Auxiliary supply within LV switchboards

Auxiliary supplies within LV switchboards, to serve a power monitoring meter, for example, are commonly taken from the main busbars via DIN or Red Spot fuses. However, the installation between the main busbars and these fuses is in no way rated for the prospective fault current. Is this practice recognised/formalised in regulations or codes of practice, somewhere? Ta!

  • In general terms BS 7671 allows fault protection not to cover short lengths of conductors (<=3m long, installed in such a way to reduce the risk of faults to a minimum, and installed in such a way that the risk of fire or danger to persons is minimised should a fault occur) - see reg 434.2.1. Within a switchboard though - the specific equipment standard would outrank BS 7671 - though I suspect it will have similar provisions.

       - Andy.

  • yes - you need to be less than 3m long, and enclosed/protected  in a way that makes accidental damage unlikely.

    Look at when protection may be ommitted (Regulation group 433.3 , and/or Regulation group 434.3)  for the guidance.


  • I'd suggest this is one for BS EN 61439 rather than BS 7671;  but this is not my specialty.

    At least it's fuses which likely have a reasonable fault withstand... occasionally suppliers propose MCBs on high Ipfc boards, which doesn't go down too well with me. Granted the impedance of the wiring will significantly reduce the prospective fault but I rarely see any calculations to justify that assumption.

  • Hager have meters taken directly off the busbars, with no over-current protection. Quite flimsy cabling too IMO.

  • It would in my view be preferable to fit relatively large fuses such as 32 amp, as close as possible to the bus bars, and then much smaller fuses such as 6 amps closer to the auxiliary equipment. The wire between the bus bars and the 32 amp fuses would be very short and and short circuits almost impossible. The wire downstream of the 32 amp fuses would be protected against faults by the 32 amp fuses. These 32 amp fuses would probably last for the life of the installation.  The 6 amp fuses would be readily replaced if needed.

    I do not like several meters of unfused small wire connected to large supplies with a FLC of hundreds of amps, and a prospective fault current of many thousands of amps. Just because you CAN do this does not mean that you should.

  • Arguably in an extreme case the thin 'unprotected' wire is its own fuse - if there is enough current there, then any fault will blow to clear without troubling the upstream ADS.

    That is fine, as a conscious design decision,where some thought is given to fixings and containment of the event and where all the burnt bits will end up... On a smaller scale, we are used to the idea that it is OK if an enclosed  piece of kit 'blows up' inside the box, so long as all the shrapnel is caught somewhere, and it does not get too hit and burn down the building.

    However that sort of design is not something to sleepwalk into - if on its way down the bits of that melting thin wire can get somewhere that they  trigger an arc between the bus bars, or something of that sort of magnitude, then it is not really a safe 'self fusing' mechanism at all.