Insulation resistance: maintenance principles of measurement

   • Verify the quality of the electrical equipment produced • Ensure that the electrical equipment meets regulations and standards (safety compliance)

　　• Determine how electrical equipment performance changes over time (preventive maintenance)

　　• Determine the cause of the failure (troubleshooting)

Resistance and conductivity

   Resistors (R) are oppositions to the flow of electricity in a conductor. Resistance is measured in ohms. The Greek letter Q represents an ohm. For larger values ​​of resistance, a prefix can be added to the D to form a compound unit, such as kiloohms (kQ) or megaohms (MQ). According to Ohm's law, resistors limit the flow of electrons (current) in a circuit. Current is measured in amperes (A). The greater the resistance, the less electrons flow; similarly, the less resistance, the greater the electron flow. See Figure 1-1. All materials have some resistance to the flow of electricity.

　　A conductor is a material that has very little resistance, allowing electrons to pass through it very easily. Most metals are good conductors. Copper (Cu) is the most common conductor. Silver (Ag) is an even better conductor than copper, but is too expensive for most applications. Aluminum (Al) is not as conductive as copper, but is less expensive and lighter for high-voltage applications like overhead power lines. For this reason, aluminum is also often used as a conductor. Conductors can be wires, power cords, or cables, and can be bare, insulated, or covered. See Figure 1-2.

　　Bare wire has no covering or insulation. Insulated wire is the most common conductor used in power systems. Insulated wire can be solid or stranded. Coverings for covered wire do not have a specific insulation rating. Coverings for wire are regulated in Section 310 of the National Electrical Code (NEC®).

　　Most wires, cables and busbars are made of copper or aluminum. Of all the metals used as conductors (except silver), copper offers the lowest resistance to the flow of electrons. Copper and aluminum can be bent and easily formed, are very flexible in small sizes, and have multi-core structures. Typical multi-core structures are 7, 19 or 37 strands.

Figure 1-1 In a circuit, the greater the resistance, the smaller the current. Similarly, the smaller the resistance, the greater the current in the battery.

Figure 1-2 shows wires, power supplies, cables, which can be exposed, insulated, or covered.

　　Factors that affect wire resistance include the wire's cross-sectional area, length, material, and temperature. A wire with a larger cross-sectional area has a lower resistance than a wire with a smaller cross-sectional area. The longer the wire, the greater the resistance. A shorter wire has a lower resistance than a longer wire of the same cross-sectional area.

　　Copper is a better conductor than aluminum and can carry more current than aluminum of the same gauge. According to the NEC®, all conductors must be copper unless otherwise specified. Temperature also affects the resistance of the conductor: the higher the temperature, the greater the resistance.

　　Power generation and transmission rely on the performance of electrical insulation. Due to energy shortages, this is critical.

　　An insulator is a material that has a very high electrical resistance. An insulator impedes the flow of electrons. Common insulating materials include rubber, plastic, air, glass, and paper. Wire insulation is classified by temperature rating, 140°F (60*C), 167oF (75*C), and 194°F (WC). See Figure 1-3. The required insulation level depends on the specific application. In high voltage applications, the insulation properties of the wire must be higher.

Figure 1-3 Wire insulation is classified by temperature grade.

　　When testing the integrity of insulation, always use a meter designed specifically for insulation resistance measurements.

　　Materials used for insulation, such as rubber or plastic, must have a very high electrical resistance. Materials used for conduction, such as wires or switch contacts, must have a very low electrical resistance. When conductor insulation degrades due to moisture and/or is damaged by overheating, its resistance decreases.

　　All electrical conductors must be protected from possible contact with other conductors, metal parts, and people. The insulation of a conductor protects the conductor from damage and isolates the electrical energy within the conductor. However, not all energized parts of a circuit are protected by insulation.

　　When live parts of a circuit are exposed, such as when wires connect to a fuse or circuit breaker panel, the distance, or air gap, acts as an insulator. The greater the distance between live wires or parts, the higher the resistance; the higher the voltage, the greater the air gap required to create resistance to prevent unwanted electron flow (such as a deadly arc).

