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Topic Title: Double and reinforced insulation
Topic Summary: Definition
Created On: 31 December 2012 12:11 PM
Status: Post and Reply
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 18 January 2013 06:42 AM
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Posts: 3751
Joined: 18 January 2003

The Wiring System formed by cables and fixings etc, can be considered as providing adequate protection, similar to that provided by double or reinforced insulation.

and in accordance with section 412 of BS 7671?

- Andy.

Surely that is exactly what 412.2.4.1 is saying.

I have not read all of the posts on this topic, as frankly, I think it has descended into naval gazing - so if what follows has already been said by others then I apologize in advance.

Chapter 41 is the implementation into UK standards of EN 61140 which itself is just the European implementation of IEC 61140. In other words we are not the sole authors of this work it actually involved most of the world. The 14th Edition was the last time we 'wrote the regs' .

So may of the criticisms aimed at BS 7671 should more correctly be direct to the IEC.

Quote from BS EN 61140 of the fundamental rule
4 Fundamental rule of protection against electric shock

Hazardous-live-parts shall not be accessible and accessible conductive parts shall not be
hazardous live
- either under normal conditions (operation in intended use, see 3.13 of ISO/IEC Guide 51,
and absence of a fault), or
- under single-fault conditions (see also 2.8 of IEC Guide 104).

NOTE 1 The accessibility rules for ordinary persons may differ from those for skilled or instructed persons, and
may also vary for different products and locations.

NOTE 2 For high-voltage installations, system and equipment, entering the danger zone is considered the same
as touching hazardous-live-part.

Protection under normal conditions (see 4.1) is provided by basic protection, and protection
under single-fault conditions (see 4.2) is provided by fault protection.

Enhanced protective provisions (see 4.2.2) provide protection under both conditions.

It goes on but you will have to either buy or arrange library access to the standard for more.

Now wiring systems such as PVC twin and earth do not, on the face of it, provide fault protection. If we stopped there we would have to cover PVCTWE with an earthed protective screen. The standard does allow enhanced provisions such as Class II etc. So if we can claim an equivalent degree of safety for PVCTWE we might just sneak under the radar and that is what 412.3.4.1 is attempting to do.


Geoff Blackwell
 21 January 2013 12:34 PM
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Joined: 13 August 2003

Thanks Geoff - I think we might be on the same wavelength!
- Andy.
IET » Wiring and the regulations » Double and reinforced insulation

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