Why are MCBs rated differently from fuses?

  • Thread starter Guineafowl
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  • #1
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Main Question or Discussion Point

Old-fashioned fuse wire comes rated at 5, 15 and 30A. The equivalent MCBs (breakers) are rated 6, 16 and 32A.

Is there a reason for this difference?
 

Answers and Replies

  • #2
CWatters
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Not 100% sure but I thinks it's because fuses tollerate slightly higher short term currents. Eg they blow at 30A continuous but tollerate 32A for short periods.

Edit: Think that might be wrong because type B MCB actually trips at 3-5 times the rated current.
 
  • #3
571
265
Not 100% sure but I thinks it's because fuses tollerate slightly higher short term currents. Eg they blow at 30A continuous but tollerate 32A for short periods.

Edit: Think that might be wrong because type B MCB actually trips at 3-5 times the rated current.
An MCB has two functions: overcurrent heating a bimetallic strip and a solenoid that actuates with very fast rise times, as in a dead short.

I’m not sure how the B (or whatever) curve rating relates to these.
 

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