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RISK CONTROL REDUCE RISK. PREVENT LOSS. SAVE LIVES. Motor electrical insulation testing Introduction Insulating material with high electrical resistance is used to confine electrical current to a desired path through conductors and to isolate the conductors from ground and from each other. The conductors are usually made of copper or aluminum. The insulating materials can be a variety of nonmetallic substances including rubbers, rubber-like polymers, thermoplastic polymers, paper, wood, cloth, varnish, mineral oil, mica, pressed mica board, glass, porcelain and other ceramics. The thickness and strength of the insulation material depends on the operating voltage of the machine and also on the level of voltage, which can occur due to lightning strikes or other faults on power lines. A breakdown of the insulation will result in arcing and current flow through the insulation. This insulation breakdown causes a short circuit and failure of the machine, and possibly extensive damage in the area around the machine. Insulation life expectancy Except for ceramics and glass, all insulation materials suffer deterioration over time from normal heat of operation. The insulation material’s operational temperature primarily determines its rate of deterioration. Deterioration lowers the material's resistance to current flow and high voltage breakdown. Deterioration may also result in mechanical breakdown of the material, allowing two conductors to come into contact. Insulation, therefore, has a defined normal life expectancy that depends on the “hot spot” temperature of the device. This hot spot is the point in the machine where the insulation is subjected to the highest temperature. The machine’s designer determines the expected hot spot location and temperature through tests or computer simulations. On larger equipment, the hot spot temperature may be monitored on a temperature meter. Insulation classes Insulation materials used in motors are classified by the National Electrical Manufacturer's Association (NEMA) by the maximum temperature at which the material can operate and still maintain its insulation and mechanical integrity over the expected lifetime of the machine. These classes are: Class A B F H Max. Temp. (Hot Spot) °C 105 130 155 220 Class A insulation: This type of material is used as the solid insulation in oil-filled transformers. It is made up of organic material such as paper, wood and varnish. Class A motor slot insulation, made up of paperboard, insulates the motor coil assemblies from ground. Class B insulation: This material is made up of inorganic materials such as mica or fiberglass and is held together with organic binders such as varnish. Class A and B windings have a seven-year life expectancy when operated at their rated temperature limits. Few motors are continuously run at their maximum temperature limit. Operation at lower temperatures will greatly extend insulation life. At normal power levels, insulation life expectancy of 20 years or more is common. Class F insulation: This is the newest type of insulation material. Class F is made of petrochemical base materials such as polyethylene, cross-linked polyethylene, silicone and synthetic rubbers. Class H insulation: This is made with Class B materials held together with silicone rubber as a binder and filler. Life expectancy for Class H Insulation operating at its maximum rated temperature is estimated to be 60 years. PAGE 1 RISK CONTROL Motor electrical insulation testing Insulation failures The most frequent cause of electrical equipment breakdowns and accidents is insulation failure. The most common cause of insulation failure is extended operation at high temperature. If the insulation is operated above its maximum rated temperature, the life expectancy of the material is reduced. Overloads, short circuits, low volt-age, phase voltage unbalance or reduction or loss of cooling can cause high temperatures. An overload exists when the conductors are carrying between 100 and 200 percent of the rated load for an extended period of time. A short circuit can allow a current flow of greater than 200 percent, but usually is of short duration. Circuit breakers or fuses will trip the equipment off-line quickly. In any motor, except hermetically sealed units, the ventilating air passages are susceptible to contamination by dust, dirt, oil, etc. This contamination interferes with the flow of ventilating air or with the heat transfer between the coils and the air. A reduction in the cooling ability of the motor makes the motor run hotter and reduces insulation life. Insulation can also fail due to contamination. Contamination by moisture, corrosive gasses or mists, or introduction of conductive dust into the device can lower the resistance of the insulation. Contamination can also cause insulation failure of ceramic or glass materials by allowing current to flow through