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431 Protection according to the nature of the circuits and the distribution system BS 7671:2018

Former Community Member
Former Community Member
hello

in regulation 431.1.2 states that in TT or TN system for a circuit supplied between line conductors and in which the neutral conductor is not distributed, overcurrent detection need not be provided for one of the line conductors, provided:

 i - there exists differential protection to detect ubalanced loads

ii - neutral conductor is not distributed from an artificial neutral point of the circuits situated on the load side of the differential protection device mentioned in (i)


my question is how can artificial neutral be supplied, and what effects does it have if it does not comply with ii
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    2 parts -

    An artificial neutral in this context is just a voltage equidistant from all 3 phases - in principle the star point of any 3 identical loads, one on each phase,  would do. In reality inductor/transformer methods are preferred as being lowest loss.


    However, this rule is really saying if you have 3 wires, you need to fuse or circuit breaker at least 2 of them to be able to make safe all possible over-current situations.   (or perhaps in a 2 wire phase split, you need at least one fuse or breaker)


    Now as soon as you create a 4th wire, with either a real or derived neutral path, then you are back to needing 3 fuses or breakers,

    Even without the pseudo neutral,  you still need to sense and disconnect faults to earth, so there are very few situtions when you would not  have something that interrupts all phases in any case, rather than just 2 out of 3 or one out of 2.

    Mike.

Reply
  • 0
    2 parts -

    An artificial neutral in this context is just a voltage equidistant from all 3 phases - in principle the star point of any 3 identical loads, one on each phase,  would do. In reality inductor/transformer methods are preferred as being lowest loss.


    However, this rule is really saying if you have 3 wires, you need to fuse or circuit breaker at least 2 of them to be able to make safe all possible over-current situations.   (or perhaps in a 2 wire phase split, you need at least one fuse or breaker)


    Now as soon as you create a 4th wire, with either a real or derived neutral path, then you are back to needing 3 fuses or breakers,

    Even without the pseudo neutral,  you still need to sense and disconnect faults to earth, so there are very few situtions when you would not  have something that interrupts all phases in any case, rather than just 2 out of 3 or one out of 2.

    Mike.

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