|Home | Articles | Forum | Glossary | Books|
8. 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 protective devices are classified as low-voltage power circuit breaker trips, MCCB trips, overload relays, and ground fault sensing and relaying equipment.
8.1 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.
8.1.1 Overcurrent Trip Units
184.108.40.206 Electromechanical Trips
These are hermetically sealed units that provide time-delay and 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 reasonably sinusoidal. Two low-voltage AC test sets, one capable of testing small size breakers and the other large frame breakers, are shown in Figures 4 and 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 adjustments 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 functioning 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.
220.127.116.11 Solid-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 con ducted, 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 facilities. 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 injection 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 message 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 electrically from as many control devices as practical. Checking the correct functioning 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.
8.2 Molded-Case Breaker Trips
MCCBs are low-voltage protective devices that are available in a wide range of sizes and ratings. They are used widely in the industry to provide a reset table circuit interrupting device. MCCBs have a good record of reliability when they are maintained and calibrated regularly and properly. A general guide on the field and verification testing of MCCBs is offered below. For a more detail discussion on the test procedures on MCCBs, the reader is referred to NEMA standard AB-4-2003, "Guidelines for inspection and preventive maintenance of molded case circuit breakers used in commercial and industrial applications." MCCBs having thermal-magnetic trips are tested with primary current injection method. Unlike other circuit breakers, the tolerances for minimum trip current values and trip times that are displayed on the time-current plots provided by the breaker's manufacturer cannot be accurately replicated using field test methods and field test equipment. For this reason, NEMA standards publication AB-4-2003 should be used as a guide for field testing of MCCBs.
MCCBs that have solid-state trip devices can be tested by secondary current injection using a test set made specifically for this purpose by the breaker's manufacturer, or primary current injection method. The primary current injection method for testing MCCBs is described in more detail in the following sections.
8.2.1 Protective Trip Testing
The testing of protective trips involves the calibration of overload (thermal) and magnetic overcurrent trips to verify that the trip units are functioning as expected and open the circuit breaker automatically. This is important from the viewpoint of protection and system selectivity.
18.104.22.168 Overload (Thermal Element) Test
The overload trip characteristics (i.e., time-current relationship) can be verified by selecting a certain percentage of breaker current rating, such as 300%, and applying this current to each pole of the circuit breaker to determine if the breaker will open in accordance with the manufacturer's specified time. The obvious goal is to see if the circuit breaker will automatically open and, further, to see if it opens within the minimum and maximum range of operating time bands. For example, ANSI/IEEE standard 242-2001, Section 15.3 specifies a test tolerance of -15% for the minimum operating time band and +15% for the maximum operating time band.
For specific values of operating times, refer to the manufacturer's manual for breakers under test. The evaluation of test results is based upon the following:
Minimum trip times: If the minimum tripping times are lower than indicated by the manufacturer's published data plus -15% for the breaker under test, the breaker should be retested after it has been cooled to 25°C. If the values obtained are still lower after retest, the breaker manufacturer should be consulted before reenergizing.
Maximum tripping time: If trip time of the breaker exceeds the maximum tripping time as indicated in the manufacturer's published data plus +15% for the breaker under test, recheck the test procedure and conditions (as shown under verification testing), and retest. If the test still indicates higher values than maximum tripping, further check the breaker for maximum allowable tripping time.
Maximum allowable tripping time: If the breaker does not trip within the allowable maximum time, the breaker should be replaced. However, if the breaker tripping time is below the maximum allowable but higher than the maximum tripping time, the breaker should be checked to see if it is below the tripping time for cable damage. If so, the breaker is providing an acceptable level of precaution.
22.214.171.124 INST (Magnetic) Test
The magnetic (INST) trip should be checked by selecting suitable current to ensure that the breaker magnetic feature is working. The difficulty in conducting this test is the availability of obtaining the required high value of test current. Again, to verify the breaker trip characteristics, precise control of test conditions is necessary; otherwise, different test results will be obtained.
