The magnetic trip element makes use of a magnetic element (electromagnet). If current reaches a preset quantity, the magnetic element attracts the trip bar and releases the latch. The thermal-magnetic trip element combines the actions of the bimetallic and magnetic elements in a single trip element. If either the bimetal element or the magnetic element reacts, the circuit breaker will trip.
Q.37 What are the three time delay ratings for circuit breakers?
Q.38 What is selective tripping and why is it used?
Q.39 If the power distribution system shown in figure 2-22 uses selective tripping, what is the time delay rating for each of the circuit breakers shown?
Q.40 What factors are used to select a circuit breaker?
Q.41 What type of circuit breaker is used on a multimeter?
CIRCUIT BREAKER MAINTENANCE
Circuit breakers require careful inspection and periodic cleaning.
Before you attempt to work on circuit breakers, check the applicable technical manual carefully. When you work on shipboard circuit breakers, the approval of the electrical or engineering officer must be obtained before starting work. Be certain to remove all power to the circuit breaker before you work on it. Tag the switch that removes the power to the circuit breaker to ensure that power is not applied while you are working.
Once approval has been obtained, the incoming power has been removed, the switch tagged, and you have checked the technical manual, you may begin to check the circuit breaker. Manually operate the circuit breaker several times to be sure the operating mechanism works smoothly. Inspect the contacts for pitting caused by arcing or corrosion. If pitting is present, smooth the contacts with a fine file or number 00 sandpaper. Be certain the contacts make proper contact when the operating mechanism is ON.
Check the connections at the terminals to be certain the terminals and wiring are tight and free from corrosion. Check all mounting hardware for tightness and wear. Check all components for wear. Clean the circuit breaker completely.
When you have finished working on the circuit breaker, restore power and remove the tag from the switch that applies power to the circuit.
Q.42 What steps are to be taken before beginning work on a circuit breaker?
Q.43 What items are you to check when working on a circuit breaker?