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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
    jbrameld said:
    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.

    Why do you need a CE marking? My understanding is that a CE marking is required to be applied by the manufacturer when selling a product within the EU/UK. I'm not aware of any requirement for the user to install only CE marked products - although I'm happy to be corrected.

  • 0 in reply to Tim Boler

    Conformity to the Electrical Equipment (Safety) Regulations 2016. The requirement is when the product is placed on the market, not installed.

    If the equipment is modified, then the original declaration by the manufacturer or importer might not remain valid, unless the manufacturer or importer approved the modification.

    BS 7671 requires equipment to conform to the relevant British or Harmonized standards ... something different in some respects to simple CE or UKCA marking, as the legislation permits a 'technical file' route in addition to a harmonized standards route. If the equipment is declared by the manufacturer using the 'technical file' route, the installer would have to make a statement about safety in the electrical certification to BS 7671 (see Regulation 511.2).

  • 0 in reply to gkenyon
    gkenyon said:
    BS 7671 requires equipment to conform to the relevant British or Harmonized standards

    Just to clarify, if the circuit-breakers were changed from 6kA to 10kA, could it not be argued that the equipment still complies with the relevant standards? The warranty might be void, and the person modifying the equipment would have to take the risk, but it's not clear to me if it would render the installation non-compliant?

  • 0 in reply to Tim Boler
    Tim Boler said:
    Just to clarify, if the circuit-breakers were changed from 6kA to 10kA, could it not be argued that the equipment still complies with the relevant standards?

    THe equipment is not as per the manufacturer's original specification, so their declaration (of safety and EMC) is void unless they approve such a modification.

    Tim Boler said:
    The warranty might be void, and the person modifying the equipment would have to take the risk, but it's not clear to me if it would render the installation non-compliant?

    It would not necessarily render the installation non-conformant, but yes the manufacturer might not want to take responsibility for the equipment thereafter.

    What could go wrong?

    I'm not saying this actually is the case ... no-one would do that without particular details of the equpment etc. ... but ...

    Yes, it could cause a safety issue ... perhaps a fire. One possible issue that might be overlooked in changing the original circuit protection in the equipment, is that the circuit protection is likely to be matched with the ratings of the semiconductor switching devices in the equipment and the sizing of the PCB tracks. If you change the breaker for another one, will the semiconductors and PCB tracks remain protected against damage ... and possible fire or other damaging event ... in a short-circuit ?

  • 0 in reply to Tim Boler

    if the circuit-breakers were changed from 6kA to 10kA, could it not be argued that the equipment still complies with the relevant standards?

    Depends on the design of the equipment. Higher rated circuit breakers tend to have larger energy let-though (bigger chunkier construction, so more mass to get moving to open contacts, so slightly slower action) - so in some cases what was adequately protected before may not be afterwards.

      - Andy.

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  • 0 in reply to Tim Boler

    if the circuit-breakers were changed from 6kA to 10kA, could it not be argued that the equipment still complies with the relevant standards?

    Depends on the design of the equipment. Higher rated circuit breakers tend to have larger energy let-though (bigger chunkier construction, so more mass to get moving to open contacts, so slightly slower action) - so in some cases what was adequately protected before may not be afterwards.

      - Andy.

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