Technical information

It is known that the value calculated in the "current x coefficient" is the

maximum magnetic protection calibration value.

For example, an “inverse time” automatic circuit breaker installed upline from a

motor with a current equal to 10 A, should have a magnetic threshold of 10 A x

250% = 25 A.Automatic circuit breakers according to European standards

normally have a magnetic threshold ten times that of the thermal threshold: thus a

breaker calibrated magnetically at 25 A is able to bring to 2.5 A capacity before

the thermal protection trips in and this is not suitable for the motor under

consideration.The nub of the problem is the fact that for the NEC and the CEC

the circuit breakers must have a fixed thermal and magnetic adjustable in a wide

range; the construction criterion is therefore opposite that of the European.

Nev ertheless NEC/CEC nev er make precise r eference to the magnetic threshold,

but only to a g eneral calibration: this leaves the margin on the use of automatic

circuit breakers approved as BCP but of a European type in which the calibration

defined on the basis of the product “current x coefficient”, is applied to the

adjustable thermal threshold.

On the contrary, the “type E” motor starters also called “self-protected

combination motor controller”, are not taken into consideration because the

magnetic tripping value is fixed constructively at 1200 to 1300% of the current of

the motor and this value is implicitly accepted in which their use is permitted. In

the “type E” motor starter, only the thermal protection is calibrated.

The NEC and the CEC provide for the following standard sizes/regulations for

fuses and circuit breakers: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125,

150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 601, 700, 800, 1000, 1200,

1600, 2000, 2500, 3000, 4000, 5000 and 6000.Additional sizes for fuses are 1, 3, 6

and 10.

BRANCH CIRCUIT