Moreover, due to large values of test current, the trip characteristics of the breaker can be influenced by stray magnetic fields. Also, the current wave shape can influence the test results. Therefore, when conducting this test, stray magnetic fields should be minimized and true sinusoidal test wave shape should be used. The magnetic trip unit may be tested as follows:
In the run-up method, one pole of the breaker is connected to the test equipment and approximately 70% of the tripping current is injected into the breaker gradually until the breaker trips. The injection of current into the breaker has to be done skillfully so that it is neither too slow nor too fast. If the injection of current is too slow, the breaker may trip owing to the thermal effect and not provide a true value of tripping current. Whereas if the current is injected too quickly, the meter reading will lag the actual current owing to damping of the meter and thus provide an erroneous test result. It is very difficult to obtain true test results from this test.
The pulse method requires equipment with a pointer stop ammeter or an image-retaining oscilloscope. This method is generally considered more accurate than the run-up method. The current to the circuit breaker under test is applied in short pulses of 5- to 10-cycle duration until the breaker trips.
The current is then reduced just below this value, and the pointer stop on the ammeter is adjusted by repeated pulses until the pointer movement is barely noticeable. The current is then raised slightly and the tripping value of current rechecked. One disadvantage of this test is that it is subject to DC offset when conducted in the field. The DC offset may be as high as 20%, and therefore the tripping current indicated by the ammeter may be 20% lower.
Because of the inherent errors in the field testing of protective trips, test results may vary from the manufacturer's published values. Therefore, the main thrust of any field testing of molded-case breakers should be to ensure, first, that the breaker is functional and, second, that its trip characteristics are within the range of values for that particular type of circuit breaker.
NEMA AB-4-2003 provides recommended tolerances for testing INST trip units in the field. These tolerances are summarized in Table 7.
TABLE 7 INST Trip Tolerances
8.2.2 Verification Testing
The verification testing of MCCBs in the field is intended to check the circuit breaker performance against manufacturer's published test data. When performing field verification testing of MCCBs the important issue is how field testing is conducted compared to the testing done at the factory to develop the MCCB time-current curves. All low-voltage MCCBs that are UL listed are tested in accordance with UL standard 489 and NEMA AB-1.
The following is a summary of conditions under which the manufacturers and UL calibration tests are conducted to obtain the trip-time curves.
Current intended for testing the MCCBs shall be essentially sinusoidal and of symmetrical waveform. When performing verification testing in the field, the conditions as stated in UL standard 489 and NEMA AB-1 must be simplified. But the simplified testing must recognize the differences in testing results that are obtained for various test setups in the field. Attempting to reproduce laboratory test conditions in the field can be expensive and difficult to achieve. The overload trip test performed at 300% current should confirm that the breaker trips within the tolerances shown in the time delay region plus some tolerances to account for the differences between the field and factory test conditions.
The INST trip test should demonstrate that the breaker will trip before the high end limit of the INST trip is reached, and will not trip prematurely before the low end of the INST trip range. In other words, the breaker should trip somewhere within the expected band which is comprised of lower limit and upper limit of breaker time-current curves. The INST trip test is prone to significant variation and duplicating the manufacturer's curves may not be straightforward process. If the data measured under the verification tests vary significantly for the INST trip, the test conditions must be verified or the manufacturer should be consulted before discarding the breaker.
8.3 Overload Relays
Overload relays usually found in motor starters or other low-voltage applications require the same attention and calibration as do low-voltage circuit breaker trips. Overload relays should be given an overcurrent test to deter mine that the overloads will open the starter contacts to provide protection to the motor at its overload pickup value. These test procedures are similar to the test conducted for low-voltage circuit breakers, except that the current injected into the overload relay should be limited to 350% or less. The frequency for testing and calibration should be checked to assure that it is selected properly. In addition, the trip setting of the relay should be evaluated to account for any ambient variations between motor location and starter location. A motor overload test set is shown in FIG. 6.
8.4 Testing of Ground Fault Sensing and Relaying Equipment
Ground fault sensing and relaying equipment is covered by UL standard 1053. It classifies ground fault protection into class I and class II. Class I ground fault protection is intended to be used with disconnecting devices at high levels of fault current, whereas class IFI ground fault protection is used with disconnecting devices with limited interrupting current capacity.
