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

Probably worth noting the data in the original post is taken from Table 41.3 for CB's exceeding 125A

For MCB's operating in conventional 0.4 second time, it should be clear that the MCB will be at or around 20 x In if the Zs data is complied with - so will be out in 0.1 seconds (from the standard) for 0.4 seconds compliance (from BS 7671)

Regards

OMS
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0 Former Community Member over 4 years ago

Probably worth noting the data in the original post is taken from Table 41.3 for CB's exceeding 125A

For MCB's operating in conventional 0.4 second time, it should be clear that the MCB will be at or around 20 x In if the Zs data is complied with - so will be out in 0.1 seconds (from the standard) for 0.4 seconds compliance (from BS 7671)

Regards

OMS
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Type D MCB 5 Second Tripping Current

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