the contamination on the surface of the insulator. Insulation resistance testing Insulators have resistances in the megohm range (1 megohm = 1 million ohms). A lowering of the resistance will indicate deterioration of the insulation. Resistance testing is a valuable tool for determining insulation condition and scheduling maintenance. Proper testing and follow-up can prevent equipment damage and unscheduled outages that result in lost production. Insulation resistance testing is done by applying a voltage between a conductor and the equipment frame or ground. On transformers, insulation resistance is also measured between the primary and secondary windings. All insulation resistance test devices contain a voltage source and a method of measuring current flow across or through the insulation. The relationship between voltage and current determines the resistance in megohms. A megohmmeter supplies a high DC voltage across the insulation being tested. One megohmmeter lead is attached to one of the conductors and the other to the device frame or ground. The voltage source on these portable units can be either an AC motor driving a DC generator, a hand-cranked DC generator or a solid state circuit to convert a megohmmeter’ s internal battery voltage to the desired test voltage. Common test voltages are 250 V, 500 V, 2,500 V, and 5,000 V. The minimum measured value of insulation resistance for continued reliable operation of the motor or generator depends on the operating voltage with higher voltages requiring a higher minimum resistance. Resistance is a function of temperature. Unless the test can always be made at the same temperature, a correction factor is needed to normalize the data to a standard temperature. The temperature should be determined by placing a thermometer at the same place on the device each time the insulation is tested. The thermometer can be held in place with electrical putty. Either mercury or dial type thermometers can be used. The following table gives the correction factors for rotating machines with Class B insulation: Temp. °C 10 20 30 35 40 45 50 55 60 65 70 75 Correction Factor 0.63 1.00 1.25 1.58 2.00 2.50 3.98 5.00 6.30 7.90 10.0 12.6 PAGE 2 RISK CONTROL Motor electrical insulation testing With these correction factors, the condition of the insulation can be compared over time without having to ensure the same operating temperature every time a reading is taken. For example, if the temperature at the designated point on a piece of equipment is 30 degrees C, and the megohmmeter reading is 180 megohms, the reading corrected to 20 degrees C (1.80 times the 1.58 correction factor) is 284 megohms. At another date, the temperature reading is 35 degrees C and the megohmmeter reading is 140 megohms. Although this reading appears to be lower than the first reading, when it is corrected to 20 degrees C (1.40 times the 2.00 correction factor) the reading is 280 megohms, very close to the first reading. The actual and corrected megohmmeter reading and the measured temperature should be recorded in the machine's permanent maintenance record. A continuous downward trend in the corrected megohmmeter reading can indicate impending problems. The table below gives the recommended test voltage and minimum insulation resistance values for motors and generators by voltage class: Motor Nameplate Voltage (AC or DC) Up to 250 volts 250 to 1,000 volts 1,000 to 2,500 volts 2,500 to 5,000 volts 12,000 to 15,000 volts Test Voltage (DC) 500 volts DC 1,000 volts DC 1,000 volts DC 2,500 volts DC 5,000 volts DC Minimum Insulation Resistance 50 megohms 100 megohms 100 megohms 1,000 megohms 5,000 megohms Doing this testing periodically and recording the results will eventually show a downward trend of the insulation resistance as it deteriorates from service, aging and contamination. The trended results can be used to forecast when remedial action should be scheduled. The purpose of testing is to prevent unscheduled outages and to reduce repair costs. In many cases an unsatisfactory value of insulation resistance can be restored by drying out, cleaning or revarnishing, and these are less expensive procedures than a rewind done after a ground fault has occurred. Dielectric absorption ratio This test can be performed with a hand-crank megohmmeter and does not require temperature correction. The megohmmeter is operated for 60 seconds. The resistance readings are recorded after 30 seconds and again at 60 seconds. The 60-second reading is divided by the 30-second reading. This 60/30 second ratio is called the dielectric absorption ratio. The test is useful while drying out wet electrical equipment. It is also useful for charting trends in the insulation condition of smaller motors in the under 100 horsepower range. The dielectric absorption ratio should be between 1.4 and 1.6. Anything under 1.4 is questionable and should be investigated. Motor drying or more extensive insulation repairs