This testing application guidance is directed toward class I ground fault relaying.
In accordance with NEMA Publication PB2.2-2004 (Application Guide for Ground Fault Protective Device and Equipment), manufacturers are required to perform design and production tests. The design tests include calibration, temperature rise, overvoltage, overload, dielectric withstand, endurance, and the like. The production tests are conducted to determine if calibration settings are within performance limits, control circuits are working properly, and current sensors have correct turns ratio. The field testing of GFP relaying equipment is discussed in the following section.
8.4.1 Preparation for Fielding Training
Review the electrical drawings for the power system, as well as the manufacturer equipment drawings, to ensure that ground fault equipment is installed as designed.
With the power off, remove the disconnect link on the switchboard to isolate the neutral of the wiring system from both supply and ground. Measure the insulation resistance of the neutral to ground with the main disconnect open to ensure that no ground connections exist downstream of the GFP devices being checked.
For a dual fed (double-ended) power system, remove all the disconnect links to isolate the neutral from both the supply and ground before measuring the insulation resistance.
Visually inspect the wiring system to confirm there is an adequate grounding connection at the service equipment upstream of any ground fault sensor, and that the neutral connection is run from the supply transformer to the service equipment in accordance with the National Electric Code. Where dual power sources are involved, confirm that the main grounding connection at the service equipment is in accordance with manufacturer's recommendations.
Once these steps have been accomplished, return all neutral and ground connections to their normal intended operating condition.
8.4.2 Field Testing
Field testing should be limited to only those tests that are necessary to determine that the installation is correct and the ground fault protection system is operational. Because of the many variables involved, field testing cannot be considered as an accurate check of the calibration of any sensing relay. Field test current sources can introduce errors owing to nonsinusoidal wave shapes, power source regulation problems, and metering accuracy. In addition, timing measurements often include additional delay times owing to the use of auxiliary relays and timers. Field testing should be limited to a go/no-go type of testing, which confirms the serviceability of the system involved.
Before field testing is initiated on any ground fault sensing and relaying equipment, the manufacturer's installation and instruction literature should be reviewed and understood. The manufacturer's field test recommendations should be followed. Although some manufacturer's test setups may be difficult to perform in the field, the configuration is very important in order to obtain good results.
If a particular device is self-powered, considerable current (100-700A for a 4000A device) may be required to activate it. This is especially true where multiple sensors are being used in a vector summation scheme. It should not be assumed that inducing current on a ground return or neutral sensor alone will be sufficient to activate the device and get accurate time-current characteristics. This practice can lead to finding a device defective when nothing is actually wrong with it.
However, as noted below, there are systems that use external control power for which this practice is acceptable.
Ground fault sensing and relaying equipment utilizing either ground return or vectorial summation sensing methods can be checked in the field by passing a measured test current directly through the sensing transformer or test windings. To confirm the proper functioning of the equipment while it is installed in the switchboard or panelboard. The following tests can be performed:
A go/no-go test for the proper tripping of the GFP devices and the interconnections between the sensor, the relay, and the disconnect mechanism can be made by following manufacturer's test instructions. The manufacturer usually provides for a test current >125% of the maximum current setting, so a test can be made anytime without disturbing the current settings.
If there is any question concerning the ability of the GFP device to operate at its minimum setting and for low ground fault currents, a test as described in simulated ground fault tests using sensors without built-in test windings can be made immediately following installation. Periodic tests using the manufacturer's test circuit should be adequate after installation.
Equipment with built-in test circuitry but without a built-in test winding Following installation, the GFP devices should be tested in accordance with "Simulated ground fault test using sensors without built-in test windings" to confirm that sensors and interconnections to the ground fault relay are functioning. Thereafter, the manufacturer's test circuit can be used to check the operation of the GFP relay and the tripping circuitry.
Test buttons and indicators
Operate test buttons to check the functions described in the manufacturer's instructions. Pilot lights and other indicators should signal ground-fault tripping or other functions as described in the manufacturer's instructions.
Zone selective interlocking function
The manufacturer should be consulted for specific instructions when this test is to be performed in the field.