I'm confused about Type C Breaker Disconnection Times

[John Lundrigan]

Hi All,
When I consult Table 41.3 for a Type C Breaker rated 16A I see s max Zs of 1.37 Ohms. However this value is defined for both a 0.4s AND 5s disconnection time. This is obviously correct so I am missing something. Fig 3A5 also shows the table 0.1s to 5s. Can anyone explain please?

Thanks

Jonel

 

[westward10]

0.10 sec disconnection times apply when the Uo is above 400v.

 

[Strima]

Also take a look at manufacturers data, these often differ from the book as they have been designed to conform to both 60898 and 60947-2.

 

[Deleted member 26818]

MCBs have two methods of tripping.
One uses a bi-metallic strip, which heats up as current passes through it.
If the current X time is correct the MCB will trip.
The other method uses a coil and if the current is great enough, the MCB will trip instantaneously (deemed to be 0.1s, though in reality is probably less, somewhere between 0.01 and 0.08s, or half to four cycles).
It just happens that the current which would cause the bi-metallic strip to trip in 5s is the same which would cause instantaneous tripping.
Fig 3A5 gives the maximum Zs required to allow an MCB to trip within the prescribed time limits for all circuits in a TN installation 0.4s for final circuits and 5s for distribution circuits.