Mcb's

[Steve100]

Can anybody help me with defining the difference between the tripping factor (1.45 times In) of a BS EN 60898 Type B MCB and its characteristics relating to tripping between 3 – 5 times the full load?

Thanks

Steve

 

[Nicky Tesla]

they are all designed to start tripping at 1.45 times full load but this may take some time to trip. The 3 to 5 times band is the area the mcb will begin to trip in less than 0.4s usually less than 0.1s

 

[Steve100]

Okay thanks that's helpful, so in that case which one would I use to signify I2? Would it be rating of the mcb x 1.45?

 

[Nicky Tesla]

I2 a current which will trip the MCB (which could be an hour for example) must not EXCEED iz the current capacity of the conductors so i2 could be lower. Check the graphs if need be http://download.hager.com/Hager.ie/files_download/catalogue/Protection_Technical.pdf

 

[Steve100]

Thanks, 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

 

[Nicky Tesla]

In must be >or= to 1.45 Iz

Your device which you choose must have characteristics that will trip the MCB or blow the fuse before the current in the conductor to which your device is connected, gets above 1.45 times its rated current.

ie; 2.5T&e clipped direct has a rating of 27 amps. Therfore your device must trip if it is left to run with 27 x 1.45 amps through it = 39.15A

But the thing to remember, is that fuses and MCBs are so rated that the 1.45 thing is already taken care of. So if you have say a 10A MCB supplying a 10A rated cable, the graphs will show that the MCB will trip if the current through it remains at 14.5A or more but it may also trip at say 12.5A. Check the graphs to find out.