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MCB surge current/device calculations

jbrameld

I have searched and found some similar threads on this topic, which give me some guidance - most of which says there's no definitive way to assess this, but if the learned community here can check my thinking that would be helpful.

We are providing some LED fitting drivers from ETC to the electrical contractor on a theatre project.  These are DMX controlled, and have a maintained (externally backed-up) supply connection and a sense connection.  If the supply at the sense connection fails they force the output to full brightness - but still fed from the maintained supply.  There is no local battery.

The contractor has asked how many of these could be put on a 10A Type B or type C MCB (let's assume to IEC 60947 as this is not a domestic environment).  Let's also assume we are looking to avoid the breaker tripping due to surge currents on switch-on or reset.  The manufacturer states that the inrush current is 27A within the first half cycle, 

A type B MCB will fast trip at 3-5x In, and a Type C will fast trip at 5-10x In.  If we take the lower end of these figures to 'guarantee' no tripping, then we would be limited to an inrush current of 30A (1 driver) for a type B and 50A (2 drivers) for a type C.  This is just by my simplistic maths, and assumes that the inrush lasts long enough (10ms or so) for the breaker to trip.  We don't have a current-time curve from the manufacturer although we have asked for it, and also for their assessment of the number of drivers that could be put on each MCB type.

I'm sure I'm missing something here - possibly more detail on the time component.  if the inrush is 'very short' how 'short' would it need to be for the breaker not to really care about it?  What else am I missing in the simple view I'm taking?

Thanks in advance.

Jason.

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  • 0
    jbrameld said:
    The contractor has asked how many of these could be put on a 10A Type B or type C MCB

    zero-volt switching and "staggered start" timers, are also used to help mitigate inrush currents.

    You'd also want to consider the inrush effect on sub-mains/distribution circuits?

  • 0 in reply to gkenyon

    Thanks Graham.

    I'll leave the electrical contractor to worry about that!

  • 0 in reply to jbrameld
    jbrameld said:
    I'll leave the electrical contractor to worry about that!

    Surely, you mean 'designer' of the installation?

    Being  a bit devilish here, but it's not just the "installer" ("contractor") ... I'm wondering who is the "designer"?

  • 0 in reply to gkenyon

    Yes indeed - you are correct.  There is an electrical consultant on the project.  The exact division of responsibilities between the consultant and the contractor, who likely has a CDP element, I am not certain of however I am certain it's all outside of our remit on the project and we're merely trying to assist.......

  • 0 in reply to gkenyon
    gkenyon said:
    zero-volt switching

    Some zero crossing switching (SSW - solid state switching) can cause large surges where there are saturable inductors/transformers in the circuits because of hysteresis which can leave the inductors pre magnetised in the same direction as the switch on surge. 

    I can't remember the details, but there was also some discussion about [2nd order style] effects on capacitors as well (might have been the roll types).

    I certainly once had a piece of US test equipment that was supplied as 110v AC (should have been '240'v AC) and the supplier quickly sent a small adapter transformer rated to average power, and it kept blowing the fuses because the unit had an SSW on-off, so it was 50:50 is to whether this transformer would have remnance the wrong way!

    So yes, beware unexpected in-rush currents!

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  • 0 in reply to gkenyon
    gkenyon said:
    zero-volt switching

    Some zero crossing switching (SSW - solid state switching) can cause large surges where there are saturable inductors/transformers in the circuits because of hysteresis which can leave the inductors pre magnetised in the same direction as the switch on surge. 

    I can't remember the details, but there was also some discussion about [2nd order style] effects on capacitors as well (might have been the roll types).

    I certainly once had a piece of US test equipment that was supplied as 110v AC (should have been '240'v AC) and the supplier quickly sent a small adapter transformer rated to average power, and it kept blowing the fuses because the unit had an SSW on-off, so it was 50:50 is to whether this transformer would have remnance the wrong way!

    So yes, beware unexpected in-rush currents!

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