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Upstream protection for short circuit current

jbrameld

We are providing some lighting dimmer panels (they also provide hard power and switched power - configurable on a per-channel basis) for a theatre installation.  All the dimmer outputs are 10A single phase circuits and the dimmer has a 3-phase supply.

The electrical consultant has stated that the worst case PFC for a fault at the dimmer outputs is approx 9kA - presumably this is on a bolted phase-phase short..  The dimmers are provided with 6kA breaking capacity Type C 10A MCBs (to EN 60898) with neutral disconnect for their output circuits. The consultant is saying that these must be replaced with 10kA devices.  This of course is not a simple matter as the product is CE/UKCA marked and such a modification would likely require a re-certification by the manufacturer with significant cost and time impacts.  Clearly we can't simply swap the devices over ourselves and the manufacturers are not keen to do it either.

The upstream protective device is a 80A Type D MCB to EN60947-2 (fixed - non adjustable), and has a breaking capacity of 10kA. Looking at the trip time curve for that device once you get to around 800A you hit the 'instantaneous' region.  Likewise for the dimmer MCB once you hit 100A you are in the 'instantaneous' region.

My argument is that if there If there is a fault current of 1kA-6kA, you're likely to be in to a race between the 80A and 10A breakers where you can't predict the winner anyway, and in the worst case of a fault current >6kA and the 10A MCB welds shut the 80A MCB will open within it's fastest possible operation anyway (sub 0.1s).

Also, on a 4mm2 CSA output cable by my calculations once you get to about 8m of cable the worst case fault current is dropped to 6kA anyway.

434.5.1 provides for a higher breaking capacity device upstream to provide protection for a downstream device, although energy let-through needs to be considered.

Is my logic correct or flawed, and is there anything else we would need to do to determine if the configuration decsribed above is compliant?  We know the very high fault current would be an edge-case in terms of likelihood but the electrical consultant is very particular on this project.

NB - We have looked at alternative manufacturers of similar products (there aren't many) and they all use 6kA MCBs.  There is a 'bigger brother' product with higher breaking capacity which won't fit physically and would have a significant cost uplift that I expect will not be feasible.

Thanks in advance.

Jason.

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  • 0

    In domestic and similar small installations, it id often considered that sub-circuit MCBs of apparently inadequate breaking capacity are OK if upstream protection is provided by a 100 amp or less BS88 fuse in the suppliers cut out.

    Could you fit 100 amp fuses upstream of the installation ? I do not feel that any form of re-certification is required to fit EXTERNAL fuses.

  • 0 in reply to broadgage

    We definitely can't do that in the supplier's cutout.  That's currently 400A per phase.  We could potentially put an 80A BS88 fuse in line between the 80A MCB feeding the dimmer panel and the dimmer itself, but then we get into more selectivity arguments.

  • 0 in reply to jbrameld

    but then we get into more selectivity arguments.

    Where you have two devices that disconnect exactly the same loads, selectivity isn't usually a worry - since the result is the same whichever opens first (or if they both do). BS 7671 doesn't demand selectivity (or discrimination) all the time - only when it's required. There are plenty of ordinary situations where there are two (or more) protective devices in series - 30mA RCDs on both caravan pitches and within caravans, fused submains that won't discriminate with supplier's cut-out fuses where the CU is remote from the intake position. Even 13A fuses and 32A MCBs don't discriminate very well at all.

       - Andy,

  • 0 in reply to jbrameld

    If in the most unlikely event of a dead short fault that really got the full 9KA, you accept that the kit may be damaged and need replacement, then I don't see the problem - you only need the flying shrapnel to be contained, you don't need the breakers or the electronics to survive.
    Any real fault will be have finite resistance - it has to dissipate or there is no energy to generate the flash  and bang, and as you note, is likely to be more than a few metres down the line so current limited.

    Broadgage's comment about fuses applies not just to houses, but also to any large installation - the PSSC is not actually limited to any specific level, but the time it is present for very much is, so the let through energy - which is what really does the damage, certainly is limited, and is often expressed as an equivalent reduced PSSC current after the fuse.
    Mike

  • 0 in reply to jbrameld

    Whilst you cant rely on the suppliers cut out in the circumstances, I believe that you COULD rely on a set of 80 amp or 100 amp fuses upstream of the equipment. Whilst belonging to the consumer and not to the DNO, they would provide equivalent protection.

  • 0 in reply to broadgage
    broadgage said:
    In domestic and similar small installations, it id often considered that sub-circuit MCBs of apparently inadequate breaking capacity are OK if upstream protection is provided by a 100 amp or less BS88 fuse in the suppliers cut out.

    CAUTION.

    This is due to the fact that (and is only valid where) the assembly in which the devices of 'apparently inadequate breaking capacity' has been type-tested according to the national annex of BS EN IEC 61439-3.

    This is not a new requirement for DBOs (consumer units) in the UK.

    The devices, in fact, have what is termed a conditional short-circuit rating of 16 kA if they are successfully type-tested as such.

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  • 0 in reply to broadgage
    broadgage said:
    In domestic and similar small installations, it id often considered that sub-circuit MCBs of apparently inadequate breaking capacity are OK if upstream protection is provided by a 100 amp or less BS88 fuse in the suppliers cut out.

    CAUTION.

    This is due to the fact that (and is only valid where) the assembly in which the devices of 'apparently inadequate breaking capacity' has been type-tested according to the national annex of BS EN IEC 61439-3.

    This is not a new requirement for DBOs (consumer units) in the UK.

    The devices, in fact, have what is termed a conditional short-circuit rating of 16 kA if they are successfully type-tested as such.

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