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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.

  • 0 in reply to jbrameld

    Yes, the Noark datasheet doesn't provide any I2t data so not much to go off. One option could be to loop 10 (or so) meters of 25mm cable from the 80A protective device around the room and tell your consultant to recalculate the new PSCC at the terminals of the dimmer. 

  • 0 in reply to mapj1

    I was wondering the same. In the product data sheet of the dimmer panel link it doesn't make any mention of a maximum short-circuit current. The standards listed are:

    EN61439-1, EN55015, EN61000-6-1, EN61000-6-2, ESTA DMX512-A/RDM.

    Doesn't the manufacturer have to give the Icw or Icc per 61439?

  • 0 in reply to Tim Boler

    CE marking does not guarantee electrical safety. It shows it meets the standards the makers declare they tested it to, nothing more or less. If they don't mention a supply PSSC, they won't have considered it at all.
    Installers can in general., and do, install what and how they like.

    Mike,

  • 0 in reply to gkenyon

    "....
    according to the national annex of BS EN IEC 61439-3. "


    That's "just" the regulatory acknowledgement of the very real energy-limiting physics of the right sort of fuses. That is something fuses get generally better at for high PSSC,  compared to  mechanical breakers (which struggle to break contacts at supersonic speeds without a lot more cunning that found inside the average MCCB).

    My point is, iff you are confident in your understanding and specification of fuses (and looking at some installations out there, that is quite an 'iff' actually), you don't strictly need an annex to cover the design process and can extend the concept to other currents and system ratings.

    For those not familiar
    nice explanation here page ten this PDF

    https://www.eaton.com/content/dam/eaton/markets/for-safety-sake/files/current-limiting-fuses.pdf

    The rest of the doc is aimed at users of higher voltages bt the principle stands - see extract showing the effect. Note that the fuse needs to be small (no more than a few % of pssc) to start to really limit.

    Mike

  • 0 in reply to AJJewsbury

    I got the energy let-through curves for the upstream devices.  Looks like around 120,000 A2s at a 9kA fault current.  This should be OK on a 4mm2 outgoing cable.  Just need verification from the manufacturer that their dimmers are OK with that let-trough value and I think we should be OK.

  • 0 in reply to W B X

    Not a simple answer to that one.  We are contracted as a specialist sound and lighting contractor to a main contractor.  A Theatre Consultant specified the dimmer panels and named the ETC product as approved - although we had the freedom to choose another if we wished. The electrical consultant is employed by the end client (venue owner) to serve as the electrical designer - I presume it will be their name in the 'design' box on the certificate.  There is an electrical contractor also employed by the main contractor.  We free-issue the dimmer panels to them for installation and testing of the electrical installation (their names in those boxes).

  • 0 in reply to jbrameld
    jbrameld said:
    PSSC at the origin of the installation is 14.4kA measured

    Wouldn’t trust that, but I suppose what else can you do. Is that PP/0.87 or PN x2. In any event, as I understand it, the reactive component is not included and thus the Ipf would likely be much less, albeit with a lower pf. 
    Below, all up to date calibrated instruments with results taken several times. 

  • 0

    It has Double Pole RCDs and well as the MCBs, which will also probably be rated at 6kA.

  • 0 in reply to jbrameld

    Hi jb, how does this consultants opinion fit into the contract? 

  • 0 in reply to Tim Boler

    The dimmers are ETC Colorsource Thrupower units: Link

    They use a Noark MCB (I haven't heard of them either) as their protective device on the outputs: Link

    You raise an interesting point.  ETC as the manufacturer would still need to validate a modification but that may be easier if a 10kA device has a lower energy let-through. There is a Schneider Aciti9 10kA device that looks like it would fit.  Energy let-through/withstand data is not easy to come by with all manufacturers but we could try. 

    We could also look at the upstream fuse suggestion as well.

    These are not yet installed, but they are procured and in our warehouse due to long contract periods and wanting to preserve the purchase price (client decision).

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