Maintenance and Testing of Low-Voltage Protective Devices
Low-voltage protective devices consist of protective devices for low-voltage MCCBs, insulated case circuit breakers, draw-out power circuit breakers, overload relays, and ground fault protective (GFP) devices. The protective devices are integral parts of the circuit breakers, and motor starters and their maintenance and testing should be coordinated with the maintenance of the circuit breakers, motor starters, and switchgear assemblies. Low-voltage pro- tective devices are classified as low-voltage power circuit breaker trips, MCCB trips, overload relays, and ground fault sensing and relaying equipment.
Power Circuit Breaker Overcurrent Trip Devices
Protective devices for power circuit breakers consist of electromechanical type and solid-state (electronic) types. The routine maintenance tests for these breaker overcurrent trip devices comprise of testing overcurrent trip units, insulation integrity, and quality of contacts.
Overcurrent Trip Units
Electromechanical Trips
Theseare hermeticallysealedunits thatprovide time-delayand INST overcurrent protection. The maintenance and testing of these devices involve checking the operation of the trip device and evaluating their trip characteristics. Before any field tests are made, the tester should be thoroughly familiar with the operating and maintenance procedures of these devices. He should also check that the breaker mechanism and trip latch are properly functioning. Ensure that the breaker is de-energized and perform the following maintenance and tests:
Mechanical check: Perform a mechanical check on the trip device to assure a successful tripping operating just before the armature reaches its fully closed air gap position. Consult the manufacturer’s information on the unit under test for conducting this check. Also assure that the time-delay escapement is operative. Visually check for missing hardware, evidence of leaking oil, and cracked trip paddles. Use manufacturers’ manuals for cleaning methods for trip units.
Overcurrent test: The purpose of this test is to determine that the trip device will open the circuit breaker to which it is applied. This test can usually be per- formed by injecting 150%–300% current of the coil rating into the trip coil. The test equipment used should be able to produce the required current and be rea- sonably sinusoidal. Two low-voltage AC test sets, one capable of testing small size breakers and the other large frame breakers, are shown in Figures 8.4 and 8.5. The following test procedure is offered as a general guide.
• Connect the test set to the upper and lower studs of one pole of the breaker. Set the LTD trip unit at 100% setting. Close the breaker and adjust the current of the test set to the desired value (i.e., 150%–300%) required for the particular trip device. Consult the manufacturers’ recommendations for exact values of the test current value.
• Shut off the test set and allow the trip device to reset. After the trip device has reset, again apply the current to it until it trips. Record the trip time and trip current.
• If repeat tests are required, allow the device to cool sufficiently between each test.
• Compare the trip time measured at the test current values with the manufacturer’s curve for the trip device being tested. Make adjust- ments to bring the trip device within factory trip curve values. However, do not exceed adjustable ranges of the trip unit when making field adjustments.
In view of the wide variation in test conditions between field tests and factory tests, it will be difficult to duplicate the factory trip curves. Judgment should be used when evaluating the test data as to whether the trip unit is function- ing within specified limits. The STD and INST tripping units may be tested similarly. The setting of these units should not be indiscriminately changed because the protection provided by these units may be compromised.
olid-State Trip Units
The field testing and calibration of solid-state trip units can be performed by either primary current injection method or secondary current injection method.
Primary current injection tests
The primary current injection method is usually preferred because it is expected that this method verifies the current sensors, wiring, and the current conduction path in the breaker. However, this method has a shortcoming in that it will not detect sensor wiring and polarity problems. This is because the primary injection test is conducted on one phase of the breaker at a time, whereas the solid-state trip logic of the breaker works on processing the signals from the three-phase sensors simultaneously. In order to identify sensor- and wiring-related problems, it is recommended that the primary current injection test be conducted simultaneously on all three phases when testing breakers with solid-state trip units. If three-phase primary injection test cannot be conducted, then it is recommended that the sensors and wiring of the breaker be tested separately to ensure that these components are working properly.
The correct functioning of the trip devices of low-voltage power circuit breakers can be tested using primary current injection as discussed above.
