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Group 1 Medical location - maximum resistance between EBB, socket outlet and exposed conductive part - part 1 of 2

Simon Bourke

The question is about the compliance with IET regulation 710.415.2.2, which is included in Section 710 'Medical locations'.

It states that: 

the measured resistance between the earth terminal of any socket-outlet (or fixed equipment) and any extraneous-conductive-part shall not exceed 0.7 ohm.

(image extract below)

In clause 9.81, referred to, states:

(a) for IT, TN and TT systems, the voltage presented between simultaneously accessible exposed-conductive-parts and/or extraneous-conductive-parts shall not exceed 25 V a.c. or 60 V d.c.;

Note the International Standard IEC 60364-7-710 states the following:

Question 1 of 2 (question 2 in a separate post)

Hence I consider that the requirements for Group 1 are as below:

0.7Ω maximum resistance within the room between the terminals of the protective equipment conductor of socket outlets and fixed equipment and the EBB, and between any extraneous  conductive part and the EBB.

25v maximum touch voltage for systems operating at a.c.

Is this assumption correct?

  • 0

    Which IET book are you referencing, when was it published, which edition of the Wiring Regulations does it cover, and more importantly where did you buy it from?

    Get your current copy of BS7671 The Wiring Regulations out and check what it says in there in for regulation 710.415.2.2.

    The extract you have posted from your IET reference book looks highly dubious.

    engx.theiet.org/.../counterfeit-iet-books

  • 0

    Hang on, the value was 0.2 Ω a few days ago.

  • 0

    I'm concerned this is a "1 of 2" ... The 'most onerous' of the requirements apply ... whatever the conditions. In the UK we apply BS 7671, and all of the requirements apply across the various conditions specified.

    It's not possible to assume that IEC(and CELENEC) documents can be rationalised directly into national codes.

    National conditions (SCNs) need to be taken into account when looking at IEC documents, 'special national conditions' (SCNs) when looking it CENELEC documents.

  • 0

    It looks like your reference is an obsolete IET book.

    The information was correct ten years ago under BS7671:2008:Amendment 3:2015.

  • +1

    However in the current edition of The Wiring Regulations BS7671:2018:Amendment 3:2024 the distinction between Group 1 and 2 locations has been removed, so the 0.7 ohms requirement is incorrect.

  • 0

    The 0.7 requirement for Group 1 locations was removed a few editions ago. It is now 0.2 for both Group 1 and Group 2. The 25v maximum touch voltage is correct.

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