Drawout circuit breaker connected inside switchgear cell. Photo: EATON.
There are many things can happen to a circuit breaker on its journey from the factory to facility, and while they are always tested by a manufacturer prior to shipping, the following checks and tests should be performed at a minimum to ensure that a new low voltage circuit breaker is ready to be placed into service.
Warning: Failure to perform these tests before de-energizing equipment may result in a serious hazard to equipment and/or personnel.
Acceptance Testing should always be performed by qualified workers using test instruments that have been calibrated within the past 12 months.
Visual and Mechanical Inspections
Before conducting electrical tests, circuit breakers are evaluated with a visual inspection to ensure the correct equipment is installed and there are no obvious signs of damage. Mechanisms and other devices such as safety interlocks are operated to ensure proper function before applying voltage.
1.) Compare all nameplate data with jobsite drawings and specifications
Pay special attention to the circuit breaker frame size, ampacity and interrupting ratings.
Verify that the catalog number is correct to ensure that the proper accessories are installed and that the specified control voltage is correct. Ensure that all of the manufacturer supplied maintenance devices for the circuit breaker are available, including charging/racking handles and lifts.
Note: Some circuit breakers may require special interlock keys or secondary blocks for testing outside of its cell. Consult manufacturers literature.
2.) Inspect the physical and mechanical condition of the circuit breaker
The breaker should be clean and arc chutes intact. Inspect the main contacts and finger clusters for abnormalities. Check that circuit breaker lubrication is in accordance with manufacturers specifications and mechanically charge, close and trip the circuit breaker multiple times.
3.) Verify that the circuit breaker fits inside of its cell and is properly aligned.
Look inside the cell for any obvious signs of bent guides or other components. Check that the circuit breaker racking mechanism operates smoothly. Do not rack the circuit breaker onto live bus until all other testing is completed!
4.) Record as found and as left operations counter readings
The operations counter should advance one digit per close-open cycle operation. Verify settings comply with coordination study recommendations and document as found protective device settings. Make sure the trip unit battery is in good working condition.
After a circuit breaker is evaluated and found to be in adequate condition, technicians will proceed with electrical tests. The following checks will evaluate the condition of insulation, contact pressure, and interrupting capabilities before being placed into service.
5.) Inspect bolted connections, fuses, and contact/pole resistance using a low-resistance ohm meter
Microhm or dc millivolt drop values should not exceed levels of the normal range as indicated in the manufacturer's published data.
If no manufacturer's literature can be found, investigate values that deviate from adjacent poles or similar breakers by more than 50 percent of the lowest value.
6.) Insulation Resistance
Perform insulation resistance tests on each pole, phase-to-phase, and phase-to-ground with the circuit breaker closed, and across each open pole. Test duration should be one minute using a voltage in accordance with manufacturers published data.
If no literature from the manufacturer is available, apply 1000 volts DC for equipment with a nominal rating of 600 volts. Apply 500 volts DC for equipment with a nominal rating of 250 volts.
Values should typically be no less than 100 Megohms for equipment with a rating of 600V, and 25 Megohms for equipment rated 250 volts or less.
Important: Units with solid-state components could be damaged if not properly isolated (via removal of plugs and/or fuses) before applying test voltage. Be sure to follow all manufacturers' recommendations when performing dielectric tests on solid state components.
7.) Control Wiring Tests
Perform insulation-resistance tests on all control wiring with respect to ground. Apply 500 volts DC for 300-volt rated cable and 1000 volts DC for 600-volt rated cable for the duration of one minute.
Insulation-resistance values of control wiring should not be less than two Megohms. Control wires should be inspected for loose connections and damaged insulation.
8.) Test all basic trip unit functions via primary injection testing
Verify and document the long time pickup, long time delay, short time pickup, short time delay, instantaneous pickup, ground fault pickup and ground fault delay using a high current test set.
Primary injection should always be performed on new breakers prior to being placed into service to ensure proper CT ratio and polarity. Secondary injection may only be used after primary injection testing has first been completed.
Ground fault protection will most likely need to be disabled by either rewiring or using the manufacturers recommended device. Saving ground fault tests for last will ensure the function is working properly before placing the circuit breaker into service.
9.) Verify the correct operation of any auxiliary features
Most of these tests, such as electrical charge and close operations can be performed while primary injection testing using the proper voltage listed on the circuit breaker nameplate. Other devices may require special equipment and procedures.
10.) Reset all trip logs and indicators after testing is completed
Verify the breaker is discharged, racked out, and that any auxiliary plugs removed for testing have been re-installed. Ensure the circuit breaker is ready to be energized and placed into service.