Insulation resistance measurement

   Before beginning an insulation resistance test, technicians first make basic voltage, current, and resistance measurements. They use insulation resistance testers to determine the integrity of windings or cables in motors, transformers, and switchgear after de-energizing the equipment. The type of equipment being tested and the purpose of the insulation resistance test determine the measurements required. The two basic insulation resistance measurements made on insulating materials are insulation resistance measurements and leakage current measurements, which ultimately produce an insulation resistance value. See Figure 1-4. The true condition of the insulation is determined by making insulation resistance measurements using an insulation resistance tester. The measurement is made between the conductor that carries current when energized and the rest of the system where no current flows during normal operation.

Figure 1-4 The two basic measurements performed on insulating materials are insulation resistance measurement and leakage current measurement.

　　Regardless of the size of the circuit, system, or electrical load, electrical conductors are designed to provide the proper voltage, current, and power. The insulation on the conductors is used to prevent the current from flowing outside the designed path. No insulator can completely prevent the current from flowing through the insulator to the ground or other conductors. All insulators will pass a small amount of leakage current. Generally speaking, the leakage current is very small and will not cause any malfunctions and can be ignored; unless the leakage current reaches a certain level and begins to cause electric shock, temperature rise, or equipment damage. The higher the resistance of the insulator, the less leakage current will flow through the insulator. The resistance of the insulator is the highest when it is first put into use.

　　Virtually all insulators degrade over time, causing their electrical resistance to decrease. Moisture, extreme temperatures, dust, dirt, oil, vibration, contamination, and other mechanical stress or damage can all cause insulator degradation. When air gaps act as insulators between electrical components, the resistance of the air gaps also decreases over time due to the formation of dust and moisture. Sudden drops in insulation resistance are usually caused by physical damage, sudden changes in the environment, extreme temperatures, or contact with corrosive materials. The total value of insulation resistance depends on the amount of conductive leakage current and capacitive leakage current in the system.

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　　Conductive leakage current is the small amount of normal current that flows through the insulation of a wire. Conductive leakage current flows from wire to wire, or from the hot wire to ground. See Figure 1-5. Conductive leakage current can be determined using the formula based on Ohm's law, or by measuring leakage current using a megohmmeter. As the insulation ages or is affected by destructive factors, the resistance decreases. An increase in conductive leakage current causes further degradation of the insulation. As conductive leakage current increases, the resistance of the wire insulation decreases. Keeping the insulation clean and dry will minimize leakage current.

Figure 1-5 Conductive leakage current refers to the normal current that flows through the insulation layer of the wire in a small amount.

　　Capacitive leakage current is the current that flows through the insulation of a wire due to the capacitive effect. See Figure 1-6. This effect occurs when two or more wires are arranged in the same conduit. In a DC circuit, when voltage is first applied to a wire, a current surge occurs and the wire acts like a capacitor, resulting in the capacitive effect. A capacitor is an electronic device used to store electrical charge. Two metal sheets separated by a dielectric material form a capacitor.

Figure 1-6 Capacitive leakage current refers to the current flowing through the insulation layer of the wire when two or more wires are arranged in a wire tube.

　　Dielectric materials are materials with relatively low electrical conductivity. Insulators are often called dielectric materials. Two wires that are very close together form a small capacitor. The insulating layer between the wires is the dielectric, and the conductor is the metal sheet.

　　In a DC circuit, wires carrying DC voltages generally produce small capacitive leakage currents because they last only a few seconds before stopping; AC voltages produce continuous capacitive leakage currents, but this can be minimized by separating the wires throughout the array.

　　Surface leakage current is the current that flows from the area where the insulation on the wire is stripped to make an electrical connection. In a circuit! Wires are connected at different points using nuts, connectors, spade lugs, terminal blocks, and other connection fixtures. The connection point where the insulation is stripped provides a low-resistance path for surface leakage current, and dust and moisture will cause greater surface leakage current. See Figure 1-7.

Figure 1-7 Surface leakage current refers to the current flowing out of the area where the insulation layer of the wire is stripped to make an electrical connection.

