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Topic Title: Does the metal ceiling or metal posts (inside the building) need bonding to the earthing system.
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Created On: 01 September 2013 01:56 PM
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 01 September 2013 01:56 PM
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mabdelhameed

Posts: 3
Joined: 08 May 2013

Sirs,
Can anyone advise whether it is mandatory or not to bond any metal works inside the building (metal ceiling, balustrade or internal structural posts...etc.) to the building earthing systems...
Thank you
 01 September 2013 02:13 PM
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Fm

Posts: 784
Joined: 24 August 2011

Are they likely to introduce a potential
 01 September 2013 03:06 PM
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mabdelhameed

Posts: 3
Joined: 08 May 2013

Lighting fixtures are installed in this metal ceiling and cabel trays are running above however these lighting fixtures and cable trays are bonded to the earthing vis the PE conductor.
 02 September 2013 10:28 AM
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AJJewsbury

Posts: 11784
Joined: 13 August 2003

Two cases here -
1. Parts that could forseeably be made live by a simple fault (e.g. insulation breakdown) - exposed-conductive-parts - need to be earthed (as distinct from bonded) - size of conductors related to the circuit & its protective device. Examples include metal conduit or trunking containing single insulated cables. Metallic wiring systems supporting sheathed cables (whether metal or insulating sheath) normally don't need to be earthed as the cable itself already provides the required two layers of protection from electric shock.

2. Parts that can introduce a potential (voltage) from outside of the the installation - typically parts that are part buried in the ground (e.g. water pipes) - these are bonded to the main earth terminal to reduce the potential difference between them, especially during faults. Bonding conductors are sized according to the characteristics (and/or supply) of the entire installation.

(There's also a variant of 2, where local bonding is applied to a small location inside an installation, e.g. a bathroom, where metal parts could introduce a potential from outside that location).

Other standards (e.g. BS EN 50310) sometimes call for all metalwork to be bonded to create a lower potential difference within a building for the benefit of some types of IT equipment, but that's not covered by BS 7671 (the usual Wiring Regs).

- Andy.
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