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Do SP+N RCBO's offer less protection than full double pole RCBO's

Alan B

This is a question that came up in a face book page, someone stated that a double pole RCBO is safer than a SP+N RCBO and personally I couldn't see why one was safer than the other.

My interpretation of the post was 

SP+N  detects L/N imbalance same as any other RCBO but switches L and N

Double pole as above but also detects overload on line or neutral.

Maybe my interpretation/understanding is incorrect?

I can see that there could be a possible advantage in 3 phase environments with 4 pole devices because without thinking too much I assume the neutral current could exceed the line current.

But in single phase circuits from my thinking at least if the line and neutral current are imbalance the imbalance would cause a trip, therefore there is no additional value in monitoring load on the neutral.

Therefore in my mind a SP+N RCD is an acceptable replacement for a double pole RCD, for example for EV charger installs.

Thinking this through further and doing a bit more digging I am coming to the conclusion that a double pole RCBO as I have described above doesn't exist and it's just manufacturers using a different name for the same thing.

Or am I missing something?

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  • 0

    "SP+N" is a bit ambiguous, but I tend to assume it refers to a device which only switches L, with a solid N - rather than one which switches both poles.

  • 0 in reply to wallywombat

    Actually, confusingly not always - there are 3 types in common use and a 4th rare one.-

    1) Un-switched neutral, sometimes called SPSN = single pole solid neutral, - until a few years ago all RCBO were like this, and it makes true isolation and fault finding quite hard, especially for N-E faults.

    and 

    2) With a full 6kA blow arc chute on the live but just late breaking early making  contacts on N also sometimes shortened to SPSN = Singe pole switched neutral. Doh!
    These are the SP+N switching, as the newer from Wylex Crabtree etc

    and

    3) Full rated simultaneous contacts on both L and N, called true double pole, usually two module width, but still sensing over-currents on the live only

    and

    4) The full monty double pole sensing which is the really rare one.

    I am not sure that even the folk who sell them are that sure of which type is needed when, and the abbreviation SPSN confusion is frankly ludicrous.

    Mike.

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  • 0 in reply to wallywombat

    Actually, confusingly not always - there are 3 types in common use and a 4th rare one.-

    1) Un-switched neutral, sometimes called SPSN = single pole solid neutral, - until a few years ago all RCBO were like this, and it makes true isolation and fault finding quite hard, especially for N-E faults.

    and 

    2) With a full 6kA blow arc chute on the live but just late breaking early making  contacts on N also sometimes shortened to SPSN = Singe pole switched neutral. Doh!
    These are the SP+N switching, as the newer from Wylex Crabtree etc

    and

    3) Full rated simultaneous contacts on both L and N, called true double pole, usually two module width, but still sensing over-currents on the live only

    and

    4) The full monty double pole sensing which is the really rare one.

    I am not sure that even the folk who sell them are that sure of which type is needed when, and the abbreviation SPSN confusion is frankly ludicrous.

    Mike.

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