　　Surface leakage current causes heat buildup at the connection point. Heat buildup causes degradation of the insulation, which can weaken the wire. Keeping all connections clean and tight will minimize surface leakage current. Surface leakage current is minimal in systems up to 600 V. In medium voltage (1 kV to 35 kV) applications, surface leakage current becomes a significant factor.

safety

   Although workplace safety is the responsibility of all personnel, maintenance technicians working on equipment are most aware of safety situations. Maintenance technicians have knowledge of tools, main switches, and emergency stops and controls. Equipment operators and other workshop personnel generally contact maintenance technicians in emergency situations. No test instrument by itself can guarantee ultimate safety. The combination of appropriate test instruments for the job, personal protective equipment (PPE), and safe work habits can provide the maximum protection required. See Figure 1-8. For appropriate safe practices, refer to National Fire Protection Association (NFPA) 70E Section 110 "General Requirements for Electrical Safety-Related Work" and Section 120 "Establishing an Electrically Safe Work Environment." Safe practices include the following:

• Work on de-energized circuits whenever possible

• Use proper lockout/tagout procedures

• Assume that all circuits under test are live. On live circuits, use appropriate personal protective equipment and safety tools, such as double-insulated tools. Wear protective clothing, goggles (with shields), rubber insulating gloves with leather protection, ear plugs, and a protective hat with a curved face shield.

Figure 1-8 Personal protective equipment includes protective clothing, head, eyes, ears, hands and feet, back, knee protection, and rubber insulating pads.

　　Also, if possible, remove any jewelry and stand on a rubber mat.

　　When measuring voltage on a live circuit, use the following steps:

   1.      Connect the ground wire.

　　2.      Connect the live wire.

　　3.      Remove the live wire.

　　4.      Remove the ground wire.

　　Minimize personal exposure to transient voltages by avoiding holding the meter as much as possible. (Use the included tilt stand or hanging strap to secure the meter whenever possible.)

　　When checking whether the circuit is open, short, or locked, please use the following steps:

   1.      Test a known live circuit.

　　2.      Test the target circuit.

　　3.      Test the known live circuit again to verify that the meter works properly before and after measuring the target circuit.

　　Keep one hand in your pocket to reduce the chance of a closed circuit through the chest and heart.

　　When performing insulation and resistance tests, please note the following: • Do not connect the insulation tester to live conductors or live equipment, and strictly follow the manufacturer's recommendations.

　　• Turn off the equipment under test by opening a fuse, switch, or circuit breaker. Lock out and tag out the equipment under test.

　　• Disconnect branch wires, ground wires, and all other equipment from the unit under test.

　　•Discharge the conductor capacitance before and after the test.

   •Do not use the insulation resistance tester in hazardous or explosive gas environments because the instrument will generate arcs when the insulation is damaged.

　　•Do not use an insulation resistance tester on electronic devices.

　　• Use insulating rubber gloves with leather protection when connecting the test leads.

　　NFPA 70E, the standard for electrical safety in the workplace, requires that each employee be evaluated for arc hazard. No outer garment with a hazard rating lower than the specified rating is worn in the workplace. The evaluation of electrical hazards is covered in NFPA 70E Section 110.7, "Electrical Safety Program, Hazard/Risk Evaluation Program."

　　Maintenance principle

   Maintenance technicians use tests such as insulation resistance testing to minimize equipment malfunction or failure and maximize productivity. By performing a high voltage insulation resistance test between a current carrying conductor and a ground conductor, maintenance personnel can eliminate the possibility of a dangerous short circuit or short to ground. This test is typically performed after new equipment is installed or refurbished. This procedure will prevent wiring errors and defective equipment from being present in the system and ensure high quality preventive maintenance tools. Over time, electrical systems are subject to a variety of environmental factors (dust, heat, pollution, vibration, etc.) that can cause the insulation to degrade. Routine monitoring of insulation resistance values ​​can provide information about the state of insulation degradation, which can cause equipment failures, electrical fires, and system downtime. Monitoring the integrity of insulation can ensure timely repair or replacement of equipment. In order to achieve efficient operation of equipment in the short and long term, insulation resistance testing procedures need to be included in a comprehensive maintenance program. Insulation resistance testing has become an important part of electrical equipment maintenance. The types of maintenance required in a plant are classified according to the equipment and functions in the plant. See Figure 1-9.

Figure 1-9 Types of test work and maintenance Check for leakage current in all de-energizing circuits through fuses, switchgear, and circuit breakers. Leakage current can cause inconsistent and erroneous readings.