Type D MCB 5 Second Tripping Current

Former Community Member over 4 years ago

In BS 7671 17th Edition A3 table 41.3 for type D MCBs gives a 5 second tripping current as 230 x 0.95 / (10In).

When cross referencing time current graphs for type D MCBs I do not get a value of 10In for a 5 second tripping time.

For example see ABB time current graph below giving a current of around 16In for a 5 second trip time.

What is the reason for the difference in values when comparing type D MCB time current charts and table 41.3 in BS 7671 17th Edition A3 for 5 second trip times?

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0 Former Community Member over 4 years ago

@‍mapj1

I'm not so concerned with the practicalities of 6 second trip time, I'm simply concerned as how we adhere to BS 7671 17th Edition A3 clause 411.3.2.3 if the values stated in table 41.3 are below that required to activate the ABB MCB trip mechanism within the required 5 seconds.

Below linked ABB document, page 59 for the graphs :

https://library.e.abb.com/public/b0b41850655145d09f4dd77b6c43acb4/2CCC451059C0203.pdf
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0 Former Community Member over 4 years ago

@‍mapj1

I'm not so concerned with the practicalities of 6 second trip time, I'm simply concerned as how we adhere to BS 7671 17th Edition A3 clause 411.3.2.3 if the values stated in table 41.3 are below that required to activate the ABB MCB trip mechanism within the required 5 seconds.

Below linked ABB document, page 59 for the graphs :

https://library.e.abb.com/public/b0b41850655145d09f4dd77b6c43acb4/2CCC451059C0203.pdf
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Type D MCB 5 Second Tripping Current

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