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Topic Title: MCB Fault Ratings
Topic Summary: Standards & Skilled Persons
Created On: 18 April 2017 03:27 PM
Status: Read Only
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 18 April 2017 03:27 PM
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nad

Posts: 399
Joined: 14 January 2005

Hello Forum Members,

Can anyone elaborate on the reasoning behind the BS EN 60947-2 & 947-2 fault ratings whereby an MCB rated at 10kA can be used to 15kA if fitted behind a locked door?

https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0ahUKEwjX7q3kma7TAhUpDsAKHQNOC54QFgg_MAQ&url=http%3A%2F%2Fwww.hager.co.uk%2Ffiles%2Fdownload%2F0%2F3180_1%2F0%2Fcommercial_installations.pdf&usg=AFQjCNGct9MT7xCA0fO8UOqyJktRCKH7Ow&cad=rja

See page 14,

All answers welcome,

Nad

-------------------------
Nad

*Regularly edited due to spell cheque misdiagnosis
 18 April 2017 03:51 PM
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CMK3PD

Posts: 70
Joined: 23 September 2016

I suspect it relates to 60947 devices being used by skilled or instructed persons as opposed to 60898 for "uninstructed", in order to reduce the possibility on unskilled persons accessing the device and switching it, the locked door comes into play
 18 April 2017 06:17 PM
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Legh

Posts: 4057
Joined: 17 December 2004

Have a look at a previous thread regarding the use of protective devices and back up devices

It doesn't seem to mention though, switching protective devices where high fault currents are present is not something that should be encouraged for non-competent persons. - possible arc flashes and all that.....

Legh

-------------------------

http://www.leghrichardson.co.uk

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 18 April 2017 06:35 PM
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mapj1

Posts: 9670
Joined: 22 July 2004

When a breaker opens, or worse is manually closed, into a near end short circuit, then it is quite possible for hot metal vapour and ionised gas to be expelled Under slow motion filming this looks rather in the manner of a bonfire night sparkler.
I must admit if the idea is that the skilled person won't operate the breaker repeatedly into a fault, but J.public might, then they may be mistaken, if its some of the 'competent persons' I have met .
Personally I'd rather see an energy limit with a fuse... I saw this video on youtube
recently, and while the start is a bit cartoon noddy, about 12-13 mins in they do come tests with large breakers working into a fault, and then some tests that re-create a many tens of kA short just at the outgoing terminals - it is quite instructive to see just how far the flash travels, and in comparison how tame looks and sounds the same test with the fuse (although they refer to it as current limiting, it is really more of a time limiting of course)

-------------------------
regards Mike
 18 April 2017 08:18 PM
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Legh

Posts: 4057
Joined: 17 December 2004

I must admit if the idea is that the skilled person won't operate the breaker repeatedly into a fault, but J.public might, then they may be mistaken, if its some of the 'competent persons' I have met .


Lol.....We've all done that at least once. Reset the breaker to clear a fault....very bad practice.
I like the youtube video, there is another very similar from the GEC stable.

Legh

-------------------------

http://www.leghrichardson.co.uk

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 19 April 2017 02:58 PM
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burn

Posts: 250
Joined: 06 June 2003

I can't open the link but suspect its to do with the difference in the standards.

BS60898 requires a CB to still be operable after breaking a fault of 75% of its rated maximum.

BS60947 only requires it to be operable after breaking a fault of 50% of its rated maximum.

Therefore a CB rated at 10kA by BS60898 is, by default rated at 15kA by BS60947.

Both standards require the CB to safely break their maximum of course, but may fall apart doing so.

burn
 19 April 2017 06:02 PM
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OMS

Posts: 22426
Joined: 23 March 2004

Current limiting fuses are, well, current limiting - by definition, a current-limiting device is one that reduces the peak let-through current to a value substantially less than the potential peak current that would occur if the current- limiting device were not used. When applied to current-limiting fuses, a current-limiting fuse is one that opens and clears the current in less than one half of an AC cycle.

So, they are time constrained but do actually reduce the current - and thus the joule energy into a particular fault (ie the Ampere square seconds)

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 26 April 2017 04:41 PM
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nad

Posts: 399
Joined: 14 January 2005

Many thanks for replies - and thanks for the link to that video - simply perfect.
So I am taking from this that the locking away of the board does not suddenly give the breakers special powers to withstand higher faults but is an indication of how well supervised the installation is. And that if the breakers are under good supervision then they will not be reused without inspection & test after an initial fault.

-------------------------
Nad

*Regularly edited due to spell cheque misdiagnosis
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