Generators onboard ships are the power suppliers for the entire vessel as they stand as the primary source of power to all running machineries including the propulsion plant. For this reason, safe and efficient running of the ship generator has to be given highest consideration. One of the important safety devices used for ship’s generator is Air Circuit Breaker (ACB).
Air circuit breaker is designed to overcome the defects and safeguard the machine before it breakdowns.
The main function of air circuit breaker is to:
– Open and close a 3 phase circuit, manually or automatically.
– Open the circuit automatically when a fault occurs. Faults can be of various types – under or over voltage, under or over frequency, short circuit, reverse power, earth fault etc.
– The main feature of ACB is that it dampens or quenches the arcing during overloading.
Air Circuit Breaker (ACB) Construction & Working
ACB has two sets of contacts i.e. main and auxiliary contacts. Each set of contact consists of a fixed contact and a moving contact. The main contact normally carries most of the load current. All the contacts are made of cadmium-silver alloy which has very good resistance to damage by arcing.
When the ACB is closed, the powerful spring is energized and the ACB is than latched shut against spring pressure. The auxiliary contact makes first & breaks last i.e. when ACB is closed, the auxiliary contact closes first then the main contact follows.
When the ACB is open, the main contact open firsts and then auxiliary contact opens. Thus the auxiliary contacts are subjected to arcing during the opening of ACB and can easily be replaced.
The main contact closing pressure is kept high so that the temperature rise in the contacts while carrying current remains within limit. Closing coil operating on D.C voltage from a rectifier is provided to close the circuit breaker by operating a push button.
How Arc Quenching is Achieved?
Quenching of arc is achieved by:
1.Using arcing contacts made of resistance alloy and silver tips for the main contacts. Arcing contacts close earlier and opens later than the main contacts.
2.When opening contacts have long travelled at high speed to stretch the resultant arc, which is transferred to the arcing contact.
3.Cooling and splitting of the arc is done by arc chutes which draw the arc through splitters by magnetic action and quickly cool and split the arc until it snaps. The circuit breaker opens when the arc is quenched.
Reference: Book on Marine Electrical equipment by H.D.McGeorge