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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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    There are a couple of points with SP+switched N devices - the lack of overcurrent protection on the N is one but also that the N contacts are typically more akin to a simple switch than the arc breaking capabilities of an MCB. In normal single phase earthed N circuits and the N contact arranged to open after the main L contacts, it's normally not an issue. Reverse polarity (e.g. in a caravan taken abroad) things could get interesting though - on a couple of counts - on L-PE faults the fault loop might be through the N side of the device rather than L - so the switching contacts don't have a suitable breaking capacity, also in that mode you're relying on the RCD element to provide fault detection - which for high PFCs could be relatively slow - (MCBs are faster for higher fault currents) - so much greater likelihood of excess energy let-through and unnecessary damage to equipment (esp cables).

    In most situations where polarity is reliable, I don't see any major issues though (unless we start considering faults from L of one circuit to N of another).

       - Andy.

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    There are a couple of points with SP+switched N devices - the lack of overcurrent protection on the N is one but also that the N contacts are typically more akin to a simple switch than the arc breaking capabilities of an MCB. In normal single phase earthed N circuits and the N contact arranged to open after the main L contacts, it's normally not an issue. Reverse polarity (e.g. in a caravan taken abroad) things could get interesting though - on a couple of counts - on L-PE faults the fault loop might be through the N side of the device rather than L - so the switching contacts don't have a suitable breaking capacity, also in that mode you're relying on the RCD element to provide fault detection - which for high PFCs could be relatively slow - (MCBs are faster for higher fault currents) - so much greater likelihood of excess energy let-through and unnecessary damage to equipment (esp cables).

    In most situations where polarity is reliable, I don't see any major issues though (unless we start considering faults from L of one circuit to N of another).

       - Andy.

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