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Type A R.C.D. 6mA tolerant.

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Can I really be confident in the use of a Type A R.C.D. that will tolerate up to only 6mA D.C. current and still operate?

I have a box of old Pifco torch bulbs as used in battery motorists' lanterns. They are filament bulbs rated at 6.2V 0.5 Amp. Their current draw is 500mA. And that for a not very bright torch bulb. That current is supplied via dry cells.

So, the Type A R.C.D.s can only work reliably with a D.C. current of up to 6mA. That is a piddly low current.

Comments please.

Z.

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    mapj1: 
     

    That is quite a nice illustration, thank you for finding it.

    The implication is that the figure shows a case where  “IRdc > 6mA”

    Is sort of true, but actually  a considerable understatement. As drawn, the example really shows the case when  IRdc >> 6mA, indeed IRdc is still  > 2.8* 30mA.

    The degree of blinding, i.e. the extra current needed to force a trip, will be comparable to the DC bias, not many times it.

    This is quite unfortunate, as I suspect many of the target audience will misunderstand it and over estimate the effect. This is  especially likely given the way magnetic theory seems to be taught to electricians (or not - given the erroneous way some text books showing arrows for the directions of magnetic fields and eddy currents, it is sad but not so surprising that the general awareness often equates with the precision of an exercise in pin the tail on the donkey)

    Mike

    PS for anyone whose maths is a bit rusty.

    >>  is vastly greater than, whereas 

    > is just ‘anything greater than’

     

    Are we to understand that the small D.C. current needed to drive a typical L.E.D. with series resistor, say 10-15mA, is enough to disable an A type R.C.D?

    Z.

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  • 0

    mapj1: 
     

    That is quite a nice illustration, thank you for finding it.

    The implication is that the figure shows a case where  “IRdc > 6mA”

    Is sort of true, but actually  a considerable understatement. As drawn, the example really shows the case when  IRdc >> 6mA, indeed IRdc is still  > 2.8* 30mA.

    The degree of blinding, i.e. the extra current needed to force a trip, will be comparable to the DC bias, not many times it.

    This is quite unfortunate, as I suspect many of the target audience will misunderstand it and over estimate the effect. This is  especially likely given the way magnetic theory seems to be taught to electricians (or not - given the erroneous way some text books showing arrows for the directions of magnetic fields and eddy currents, it is sad but not so surprising that the general awareness often equates with the precision of an exercise in pin the tail on the donkey)

    Mike

    PS for anyone whose maths is a bit rusty.

    >>  is vastly greater than, whereas 

    > is just ‘anything greater than’

     

    Are we to understand that the small D.C. current needed to drive a typical L.E.D. with series resistor, say 10-15mA, is enough to disable an A type R.C.D?

    Z.

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