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Do I need to do anything about single pole Isolator owned by DNO

Alan B

I was changing a consumer unit today on a property where there was an isolator supplied by the DNO (owned by _______ molded in to it) but it was only single pole. The supply was TNCS and the consumer unit has a double pole isolator. The property is about to be put on to the rental market.

My initial thought is that the double pole isolator switch in the CU provides the isolation function and its a TNCS supply with limited risk of dangerous voltages on the Neutral. Therefore the single pole isolator upstream of the CU isn't ideal but not immediately dangerous and I should just note it on the installation certificate.

Or should I be highlighting the problem to the owner and suggesting they contact their DNO to request a change and what is the likely response from the DNO?

Looking forward to your views and advice.

Thanks

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  • 0

    Why do you need to isolate a conductor which is connected to all the extraneous-conductive-parts of the installation in a TN earthed installation?

  • 0 in reply to Sparkingchip

    In an installation with PME earthing arrangements, as follows:

    • Equipment outdoors (where there is no "equipotential zone", although we don't use that term now).
    • Installations with no extraneous-conductive-parts (because all incoming services are non-conductive).
    • Where a measurement of external earth fault loop impedance is required (Regulation 643.7.3), or an earth fault loop impedance tester is used to measure the earth electrode resistance of a consumer's earth electrode [permitted by Note to Regulation 411.4.2, and may be used for other purposes, such as described in Regulation 722.411.4.1 (ii)] before energisation (Regulations 643.1 and 643.7.2).

    In a TN-S installation ... well, there may be a break in the PE conductor of the supply to the installation.

    All things in the "toolbox" for risk assessment according to EAWR ... although with skilled persons at least, the disconnection may be made "after the fact" and this is generally recognised by BS 7671.

    In TN-C-S installations, I'm not sure the requirement in dwellings and similar for isolation of the neutral achieves much, though ... as you say.

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  • 0 in reply to Sparkingchip

    In an installation with PME earthing arrangements, as follows:

    • Equipment outdoors (where there is no "equipotential zone", although we don't use that term now).
    • Installations with no extraneous-conductive-parts (because all incoming services are non-conductive).
    • Where a measurement of external earth fault loop impedance is required (Regulation 643.7.3), or an earth fault loop impedance tester is used to measure the earth electrode resistance of a consumer's earth electrode [permitted by Note to Regulation 411.4.2, and may be used for other purposes, such as described in Regulation 722.411.4.1 (ii)] before energisation (Regulations 643.1 and 643.7.2).

    In a TN-S installation ... well, there may be a break in the PE conductor of the supply to the installation.

    All things in the "toolbox" for risk assessment according to EAWR ... although with skilled persons at least, the disconnection may be made "after the fact" and this is generally recognised by BS 7671.

    In TN-C-S installations, I'm not sure the requirement in dwellings and similar for isolation of the neutral achieves much, though ... as you say.

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