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What's IMOD_MAX_OCPR ?

AJJewsbury

I've been trying to make some sense of the newly re-written section 712 (PV) - but I'm struggling with a few things - firstly can anyone tell me what "IMOD_MAX_OCPR" represents? (e.g. in regs 712.431.101 and 102). Unlike most one-off symbols (like Np) it's not explained below, nor can I find it on page 46 (Symbols used in BS 7671). It seems to be something to do with an overcurrent protective device, but seemingly not it's nominal rating, since part of the requirement is that  In ≤  IMOD_MAX_OCPR, nor I guess breaking capacity since 1.35 IMOD_MAX_OCPR ≤ (Ns -1) ISC MAX.

   - Andy.

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    Yes the DC and AC breaking ratings of fuses, like switches are different, for all but the smallest sizes. However if it is a supply with a current limit of the kind where the voltage collapses,  DC does not form much of an arc either, as it sort of provides its own zero-crossing. Well, not crossing exactly  but more of a dive towards zero.
    In fuses with no sand fill, like at drawn arcs, the size of the plasma fireball before is stretches to failure and breaks is broadly of the form volume is proportional to total energy delivered in the last plasma decay time, so volt drop times amps times a few msec..How many milliseconds depends a bit which materials are in the fireball but for air and a bit of copper vapour it is quite a bit less than half a mains cycle. On the other hand for welding rods things are deliberately added to the rod coating that keep enough slow cooling plasma in the mix that the arc stays conductive during the whole AC cycle, or at least  does not need a re-strike from cold 100 times a second. In a fuse you need the reverse.

    Mike

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  • 0

    Yes the DC and AC breaking ratings of fuses, like switches are different, for all but the smallest sizes. However if it is a supply with a current limit of the kind where the voltage collapses,  DC does not form much of an arc either, as it sort of provides its own zero-crossing. Well, not crossing exactly  but more of a dive towards zero.
    In fuses with no sand fill, like at drawn arcs, the size of the plasma fireball before is stretches to failure and breaks is broadly of the form volume is proportional to total energy delivered in the last plasma decay time, so volt drop times amps times a few msec..How many milliseconds depends a bit which materials are in the fireball but for air and a bit of copper vapour it is quite a bit less than half a mains cycle. On the other hand for welding rods things are deliberately added to the rod coating that keep enough slow cooling plasma in the mix that the arc stays conductive during the whole AC cycle, or at least  does not need a re-strike from cold 100 times a second. In a fuse you need the reverse.

    Mike

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