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Modifying existing power

James Edward Duncan

A client of mine needs to re-route part of a number of small power circuits (230v, low amps). It is preferred not to replace the circuits in their entirety, due to hazardous materials in the location.

So, cutting the circuits and jointing the now 2 new ends with a new cable is the option available.

For me I would test the entire circuit end to end, however my client only seems to want to test the new section! How would I persuade them that this is the wrong thing to do?

Also, the older parts of the circuits are potentially 20+ years old. So when I do the insulation test, do I still test to current regs? 

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    my client only seems to want to test the new section

    Up to a point, that might be right - in that technically you're only responsible for your own work rather any existing parts of the installation.

    However in this case you're making an alteration as well as an addition, so you need to ensure that the alteration doesn't adversely affect the existing installation - which it is quite likely to do if you're effectively lengthening the cables. For instance you need to be sure that Zs and voltage drop at the far end of the circuit are still within acceptable limits (or no worse than they were before if they were already beyond current limits).

    In any event, some tests - e.g. insulation tests cannot be done without involving the existing installation - if you make the test prior to connection to the existing then you're not testing part of your new work. Ditto for c.p.c. continuity.

    The distinction between old and new can be useful with some tests however - say the existing installation insulation tested to something awful (<< 0.5Meg say) - provided you tested both before and after you should be able to verify that your work had a perfectly adequate insulation resistance even if the rest of the circuit didn't. (Hence you can still fill out a cert (if with something rude in bit letters written in the comments on the existing installation box) finish the job and get paid even if the customer refuses to have the existing improved.)

       - Andy.

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  • 0

    my client only seems to want to test the new section

    Up to a point, that might be right - in that technically you're only responsible for your own work rather any existing parts of the installation.

    However in this case you're making an alteration as well as an addition, so you need to ensure that the alteration doesn't adversely affect the existing installation - which it is quite likely to do if you're effectively lengthening the cables. For instance you need to be sure that Zs and voltage drop at the far end of the circuit are still within acceptable limits (or no worse than they were before if they were already beyond current limits).

    In any event, some tests - e.g. insulation tests cannot be done without involving the existing installation - if you make the test prior to connection to the existing then you're not testing part of your new work. Ditto for c.p.c. continuity.

    The distinction between old and new can be useful with some tests however - say the existing installation insulation tested to something awful (<< 0.5Meg say) - provided you tested both before and after you should be able to verify that your work had a perfectly adequate insulation resistance even if the rest of the circuit didn't. (Hence you can still fill out a cert (if with something rude in bit letters written in the comments on the existing installation box) finish the job and get paid even if the customer refuses to have the existing improved.)

       - Andy.

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