may be needed. Polarization index This is another version of the dielectric absorption ratio test. This is a ratio of a 10-minute megohmmeter reading to a oneminute reading. An AC line operated motor-driven or a solid state megohmmeter is required since it is difficult to maintain constant test voltage using a hand-cranked instrument over a 10-minute period. The polarization index, or PI, is used to trend insulation condition trends for motors with more than 100 horsepower or up to 5,000 volts terminal voltage. A PI reading between 2.0 and 4.0 is acceptable. A PI between 1.0 and 2.0 might mean the condition of the insulation is marginal and should be investigated further. A PI between 1.0 and 2.0 is acceptable if the temperature corrected one-minute reading is greater than the minimum resistance for the voltage class from the table above. Equipment insulation with a PI of less than 1.0 requires immediate attention. High potential tests Large or medium voltage motors and generators and medium voltage cables require high potential testing to determine insulation condition. There are both AC and DC step voltage methods. The tests should be carried out and results evaluated by trained specialists. Improper testing can damage the insulation. PAGE 3 RISK CONTROL Motor electrical insulation testing Safety warning Whenever a source of high DC voltage is applied to a piece of equipment, any capacitance in the system is charged. This stored energy can cause serious injury. Some megohmmeters are equipped with a discharge switch to safely drain the stored energy at the end of the test. If the tester does not have this feature, a separate discharge stick should be used at the end of the test. The stick should be applied to the terminals for at least four times as long as the duration of the preceding test. Capacitors and DC machines may have a stored charge from previous operation and these should be carefully discharged both before and after the insulation testing. Insulation maintenance and resistance restoration If the insulation test results indicate a problem, cleaning or drying can often restore insulation resistance. Occasionally, repair or replacement of insulation is needed. The insulation of induction motor stators can often be restored by cleaning and drying. Thermally degraded insulation can often be restored by dipping the core and coil assembly in a vat of insulating varnish, pulling a vacuum on the varnish tank to draw out air bubbles, and then baking the core and coils in an oven. Whenever equipment is disassembled for this type of work, the insulation should be visually inspected for physical damage, deterioration or evidence of arcing or corona damage. Summary Usage and care determine the life expectancy of electrical insulation. Preventive maintenance procedures, including testing and monitoring programs, can be used to maximize insulation life, reduce equipment failures and prevent unscheduled outages. References • • • • Inter-National Electrical Testing Association standard NETA-MTS ANSI/IEEE 43-2000: Recommended practice for testing insulation resistance of rotating machinery ANSI/IEEE 95-1977: Recommended practice for testing of large AC rotating machinery with high direct voltage ANSI/IEEE 432-1992: Guide for insulation maintenance for rotating electrical machinery (5 to 10,000 horsepower) To learn more, visit BoilerRe.com. boilerre.com The Travelers Indemnity Company and its property casualty affiliates. One Tower Square, Hartford, CT 06183 The information provided in this document is intended for use as a guideline and is not intended as, nor does it constitute, legal or professional advice. Travelers does not warrant that adherence to, or compliance with, any recommendations, best practices, checklists, or guidelines will result in a particular outcome. In no event will Travelers or any of its subsidiaries or affiliates be liable in tort or in contract to anyone who has access to or uses this information. Travelers does not warrant that the information in this document constitutes a complete and finite list of each and every item or procedure related to the topics or issues referenced herein. Furthermore, federal, state or local laws, regulations, standards or codes may change from time to time and the reader should always refer to the most current requirements. This material does not amend, or otherwise affect, the provisions or coverages of any insurance policy or bond issued by Travelers, nor is it a representation that coverage does or does not exist for any particular claim or loss under any such policy or bond. Coverage depends on the facts and circumstances involved in the claim or loss, all applicable policy or bond provisions, and any applicable law. © 2014 The Travelers Indemnity Company. All rights reserved. Travelers and the Travelers Umbrella logo are registered trademarks of The Travelers Indemnity Company in the U.S. and other countries. 911-bre PAGE 4