Megohmmeters generate high voltage, low current signals for testing the breakdown strength of electrical insulation. They are called megohmmeters because the insulating resistance of a properly designed product is expressed in megohms (MO). Insulation-resistance (IR) is the DC resistance between two defined points at a specific voltage in a controlled environment. Typically, the applied voltage is 500 VDC and the environment is 25° C with less than 50% relative humidity. EN 60204-1, a standard for machinery safety, states that for test voltages at 500 VDC, the minimum insulation resistance is 1 MO. EN 60335-1, a safety standard for household and similar electrical appliances, states that for test voltages at 500 VDC, the minimum insulation resistance is 5 MO.
Important specifications for megohmmeters include DC output voltage, DC output current, current leakage rate, resistance range, and test time. The type of electrical safety test determines the level of voltage or current that is supplied. For example, hipot testing usually produces very high voltage outputs and very low current outputs. Current leakage, another important specification for megohmmeters, is the rate at which current flows through a ground conductor and is borne away safely. Resistance range measures a material’s opposition to the flow of electric current. Test time is the time required to complete the test.
Megohmmeters vary in terms of display and interface technologies. Analog meters display values on a dial, usually with a needle or pointer. Digital meters display values with a numeric readout. Light emitting diode (LED) and liquid crystal display (LCD) devices are common types of digital meters. In terms of interface technologies, several types are available. General-purpose interface bus (GPIB) is designed to connect computers, peripherals and laboratory instruments so that data and control information can pass between them. RS232 or ANSI 232 is used for serial communications between instruments and computers. Megohmmeters with printer ports and scanner ports are also available. Devices with storage capabilities can provide printouts of formatted information.
Megohmmeters provide many features. Remote controls permit control by computers or other instruments. Build-in calibration allows operators to calibrate test instruments without removing them from the test location. Warning indicator lights indicate pass/fail conditions and often work in conjunction with audible buzzers or alarms. In automated test environments (ATEs), megohmmeters are often interfaced with programmable logic controllers (PLCs). Rapid cutoff circuitry protects devices from excessively high levels of voltage or current. User-selectable output frequencies, typically 50 Hz or 60 Hz, are commonly available. Front panel lockouts enable users to password-protect megohmmeters. Some manufacturers call this tamper-proof operation.
There are a variety of applications for megohmmeters. Some devices are used to test the insulation resistance of wires, cables, transformers, and electrical motors. Others are used to verify that devices such as aircraft fuel pumps meets manufacturer IR specifications. Megohmmeters are also used to test the insulation-resistance of cable reels, rectifiers, solid-state diodes, and other electrical components.