1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-04-20a71 Center Point of Contact: KSCa71 Submitted by: Wil HarkinsSubject: Maintenance molded-case, draw-out, high-voltage, etc.). Once the circuit breaker types are identified, one can start to identify the criterion for cir
2、cuit breaker inspection, test, and maintenance. There are many sources of information available to assist one in defining this criterion (example: Federal electric shock and secondary injuries caused by contact with energized circuits or devices that are electrostatically charged after removal of te
3、st potential. Also physical damage resulting from short circuits, insulation failures, or faulty equipment.Identification of safety equipment that is required when working on circuit breakers should be provided. This equipment may consist of nonmetallic hardhats with chin straps, safety goggles, rub
4、ber gloves, sleeves, blankets/mats as required, area barricades and signs.Safety precautions that personnel should be aware of should be identified. For example; turn off power, look out, and disconnect sources of electrical energy before working on equipment. Use of insulated hand tools designed fo
5、r specific work. Use of protective equipment such as safety approved rubber-insulated gloves with protectors, insulating blankets/mats, and non-metallic hardhats. Wear safety eye protection glasses if there is a possibility of arcing or sparking. Ensure that non-current carrying metallic parts of eq
6、uipment are grounded. Inspect visually or conduct tests to determine the adequacy of grounds.The following is extracted from KSC GP-1002 and illustrates how that practice addressed one of the types of circuit breakers found at KSC.Molded-Case Circuit BreakersProvided by IHSNot for ResaleNo reproduct
7、ion or networking permitted without license from IHS-,-,-1. Application. Molded-case circuit breakers are used to interrupt fault currents, to provide overload protection, and to switch the electrical circuit.2. Construction. Molded-case circuit breakers contain two elements: a switch consisting of
8、a set of contacts with suitable mechanical linkage to operate the contacts; and a trip unit which is an overload and fault sensing device.3. Operation. When the thermal or magnetic trip senses an overload condition in the electrical circuits, a spring-loaded latch is tripped. The released spring mec
9、hanism rapidly opens the breaker contacts and de-energizes the faulted circuit. Most molded-case circuit breakers are equipped with a thermal element composed of a bimetallic strip to provide a time delay and establish the tripping characteristics.The instantaneous-trip device is magnetic. High curr
10、ent flow through the breaker picks up the armature of the magnetic trip device. Movement of the armature releases the latch and opens the breaker contacts. Some breakers are available with an adjustable instantaneous trip. The pickup of this trip may be changed in the field from approximately 5 to 1
11、0 times the continuous rating of the breaker.Breakers labeled “non-automatic have no trip unit and are used as load interrupting switches only.4. Maintenance. (1) Preventive Maintenance. Circuit breakers shall be exercised at intervals recommended in Table 1. If the circuit breaker is equipped with
12、a trip indicator, exercise it to verify that it also operates properly. During routine maintenance, check molded-case circuit breakers for improper terminal connection at the time of installation, distorted plug-in tabs or sockets, poorly cleaned or corroded conductors, improper conductors for the l
13、ugs in use, and loose terminations. All of the above are fault conditions which may cause heating and deterioration of the circuit breaker and its response characteristics.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD Table
14、1. Recommended Maintenance/Test Intervals for Molded-Case Circuit Breakers Note: The above are minimum recommended maintenance/test intervals. Additional tests may be made when the operating agency has experience to indicate need for concern.If overheating of connections is evident through discolora
15、tion or arcing, the breaker should be removed from service and all conducting surfaces cleaned. The breaker should then pass operations tests and the installation megger test before reinstallation.Those breakers with adjustable instantaneous magnetic trip devices should be set at the highest setting
16、 unless otherwise directed.(2) Corrective Maintenance. Perform corrective maintenance on molded-case circuit breakers Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-after fault conditions occur. Follow the procedures given in paragraph 4.(1) and per
17、form the tests required in Table 1 (see paragraph 5). If a molded-case circuit breaker interrupts a fault current whose magnitude is equal to its interrupting rating, the breaker may not be operable.5. Tests. (1) Insulation Megger Tests. Perform insulation megger tests at the intervals indicated in
18、Table 1 and the following instructions.De-energize and, if necessary, remove the breaker from the panel for test. Do not place the megger on large masses of iron nor near strong magnetic fields. Set the megger scale range selector switch in a position that gives a reading in midscale or above. Obser
19、ve megger readings after a minimum of one minute or when the pointer has reached an apparent stabilized value; record the readings. After completion of the test, apply temporary grounds to all test materials and disconnect all leads.This procedure tests one phase to ground with all other phases grou
20、nded and is equivalent to a phase-to-phase test besides the phase-to-ground test. Each phase must be tested individually. Table 2 provides voltage values for insulation megger tests. If a megger reading of 1 megohm minimum is not obtained for each voltage value replace the circuit breaker.refer to D
21、 descriptionD Table 2. Voltage Values for Insulation Megger Tests-Molded-Case Circuit Breakers (2) Operations Tests. Perform operations tests at the intervals suggested in Table 1 and with the following instructions. These tests include overcurrent testing for both time-delay and instantaneous trip.
