Why are MCBs rated differently from fuses?

  • Thread starter Guineafowl
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In summary, old-fashioned fuse wire comes in ratings of 5, 15 and 30A, while the equivalent MCBs (breakers) are rated 6, 16 and 32A. This may be due to the fact that fuses can tolerate slightly higher short term currents, blowing at 30A continuous but tolerating 32A for short periods. However, this may not be entirely accurate as type B MCBs actually trip at 3-5 times the rated current. The B curve rating of an MCB relates to its two functions of overcurrent heating a bimetallic strip and a solenoid that actuates quickly in a dead short.
  • #1
Guineafowl
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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?
 
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  • #2
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
CWatters said:
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.
 

1. Why are MCBs (miniature circuit breakers) rated differently from fuses?

MCBs and fuses are both electrical protection devices used to prevent overloading and short circuits in a circuit. However, they function differently and therefore have different ratings. MCBs are rated based on their breaking capacity, which is the maximum current they can safely interrupt without damage. Fuses, on the other hand, are rated based on their current-carrying capacity, which is the maximum current they can safely handle without overheating. This is why the ratings for MCBs and fuses are not directly comparable.

2. How are MCBs and fuses different in terms of tripping mechanism?

MCBs use an electromechanical tripping mechanism, which means they detect an overcurrent and trip the circuit by physically opening the contacts. Fuses, on the other hand, use a thermal tripping mechanism, where the fuse element heats up and melts to interrupt the circuit. This difference in tripping mechanism also affects their ratings, as MCBs can typically handle short surges of current better than fuses.

3. Do MCBs and fuses protect against the same types of faults?

Yes, both MCBs and fuses are designed to protect against overcurrent and short circuits in a circuit. However, MCBs can also provide additional protection against other types of faults, such as earth leakage and overload, depending on their type and features.

4. Can MCBs and fuses be used interchangeably?

No, MCBs and fuses cannot be used interchangeably. As mentioned earlier, they have different ratings and tripping mechanisms, and therefore serve different purposes. It is important to select the appropriate device for the specific circuit and its requirements.

5. Are MCBs more expensive than fuses?

In general, MCBs tend to be more expensive than fuses. However, they also offer more features and benefits, such as easier reset and better protection against multiple types of faults, which can make them a more cost-effective choice in the long run. It ultimately depends on the specific needs and requirements of the circuit.

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