Electrical insulation is not just the plastic polymer material wrapping the wires; it is a complete system composed of cable insulation layers, bushing insulators, space within conduits, motors, and general equipment. Mechanical stress, contamination, and temperature changes can cause these components to deteriorate over time, allowing current to leak. Current leakage can lead to the following problems:
1. When current passes through the insulation layer, it generates heat, which deteriorates the insulation until it eventually fails, posing a fire hazard.
2. Leakage current must return to the source; it will flow through any available conductor, conduit, pipe, water, or earth to return to the source. This undesirable current can produce dangerous voltages.
3. Leakage current is inefficient. The current leaking through the insulation does not drive motors, produce light, or generate heat, but it still results in consumption.
4. Leakage current can cause overcurrent protection devices to trip, leading to overheating of motors and transformers.
The result is that poor electrical insulation causes equipment failures and production line shutdowns. Nowadays, no factory can afford unplanned downtime. "Customers need to quickly diagnose faults. No one wants to lose production time, and many customers are eager to know whether they need to repair the equipment or simply replace it."
An electrical system is like a piping system: voltage is analogous to liquid pressure, current is analogous to liquid flow rate, and electrical insulation is analogous to the pipe wall. Insulation prevents electrons from leaking from conductors – its effectiveness is expressed in terms of insulation resistance. An effective insulation resistance system has a high resistance value, typically greater than a few megohms (MΩ). A poor insulation system has lower insulation resistance.
To detect leaks in a piping system, you need to pressurize it. Since leaks are most easily found when water pressure is highest, you cannot turn off the water supply to check for leaks. However, you would limit the available water so that when a large leak is found, you don't spray too much water around. The ideal test is to provide a limited amount of water at high (but not extremely high) pressure. This is exactly what an insulation resistance tester does.
An insulation tester (megohmmeter) applies a DC voltage to the insulation system and measures the resulting current. This allows it to calculate and display the insulation resistance value (the degree to which the insulation confines current to the wire, or prevents current leakage).
Portable insulation testers (megohmmeters) typically output test voltages of 50 V, 100 V, 250 V, 500 V, or 1000 V. As in the piping system, the goal is to provide a pressure (voltage) that is not extremely high. We want to detect existing leaks, but we do not want to overstress the system and create new leaks. Lower voltages are used for low-voltage systems, such as telephone, network, or control circuits; higher pressures are used to test power system insulation.
By testing the insulation resistance of different components in the system (transformers, switchgear, wires, motors), technicians can isolate and repair faulty components.
Technicians use testing to verify high insulation resistance between wires and ground or adjacent wires. Two common examples are testing the insulation between motor windings and the motor base, and checking the resistance between phase conductors and the ground wire/equipment frame.
Before energizing a system, using insulation testing to verify its integrity can improve system performance; insulation testing can detect manufacturing process issues and equipment defects that are generally not discovered until equipment failure. In the European Union, this test is mandatory, even for the smallest residential systems.
1. Rotate the rotary switch to any voltage position; the power is then turned on.
2. In the test number selection state, press the setting key (12) to enter the test result query state.
3. Press the up/down keys (12) to increase or decrease the test result number. (The corresponding number is highlighted in black.)
4. After selecting the number, press the test key (10) to enter the test result for that number; press the up/down keys (12) to query the test results for that number (current resistance, 15-second resistance, 60-second resistance, 10-minute resistance, absorption ratio, polarization index).
5. After querying, press the setting key (12) to return to the ready-to-test state.
6. If you need to query again, press the setting key (12) again.