22、Molded-case circuit breakers use two basic methods, sometimes combined, to achieve overload protection: thermallic strip and electromagnetic. Check the circuit breaker manufacturers instruction book to determine the type of overload device, the method of adjustment, and the proper breaker connection
23、s. Use a low-voltage high-current source such as Multi-Amp Model MSA for small breakers of ratings up to 100A and Multi-AMP Model CB-225 for larger breakers. Consult the Multi-Amp Tester instruction book to set up tester controls. De-energize and remove the breaker from the panel for test. Record al
24、l data obtained by these tests.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Individually test each pole that is equipped with an overload device. Operate (open and close) the breaker two or three times before applying the overcurrent. Connect the
25、Multi-Amp tester across one pole of the breaker with the breaker closed.If the overload protective device of the breaker under test is the thermal bimetallic strip type, allow sufficient time for the heater element to cool to near ambient temperature before starting a second test. No waiting period
26、between tests is necessary for the electromagnetic type of overload protective device.(3) Time Current Trip Test. If the breaker is electrically operated, connect temporary power to the breaker control circuits. Check for proper voltage and if alternating or direct current. The tests currents provid
27、ed in Table 3 should be used. If the breaker being tested is not listed in Table 3, the test currents should be approximately 3 times the normal trip rating of the breaker. Perform the time current trip test in accordance with Multi-Amp instructions. Observe and record the time shown on the timer as
28、 the operating time of the breaker at that percent of rated overload. Compare this test time with times given in Table 3. If the operating time of the breaker does not agree with the curve or Table 3, replace the breaker. If the breaker is not listed in Table 3, check the manufacturers time current
29、curves for the appropriate trip time.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionDProvided by IHSNot f
30、or ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionDProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionDProvided by IHSNot for ResaleNo reproduction or networking permitted without l
31、icense from IHS-,-,-refer to D descriptionDProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionDProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionDProvided by IHS
32、Not for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionD Table 3. Trip Characteristics for Molded-Case Circuit Breaker(4) Instantaneous Trip Test. Connect the breakers to the Multi-Amp tester and select the proper ammeter range (about 10 times the no
33、rmal trip current rating of the test breaker). Do not rotate the main control while large currents are flowing. Alternately increase the output current and jog the tester until the breaker trips or until the maximum test current is reached. If the breaker does not trip at the desired current value,
34、replace the breaker.The above is only an example of one type of circuit breaker. The criteria should address all circuit breakers that an organization supports.6. References:Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-1. Kennedy Space Center (KSC
35、) General Practice (GP) 1002, “Maintenance and Test Criteria for Circuit Breakers.“Impact of Non-Practice: While this practice may seem to be one that would routinely be found in a maintenance program, in reality thats not so. All too often it is assumed that a new part is not only serviceable, but
36、that it also meets the specification requirements as advertised. However, due to process variables, inspection techniques, and even fraud, this is not always the case. Example: in February 1992 KSC was alerted by NASA HQ/Code QR that the Nuclear Regulatory Commission working with the Inspector Gener
37、al was issuing a GIDEP/Alert on suspect circuit breakers. The situation described that a particular vendor took used circuit breakers from various manufacturers, refurbished the exterior only, falsified circuit breaker data and sold them as new components. These breakers had experienced premature tr
38、ip, failure to trip (some breakers were not of proper amperage, i.e., mislabeled), and some had exploded. Had KSC not had practice GP-1002, “Maintenance and Test Criteria for Circuit Breakers,“ in place, suspect circuit breakers could have easily been introduced into KSCs GSE and Facility equipment
39、supporting the Space Shuttle Program. A maintenance program that provides for testing of circuit breakers prior to or upon installation will eliminate these types of failures.Related Practices: N/AAdditional Info: Approval Info: a71 Approval Date: 2000-04-20a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
