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Type B RCDs

M. Joshi
Increasingly, I am seeing a requirement for Type B RCDs in industrial applications, particularly where servo drives are used.


The preference is Type B RCBOs however, these are quite rare and not all manufacturers make them.


Aside from the high cost and short supply due to demand from EV and PV applications, how do others address this situation?


If the earthing is adequate, cables to the motors are run in trays and there is additional bonding to the motor housing, is there really a need for an RCD?
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    be careful that type 'B' about a RCBO usually relates to the speed curve of the MCB part, not the DC sensitivity of the RCD part. Check maker's data carefully (if only different committees liaised, we would not use the same letter for two very distinct characteristics.)


    It rather depends where the requirement has come from - most designs of VSD rectify the incomong mains to a DC bus. Some designs  may credibly under some odd condition produce a DC fault to earth of the kind that would 'blind' a normal RCD. The question is what happens if the RCD is disabled.

    On a TT supply where the RCD is the only form of fault detection, that could be very dangerous. On a TNs supply it may be that the failure of the VSD to the point where a large earth current flows and then a fuse blows instead is acceptable.

    The only folk who can really answer that are the maker of the VSD, in terms of the possible failure modes and someone familiar your local installation, as to if RCDs being blinded by DC is an Issue or not.


    Personally I suspect you are correct, and the RCD is not always needed, but you need more than a gut feel to back you up before you omit it, especially if it is in the maker's instructions,  as in some cases it will matter.

    Mike.
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  • 0
    be careful that type 'B' about a RCBO usually relates to the speed curve of the MCB part, not the DC sensitivity of the RCD part. Check maker's data carefully (if only different committees liaised, we would not use the same letter for two very distinct characteristics.)


    It rather depends where the requirement has come from - most designs of VSD rectify the incomong mains to a DC bus. Some designs  may credibly under some odd condition produce a DC fault to earth of the kind that would 'blind' a normal RCD. The question is what happens if the RCD is disabled.

    On a TT supply where the RCD is the only form of fault detection, that could be very dangerous. On a TNs supply it may be that the failure of the VSD to the point where a large earth current flows and then a fuse blows instead is acceptable.

    The only folk who can really answer that are the maker of the VSD, in terms of the possible failure modes and someone familiar your local installation, as to if RCDs being blinded by DC is an Issue or not.


    Personally I suspect you are correct, and the RCD is not always needed, but you need more than a gut feel to back you up before you omit it, especially if it is in the maker's instructions,  as in some cases it will matter.

    Mike.
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