what is the meaning of A, B, C & G
- Thread starter venkatesh
- Start date
I dont think you mean breaking capacity - thats a different thing.
Its the operating curve of the circuit breaker.
These are referred to in graphs & tables in BS7671 and they will tell you the disconnect and overload characteristics for circuit breakers (& fuses).
You use these tables to determine that the cb will disconnect within the correct time (eg 0.4 or 5 secs) when a particular fault current is applied.
These days its a choice of B C or D. Dont remember a G and A no longer exists.
TTC
Its the operating curve of the circuit breaker.
These are referred to in graphs & tables in BS7671 and they will tell you the disconnect and overload characteristics for circuit breakers (& fuses).
You use these tables to determine that the cb will disconnect within the correct time (eg 0.4 or 5 secs) when a particular fault current is applied.
These days its a choice of B C or D. Dont remember a G and A no longer exists.
TTC
It determines Ia, defined in BS 7671 as, "current causing automatic operation of protective device within the time stated"
For type B this is 5 x In
For type C this is 10 x In
For type D this is 20 x In
Where In is the nominal rating of the breaker.
You can think of it as the instantaneous (<0.1 sec) trip current. So, for example, a 6A type B would require a current of 5 x 6 = 30A to guarantee an fast trip, a 32A type C would need 32 x 10 = 320A and a 20A type D would require 20 x 20 = 400A.
Much higher start-up, or 'inrush' currents are a feature of things such as motors, discharge lighting, welders, etc so you select a breaker nominal rating to handle the continuous load and type to deal with the short-term inrush current.
(There is no Type A, which would cause confusion with the use of A for amperes.)
For type B this is 5 x In
For type C this is 10 x In
For type D this is 20 x In
Where In is the nominal rating of the breaker.
You can think of it as the instantaneous (<0.1 sec) trip current. So, for example, a 6A type B would require a current of 5 x 6 = 30A to guarantee an fast trip, a 32A type C would need 32 x 10 = 320A and a 20A type D would require 20 x 20 = 400A.
Much higher start-up, or 'inrush' currents are a feature of things such as motors, discharge lighting, welders, etc so you select a breaker nominal rating to handle the continuous load and type to deal with the short-term inrush current.
(There is no Type A, which would cause confusion with the use of A for amperes.)
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