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single phase PEN protection within three phase supplies

David Slate

Good Morning all,

We have used MATTe devices many times for PEN protection when the charger has no inbuilt protection. 

The MATTe devices for single phase rely on protection settings as described in indent (iv) of 722.411.4.1

MATTe sate on their website the following:

 BS 7671 does not permit a protective device as described in indent (iv) of Regulation 722.411.4.1 to be used to protect single-phase charging equipment in three-phase installations.

Quite a few devices rely on ident (iv) but other than MATTe non of them highlight this issue?

What im after if anyone would be so kind, is an explanation as to why this is the case, and why a single phase MATTe can be used on a single phase supply on a terraced house where next door will be (most likely on a different phase of the same supply) but not within a 3 phase installation.

I have an idea it is due to unbalanced 3 phase loads  and voltage to neutral on one phase can appear to be ok masking any loss of neutral should it occur. But a better explanation would be great!! maybe even links to a diagram to be able to show to project managers!

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    If you have a 3 phase supply then there is a far better way to detect loss of PEN than looking for L-N voltages being a bit wild and you should use that.

    Consider 3 equal resistors (or capacitors) between the 3 phases in 'star'. The centre of that star is a pseudo-neutral voltage, and an alarm wired between that and the supply neutral set to trigger at say 12V is a far cleaner indicator of PEN faults - much like the reference earth electrode idea.  (if there is a truly horrific overload so own phase dips very low or vanishes altogether it may also trip but that is probably an acceptable false alarm.)

    Mike.

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  • 0

    If you have a 3 phase supply then there is a far better way to detect loss of PEN than looking for L-N voltages being a bit wild and you should use that.

    Consider 3 equal resistors (or capacitors) between the 3 phases in 'star'. The centre of that star is a pseudo-neutral voltage, and an alarm wired between that and the supply neutral set to trigger at say 12V is a far cleaner indicator of PEN faults - much like the reference earth electrode idea.  (if there is a truly horrific overload so own phase dips very low or vanishes altogether it may also trip but that is probably an acceptable false alarm.)

    Mike.

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