2.5.7.2.3 Discrimination examples (b) and (c)

This includes types of overcurrent, Short Circuit and RCD protection
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JamieP
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2.5.7.2.3 Discrimination examples (b) and (c)

Post by JamieP »

"Discrimination on overload and instantaneous operation may be
deemed to be provided when C1 ≥ 2 × C2, e.g. C1 curve 63 A
with C2 curve 32 A (see Figures 2.11 and 2.12.)"

This says a 63A with a 32A but using the method of calculation 2*32=64 so a 63A MCB is not ≥ a 32A MCB

"(A) the overload setting of C1 ≥1.6 × C2, e.g. 1000 A with 630
A; and
(B) the instantaneous setting of C1 ≥1.6 × C2, e.g. C1=5 ×
1000 A with C2=5 × 630 A."

630*1.6=1008
630*5*16=5040

1000*5=5000

Same here for the 250 to 799 requirements

Am I missing something or screwing something up or is this some sort of mistake? I highly doubt it's a mistake when it happens in 2 cases
JamieP
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Re: 2.5.7.2.3 Discrimination examples (b) and (c)

Post by JamieP »

I see both these have been changed to actually work properly in the 2018 version
AlecK
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Re: 2.5.7.2.3 Discrimination examples (b) and (c)

Post by AlecK »

They key to this apparent anomaly is the wording "deemed to be provided".
That means it doesn't matter whether the actual long-form calculation works, all that matters is the short-form "x2" calculation
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Re: 2.5.7.2.3 Discrimination examples (b) and (c)

Post by JamieP »

I don't understand what you mean sorry Alec

The short form x2? But the resulting 63A is less the 32A x2
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Re: 2.5.7.2.3 Discrimination examples (b) and (c)

Post by AlecK »

The long form would be a detailed calculation based on the operating curve data for the particular devices. The short form lets us get on the the job using a 'deemed to comply"; regardless of whether it actually complies.
The fact that the example doesn't quite match (by 1 A or 3.1%) doesn't invalidate the deemed-to-comply method, it's just not a very good example. However i believe it was deliberately chosen to indicate that such a small variance doesn't actually matter, when we're already in a 'deemed-to-comply" situation.
That's because all such are designed to be on the safe side.

The change between current and latest editions if from x2 to x 1.5; which is a change rather than a clarification or correction, and cuts us even more slack.
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Re: 2.5.7.2.3 Discrimination examples (b) and (c)

Post by JamieP »

Ah, ok right, I see what you mean, thank you!
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