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Amtech, Earth Loop Impedance and multiple earth paths

Paul McGee

im sure this question will have been asked many time before but i cannot find the answer on here.

i am currently working on a project, and when calculating a cable size through amtech for a motor cable, it is coming out at a very large cable size purely down to earth loop impedance.

in the past i have simply used the "add RCD" button and enabled earth fault protection through the intelligent starter.

however, this client does not want to do this.

the circuit in question has a power to the motor, and a control cable to the local control station (both armored cables), these are then equipotentially bonded, also bonded to the pump base, the pump base is locally bonded to a local earth bar which is connected to multiple other earth bars and multiple earth pots, these earth bars also have other pumps connected to them, which go back to the MCC.

as you can appreciate, the earth loop impedance on this circuit is going to be minimal as there is so many earth paths, however, amtech (As far as im aware) cannot compute this and just looks at the one cable.

i am happy to write in my report something along the lines of "calculation fails due to Earth Loop Impedance, however considering XYZ, the cable is considered sufficient and shall be tested on site prior to energisation to ensure that it meets all criteria from BS7671".

however, i am wondering if 1. any body knows a way around this in amtech, 2. is there a statement in any standards or guides that details this i can reference and 3, any issues that anybody can see.

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  • 0

    You can add an additional CPC and input the size within Amtech.

  • 0 in reply to Stuart Urban
    Stuart Urban said:
    You can add an additional CPC and input the size within Amtech.

    But we need to be careful.

    The 'cpc' needs to be run in the same wiring system or its immediate vicinity ... see Regulation 543.6.1.

    So whilst lower resistances happen when we add 'bonding' and other paths, ' we can't take all of that into account for ADS. (It would be like relying on fortuitous earthing of extraneous-conductive-parts to shore up the intended consumer earth electrode in a TT system, for example).

    I'd be concerned that some of the components the OP stated would not really be considered suitable as a cpc.

  • 0 in reply to Stuart Urban

    Yes aware of that, only trouble is that these are existing cables burried under ground over hundreds of meters in the middle of an chemical plant, want to avoid replacing if i can as i know on site the ELI test will pass with flying colours.

  • 0 in reply to gkenyon

    Agreed, however, Reg 543.2.6 states

    an extraneous-conductive-part may be used as a
    protective conductor if it satisfies all the following requirements:
    (i) Electrical continuity shall be assured, either by construction or by suitable connection, in such a way as to
    be protected against mechanical, chemical or electrochemical deterioration
    (ii) The cross-sectional area shall be at least equal to that resulting from the application of Regulation 543.1.1
    (iii) Unless compensatory measures are provided, precautions shall be taken against its removal
    (iv) It has been considered for such a use and, if necessary, suitably adapted.

    for which id argue local earth bars, connections to steelwork, and meshing of other earthing cables would meet the above criteria

  • 0 in reply to Paul McGee
    Paul McGee said:
    for which id argue local earth bars, connections to steelwork, and meshing of other earthing cables would meet the above criteria

    Indeed they could, but we need to be sure all of (i) to (iv) are met ... especially (iii).

    As I said, care is needed.

  • 0 in reply to Paul McGee

    If you are confident you have extraneous conductive parts that meet the requirement for a protective conductor (i would be concerned about iii as a designer who can't control what happens throughout the installation's life), can't you have the measurement taken and use it to inform your design?

  • 0 in reply to chrisbryant

    there is that many earth routes back that the only way i can see anything effecting the earth loop impedence value is if the area of the plant was de-comissioned and everything removed.

    also, I am having a IR cable test completed to confirm the cable integrity prior to us starting work as these are existing cables, however, while we are at it i will get a ELI test completed and this can be referenced also.

  • 0 in reply to Paul McGee

    You know the installation and client better than us all, so are best placed to make the assessment, but playing devil's advocate - What if someone disconnects the bonding to the pump base on the basis it is not an extraneous conductive part or exposed conductive part (assuming it is neither)? 

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  • 0 in reply to Paul McGee

    You know the installation and client better than us all, so are best placed to make the assessment, but playing devil's advocate - What if someone disconnects the bonding to the pump base on the basis it is not an extraneous conductive part or exposed conductive part (assuming it is neither)? 

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