# Re: How does a Circuit Breaker work?

Home Electrical Engineering Forum General Discussion How does a Circuit Breaker work? Re: How does a Circuit Breaker work?

#12091
Spir Georges GHALI
Participant

Dear Mr. Nasir ;

Refer to your 2nd Topic about the Circuit Breaker, and as we talk about “ Low Voltage Circuit Breakers ”, please let me clarify again some points, that are :

1- It’s written “ To prevent this larger amount of current from entering the circuit ahead of its path ” !!! ???  We know all that any current “ Load Current  or  Overload Current  or  Short Circuit Current ” flows in the circuit and go ahead to the source, because the circuit should be completed from the fault point till the source to have the fault current, so, how the Circuit Breaker can prevent the Short Circuit Current going ahead ( entering the circuit ahead ) ?

2- It’s written “The working of a circuit breaker is more or less similar to a fuse ” !!! ??? : if you talk about protection issue “ Yes ” that’s right, but if that’s about working way, sorry to say that’s not right, because we know all that the working way of a Circuit Breaker is completely different than the Fuse as the Fuse-link in any Fuse will melt when the current flows during a predefined time is bigger than a predefined value, but in a Circuit Breaker no piece will melt.

3- Most of the Circuit Breakers ( 95% or even more ) contain 2 kind of Protections, that are :

• “ Thermal Overload Release ” : used to protect the electrical circuit against the “ Overload Faults ” can happen in an electrically undamaged circuit, that means those faults are provoked by the connected equipment or loads ( IEC 60947-1 / definition 2.1.7 ). Noting that the Tripping Order of this protection is “ Delayed ( Inverse-time ) ”.

• “ Magnetic  Release ” : that’s used to protect the electrical circuit against the “ Short Circuit Faults ” can happen in any part of an electrical circuit “ components or connected equipment or loads ” ( short circuit between 2 or more live conductors ) ( IEC 60947-1 / definition 2.1.5 ). Noting that the Tripping Order of this protection is “ Instantly or Delayed ( up to 600 ms ).

Noting that the Operation way of those 2 Protections Releases is totally different.

4- It’s written “ When the current decreases back, the electromagnet is de energized and the contact is maintained again and the circuit operations safely ” !!! ???  There are in this sentence 3 points that :

• We know all that the Fault Current doesn’t decrease by itself, except the Over-load’s Current in the case of without tripping and when the load re-becomes normal, but the short circuit current doesn’t, never ever, decrease without doing the necessary maintaining of the damage circuit.

• In the standard operating of any Circuit Breaker when it trips according to an order of Magnetic Release, the electromagnet is immediately de-energized as there’s no more flowing current to energize it.

• If you mean that the contacts will close again, sorry to say that’s not right, because in the standard operating of any Circuit Breaker when it trips the contacts doesn’t reclose by themselves, except if the Circuit Breaker is “ Electrically Operated ” ( equipped with Electric Motor, Closing & Opining relays, with a Control System ). Even more, we shouldn’t re-close any Circuit Breaker if the tripping was provoked by a short circuit current, because the Making Short Circuit Current will be, at the first cycles ( 5 to 10 cycles ) of current, bigger than the short circuit current.