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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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    The other thing that may be easier is for the lamp manufacturer to tell you the value of the (flat) capacitor that gets dropped across the mains and has to charge suddenly, perhaps to over 300V, if the switch on moment is near the top of the sine wave.

    We an then pretend htat the curret that flows is a rectange of constant current rather than something that tails off as the capacitor fills, to get an under estimate of the time, but an over estimate of the I2C, which you can compare to the breaker let through to give an upper bound.

    This is not precise, but it will be within a factor of two or so, do realise it is one of those 'pretend the horse is a sphere' type analyses and should be treated as that.

    This is effecively an amount of charge that needs to be transferred, and will try to do so at current set by the (generally very low)  circuit impedance.

    a 100uF capacitor charging at 30Amps, for example, reaches a 300V charge when I*t =C*V = 300* 100E-6 = 3E-2C 

    so in 3E-2/30= 1 millisecond.
    now if we look at the I2t of that rectangular current burst.

    30A*30A * 1E-3 = 0.9 joules per ohm.

    Consider that the highest permitted let-though of even a small MCB is a ~ 15 thousand times that the lowest no trip let through is likely to be some thousand or they could have made the limit lower.

    But you may have a lot more than 100uF in several LED power supplies in parallel.

    Mike

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  • 0

    The other thing that may be easier is for the lamp manufacturer to tell you the value of the (flat) capacitor that gets dropped across the mains and has to charge suddenly, perhaps to over 300V, if the switch on moment is near the top of the sine wave.

    We an then pretend htat the curret that flows is a rectange of constant current rather than something that tails off as the capacitor fills, to get an under estimate of the time, but an over estimate of the I2C, which you can compare to the breaker let through to give an upper bound.

    This is not precise, but it will be within a factor of two or so, do realise it is one of those 'pretend the horse is a sphere' type analyses and should be treated as that.

    This is effecively an amount of charge that needs to be transferred, and will try to do so at current set by the (generally very low)  circuit impedance.

    a 100uF capacitor charging at 30Amps, for example, reaches a 300V charge when I*t =C*V = 300* 100E-6 = 3E-2C 

    so in 3E-2/30= 1 millisecond.
    now if we look at the I2t of that rectangular current burst.

    30A*30A * 1E-3 = 0.9 joules per ohm.

    Consider that the highest permitted let-though of even a small MCB is a ~ 15 thousand times that the lowest no trip let through is likely to be some thousand or they could have made the limit lower.

    But you may have a lot more than 100uF in several LED power supplies in parallel.

    Mike

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