However, because primary current injection testing is a relatively expensive service, it is usually performed only on circuit breakers that are components of a critical process or engineered safety system. Circuit breakers that have thermal-magnetic or electropneumatic trip devices are more likely to be tested using primary current injection because it is the only means available for verifying their correct functioning. Circuit breakers that have solid-state trip systems can be tested using secondary current injection, which is less expensive and uses less time to perform the test. Since, secondary current injection test cannot verify the correct functioning of the current sensors of a solid-state trip system, primary current injection is used during commissioning (start-up) to supplement a program of periodic secondary-current testing. The method of primary current injection testing is to make a programmed sequence of overload and fault magnitude currents flow in a circuit breaker and measure the periods of time that are required for the trip device to activate. When these tests are performed at a factory or repair facility, current is injected into all three poles of a circuit breaker at the same time. Start-up and maintenance tests are performed using a primary current injection test set that is specifically designed to be lighter in weight and more portable than factory test equipment. Consequently, this portable test set has insufficient capacity, in most cases, to inject current into all three interrupters of a circuit breaker simultaneously. Because of this shortcoming and other factors that make field testing generally less accurate than factory testing, single-pole testing of low-voltage circuit breakers is almost universally accepted as a reasonable compromise. A complete description of the methods and interpretation of field testing of MCCBs can be found in NEMA standard AB-4-2003. The test set has a built-in high-current transformer that supplies the simulated overload or fault current. Test sets are built with current ratings ranging from 500 to 100,000 A.
Secondary current injection
The secondary current injection test of solid-state units can be performed by a specially designed power supply unit. It should be noted that the secondary current injection method only tests the solid-state trip unit logic and components, and does not test the current sensors, wiring, or the breaker’s current- carrying components as is done during primary current injection method.
Therefore, in this respect the primary current injection test method is superior to the secondary current injection method. Most solid-state trip units have terminal blocks that are equipped with test plug terminals for making the calibration test. The test set allows checking of the solid-state trip unit operation without using primary current method. The test set will pass enough current to check any desired calibration point. The breaker must be disconnected from the bus before checking the operation of the solid-state trip units.
If the test set shows that the solid-state trip unit is not functioning properly, the trip unit should be replaced and the defective unit returned to the manufacturer for repairs. It is recommended that the reader refer to the instructions
of a particular secondary current injection test set for operating procedures.
Secondary current injection tests are performed for the same reason as primary current injection current tests, i.e., to verify the correct functioning of breaker trip devices during startup inspections or maintenance inspections.
Secondary current tests can be performed on the solid-state trip and electronic trip devices as follows:
Using the self-test facilities of solid-state trip devices: Solid-state trip devices and protective relays of recent manufacture contain built-in self-test facili- ties. Typically, a self-test can be conducted in two different modes:
1. No-trip mode: The trip functions of the solid-state electronic trip device can be tested, but the trip device will not send a trip signal to the circuit breaker’s trip actuator. Because a no-trip test will not cause the circuit breaker interrupters to open, it can be performed while the circuit breaker is energized (i.e., carrying load current).
2. Trip mode: The functions of the solid-state electronic circuit are tested in the same way as in a no-trip mode test, but the trip device will send a trip signal to the circuit breaker’s trip actuator. Because a trip test will open a circuit breaker, it is typically performed only when a circuit breaker is withdrawn from its compartment and therefore disconnected from the switchgear bus. For a circuit breaker
that cannot be withdrawn from its compartment, an interruption of power must be expected. Self-tests are easier to perform and can be performed more frequently. For example, no-trip tests can be per- formed monthly. A trip test is very useful for troubleshooting a suspected circuit breaker malfunction. Like secondary current injec- tion tests, self-tests do not verify the correct functioning of the trip system’s current sensors and the associated current wiring. Additionally, some of the internal components of the trip device that carry secondary current cannot be functionally tested. For these reasons, self-testing is occasionally supplemented with secondary current injection testing or primary current injection testing. Many modern solid-state trip devices continually execute a programmed sequence of self-diagnostic checks. A distinguishable change on the display panel of the trip device, such as the cessation of the flashing of a status lamp or the appearance of an alpha-numeric fault mes- sage is an indication of potential problems in the trip unit. Additionally, the trip device is able to communicate its alarm or fault condition via a built-in relay contact or digital communication system if such a feature is bought with the trip unit.
Functional tests of the electric controls: Before installing a new circuit breaker or returning it to service after a maintenance inspection, it should be installed in its test position in its compartment and operated closed and open electri- cally from as many control devices as practical. Checking the correct func- tioning of a circuit breaker’s electric control verifies the integrity of control wiring, control components, and the source of control power. When a circuit breaker is in its test position in its compartment, closing its interrupters will not connect the associated load circuit with the switchgear’s power source circuit. It should be noted that the functional testing described in this section may not be performed while personnel are performing work on electrical equipment that is connected to the breaker’s load circuit.