MCB's - I2 ≤ Iz1.45

[steve100]

Can anybody help me with this......I understand the concept of the equation I2 ≤ Iz1.45, however what I’m not sure about is how we obtain I2?

The way I understand it could be either by multiplying In by 1.45 or multiplying In by 5 (which would be the worst case scenario of the MCB’s tripping characteristics 3 – 5).

So which would you use and what is the difference between the two is what I’m struggling to understand, sorry if I wasn’t very clear?

Thanks

 

[Richard Burns]

I2 is the current causing operation of the device and is the 3-5 * In for type B MCBs that you mention, however the regulations state that the I2 <= Iz * 1.45 is deemed to be complied with if using standard circuit protective devices so it is not necessary to ever know I2.

 

[Sparky Ninja]

Effective operation means regardless of time.

Look at the time curve characteristics in BS 7671 Appendix 3.

For example, A 20A BS 3036 will operate with as little as 40A applied to it.

The fusing factor is the ratio of a fuse's minimum fusing current (the smallest current that will cause the fuse to melt, or I2) and the nominal rated current (usually the number in Amps marked on the fuse body).

BS 3036 fuses have high fusing factors, for example 40A/20A = fusing factor of 2.

The typical fusing factor for a protective device is around 1.45 - 1.8, because the BS 3036 fuse take longer to operate we apply the rating factor, Cf, of 0.725 - which is obtain by diving the 1.45 common factor by the 3036 factor of 2 - when the device is required for overload protection.



I've waffled again.

 

[telectrix]

waffled maybe, but a clear and concise explanation.

 

[drew35]

I like waffles :smug:

 

[AbdulS]

Effective operation means regardless of time.

Look at the time curve characteristics in BS 7671 Appendix 3.

For example, A 20A BS 3036 will operate with as little as 40A applied to it.

The fusing factor is the ratio of a fuse's minimum fusing current (the smallest current that will cause the fuse to melt, or I2) and the nominal rated current (usually the number in Amps marked on the fuse body).

BS 3036 fuses have high fusing factors, for example 40A/20A = fusing factor of 2.

The typical fusing factor for a protective device is around 1.45 - 1.8, because the BS 3036 fuse take longer to operate we apply the rating factor, Cf, of 0.725 - which is obtain by diving the 1.45 common factor by the 3036 factor of 2 - when the device is required for overload protection.

Thank you