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Coordination of insulation monitoring devices

Jam
Hi all,


I occaisionally come up againt a requirement to provide an IMD where there already is one elsewhere on the same network (for example, on an IT system where the downstream equipment contains its own IMD out of the box, but has been installed in such a way that it might be disconnected with the distribution circuits still energised, leaving the latter unprotected)


In the past I've been advised by manufacturers that you can't have two separate IMDs monitoring the same network as they'll see each other as faults, so we've ended up with convoluted arrangements to priorise devices such that only one is in use at any time, or simply removed downstream devices.


But this doesn't feel satisfactory to me, and I suspect I'm missing something... Is there a better way to coordinate IMDs?
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  • 0
    PV central inverters are the classic that comes to mind, with an IMD primarily looking into the DC from the AC side (but also seeing the AC). Normally connected directly to the transformer IT0N, all good. But sometimes one comes across cases where there's interposing switchgear, perhaps to supply auxiliary equipment and controls (normally via isolation transfomrers) meaning that you can switch off the protection to the upstream circuit(s)... Not uncommon if the maintenance contractor wants the lights on to service the equipment, and may concievably persist for a while if there's an equipment fault.

    Yes, one could look at other protective measures, but that's not the question. Similarly thinking about more complex arrangements where old-skool "No fault zones" are not sufficient.

    Other variations on this theme exist.
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  • 0
    PV central inverters are the classic that comes to mind, with an IMD primarily looking into the DC from the AC side (but also seeing the AC). Normally connected directly to the transformer IT0N, all good. But sometimes one comes across cases where there's interposing switchgear, perhaps to supply auxiliary equipment and controls (normally via isolation transfomrers) meaning that you can switch off the protection to the upstream circuit(s)... Not uncommon if the maintenance contractor wants the lights on to service the equipment, and may concievably persist for a while if there's an equipment fault.

    Yes, one could look at other protective measures, but that's not the question. Similarly thinking about more complex arrangements where old-skool "No fault zones" are not sufficient.

    Other variations on this theme exist.
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