductive-part.
Assuming that it does most cables are not actually double insulated because the outer sheath is there for mechanical protection, not insulation.
That said, BS 7671 does recognize that the outer sheath does provide insulation and this is indicated by 412.2.4.1.
Next the trunking and tray question - leave trunking to one side for the moment and just consider tray.
GN 8 addresses the issue in 10.12 (page 151) - it considers tray and cable basket.
The basic argument is that wiring systems are either Class I - such as bare MI or class II such as any wiring system installed to chapter 52 that has PVC outer sheath.
Given this the tray or basket is not considered to be an exposed-conductive-part and does not require earthing (please note I am not talking about bonding). That is, for the class I cable its sheath is already earthed and that is all that is required, and for the class II case fault protection is provided by 'double insulation'.
The next question concerns bonding. Is the tray or basket an extraneous-conductive-part. The general answer is no because it would not normally introduce a potential that does not already exist within the installation. We may have an issue with very extensive systems as (just like "Other Installation Pipework and Ducting") they may pick up the earth potential from the fabric of the building. GN 5 and GN 8 provide a test that could be used for this - its just guidance but it is widely accepted. This is measuring the resistance between the suspected item and earth - if the value exceeds 22 k Ohms don't bother bonding (GN 8, page 56).
Now to trunking - you might apply the arguments above to trunking but this would not hold if single insulated cables were installed in it. Now given that this could occur at some time in the future most would probably want to earth it - that is treat it as an exposed-conductive-part.
H
holmslaw
..
On what grounds, bar being rough and leaving a 10mm gap?
it also depends on the specification of the client..
we re-used some 3 compartment trunking on a hospital job, and they insisted that every single piece of trunking was earthed via a 16mm earth ring..
drilled, ground the paint off and tapped a roofing bolt into each length in the middle and used 16x6 ring crimps to run the earth ring round it..
shakeproof washer and locknut on every bolt...
we re-used some 3 compartment trunking on a hospital job, and they insisted that every single piece of trunking was earthed via a 16mm earth ring..
drilled, ground the paint off and tapped a roofing bolt into each length in the middle and used 16x6 ring crimps to run the earth ring round it..
shakeproof washer and locknut on every bolt...
My favorite here is seeing nicely wrapped coils of green and yellow cable that are allegedly earthing cable trays.
What is that about, and do they not understand the concept of impulse impedance? To my mind they should be straight, and anything done in a coil is going to add additional impedance at the instance of failure that will increase the time before the RCD trips.
What is that about, and do they not understand the concept of impulse impedance? To my mind they should be straight, and anything done in a coil is going to add additional impedance at the instance of failure that will increase the time before the RCD trips.
it's for vibration and expansion reasons... to stop the crimps breaking off..
no need to coil the wire, but a loop is often needed.
no need to coil the wire, but a loop is often needed.
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