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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

    Thanks all,

    I had noticed the 'lower end' curves, and if we use those with the simplified maths we don't get to put many drivers on an MCB.

    On pressing the manufacturer, they decided their data sheet was wrong and the inrush is 15A and not 27A, so on the very simple maths (B10 x 3 ->30A trip -> 2 drivers; C10 x 5 -> 50A trip -> 3 drivers).

    The manufacturer is attempting to get a current-time curve from their PSU manufacturer so we'll see if that gives us more information on the time component.

    Jason.

  • 0 in reply to jbrameld

    If you are waiting for them, it may be faster if you have a breaker and some fittings available to try it, adding units to the point of trouble, and then say halving that no. Or adding inrush protection, so you know it is under your control. In the past have used these in small metal box on ceramic choc block* to define the cold inrush of problematic equipment. It only adds a couple of ohms when cold, but the effect of depressing and spreading the inrush current is remarkable. I think on the last 16A circuit I did I had two SG420 in series, to give ~ 4 ohms cold, so even in to dead flat, maximum 60A inrush, and a touch under 50 milli-ohms hot. (that model will take a oneshot surge of 250Joules each per start up - and 1/2.C.V^2 can be compared to that, or 1/2.I^2.L for transformers and motors)
    Be aware that on cold test, they mess with the R1+ R2 results, and rather like a dimmer or similar, that needs factoring out of the design. RCDs are your friend there! 
    Mike.
    * needs to be ceramic - they run uncomfortably  hot for long term use with polythene or polypropylene terminals.

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  • 0 in reply to jbrameld

    If you are waiting for them, it may be faster if you have a breaker and some fittings available to try it, adding units to the point of trouble, and then say halving that no. Or adding inrush protection, so you know it is under your control. In the past have used these in small metal box on ceramic choc block* to define the cold inrush of problematic equipment. It only adds a couple of ohms when cold, but the effect of depressing and spreading the inrush current is remarkable. I think on the last 16A circuit I did I had two SG420 in series, to give ~ 4 ohms cold, so even in to dead flat, maximum 60A inrush, and a touch under 50 milli-ohms hot. (that model will take a oneshot surge of 250Joules each per start up - and 1/2.C.V^2 can be compared to that, or 1/2.I^2.L for transformers and motors)
    Be aware that on cold test, they mess with the R1+ R2 results, and rather like a dimmer or similar, that needs factoring out of the design. RCDs are your friend there! 
    Mike.
    * needs to be ceramic - they run uncomfortably  hot for long term use with polythene or polypropylene terminals.

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  • 0 in reply to mapj1

    Thanks Mike.

    They are interesting looking little devices.  I'll hold that information in reserve if the electrical contractor is still struggling.

    I'll also leave them to the empirical tinkering (I'm a great believer in that approach) if they feel the need to do so.

    Thanks again for the detailed advice.

    Jason.

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