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AMD 4 DPC - new section 545 - functional earthing

AJJewsbury

Has anyone had a look at the proposed new section 545 (functional earthing) in AMD 4 DPC?

At first glance, it seems rather confusing (to little heads like mine at least). I can see the general point of trying to keep protective conductor currents (whether due to normal conditions or faults) from flowing along functional earthing conductors - so AFARP connect to the protective earthing system at one point only - at least for each "cluster" of interconnected equipment.  It seems to be rather prescriptive though - e.g. "If there are multiple functional bonding conductors present in the electrical installation, a separate main functional earthing terminal (MFET) shall be installed for ease of connection for these conductors. The main functional earthing terminal shall be connected to the main earthing terminal only once." - so if I had two clusters of equipment at opposite ends of a large installation, would this prevent me connecting each cluster independently to a local PE (say a nearby DB) but rather have to run all the functional earthing/bonding conductors back to one central MFET? It doesn't seem to be a good fit with the proposed 545.2 "The main functional earthing terminal MFET and the MET may be combined" nor the methods described in 444 (e.g. Fig A444.2).

Or have I missed something obvious? (again)....

   - Andy.

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  • Thanks all. I think you're reinforcing my original thought - that in many cases bringing all FB conductors back to a single per installation MFET could be counter productive. Perhaps similar to the idea of deliberately connecting bathroom supplementary bonding back to the MET - it's as likely to skew things by puling down one corner as have any reliable benefit. Maybe they mean A FET per cluster rather than a MFET per installation, but the words don't seem to say that to me, and even then it doesn't always seem appropriate (e.g. where the cluster has its own mesh). It all feels a bit unclear for a "shall" regulation.

       - Andy.

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  • Thanks all. I think you're reinforcing my original thought - that in many cases bringing all FB conductors back to a single per installation MFET could be counter productive. Perhaps similar to the idea of deliberately connecting bathroom supplementary bonding back to the MET - it's as likely to skew things by puling down one corner as have any reliable benefit. Maybe they mean A FET per cluster rather than a MFET per installation, but the words don't seem to say that to me, and even then it doesn't always seem appropriate (e.g. where the cluster has its own mesh). It all feels a bit unclear for a "shall" regulation.

       - Andy.

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