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 dis- played 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 injec- tion method for testing MCCBs is described in more detail in the following sections.
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.
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 automati- cally 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 indi- cated 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.
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 8.7.
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.
• Time–current curves are based on 40°C ambient temperature
• Time–current curves are based on current flowing in all three poles
• Circuit breakers are tested in open air
• Trip values of circuit breaker are measured from cold start
• Calibration tests are made with UL specified size conductors connected to line and load terminals
• Current must be held constant without variation over the entire test period
• Rated maximum interrupting current for testing magnetic trip is 5000 A or more
• 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 con- ditions 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.
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 fre- quency 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 Figure 8.6.