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Topic Title: MCB overload
Topic Summary: When will they trip?
Created On: 23 February 2010 04:57 PM
Status: Post and Reply
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 23 February 2010 04:57 PM
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rikhill

Posts: 411
Joined: 18 October 2007

If subject to a small but permanent overload when (If at all) will an MCB trip?

From my reading of the time current curves in BS7671 they would seem to show a vertical line below approx 1.45ln so I would not expect them to ever trip.

On another thread rocknroll challenged this view and although I wasn't convinced by the physics presented I suspect he might be right and wondered whether anybody who enjoys playing with their test benches could settle it for me.
 23 February 2010 05:23 PM
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AJJewsbury

Posts: 11775
Joined: 13 August 2003

From my reading of the time current curves in BS7671 they would seem to show a vertical line below approx 1.45ln so I would not expect them to ever trip.

Ah, but BS 7671 only shows the slowest allowable trip times (i.e. worst case for shock protection) - look at manufacturer's data (e.g. http://www.mkelectric.co.uk/Do...0Circuit%20Breaker.pdf ) and you'll see that there's an entire range of acceptable times for any given current.

- Andy.
 23 February 2010 05:37 PM
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rikhill

Posts: 411
Joined: 18 October 2007

Thanks Andy - Clearly a much easier to read graph than the BS ones, but that vertical line is still there (Just at 1.13 rather than my incorrectly assumed 1.45) - does that mean that a 6A type B running at 6.5A will hold for ever?
 23 February 2010 05:55 PM
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perspicacious

Posts: 7267
Joined: 18 April 2006

"does that mean that a 6A type B running at 6.5A will hold for ever?"

Even 6.78 A

Hence 433.1

Regards

BOD
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