1 Megohm Rule

Although widely used, the best way to determine the correct value is to review the classification rules that apply to your installation. If the device you`re testing has a very low capacity, such as short internal wiring, point reading is all that`s needed. For many years, maintenance professionals have used the rule of a megohm to set the permissible lower limit for insulation resistance. The rule can be defined: the insulation resistance must be about one megohm per 1,000 volts of operating voltage, with a minimum value of one megohm. For many years, maintenance professionals have used the rule of a megohm to set the permissible lower limit for insulation resistance. For example, a motor with a rated voltage of 2,400 volts should have a minimum insulation resistance of 2.4 megaohms. In practice, the values measured by Megohm are usually significantly higher than this minimum value in new devices or with good insulation. A megohmmeter is usually equipped with three terminals. The Megger insulation tester is a small portable device that allows you to directly read the insulation resistance in ohms or megohms. For good insulation, the resistance is usually of the order of Megohm. Insulation resistance measurement is a routine routine test performed on all types of electrical wires and cables. Its purpose is to measure the ohmic value of the insulator under a DC voltage of high stability, usually 50, 100, 250, 500 or 1000 VDC. The ohmmic value of the insulation resistance is expressed in megohms (MΩ).

In order to comply with certain standards, the insulation resistance test can be performed under voltages up to 1500VDC. Due to the stability of the voltage source, it is possible to adjust the test voltage in increments of 1 volt. The rule can be defined: the insulation resistance must be about one megohm per 1,000 volts of operating voltage, with a minimum value of one megohm. On the other hand, a megameter, also called a megger, is often used to measure the insulation resistance of an isolated body. For its operation, it uses a DC generator or battery that can generate output voltage values of up to 5000V. The results obtained in the ohm test refer to the insulation resistance that has an isolated element that refers to an active or conductive element. A well-known method for determining the minimum permissible insulation resistance used by many professional engineers and electricians is the “mega-ohm rule”. This stipulates that the minimum insulation resistance for each operating voltage of 1000 volts must be greater than 1 mega-ohm. However, the measurement of insulation resistance is non-destructive under normal test conditions. It is carried out by applying a DC voltage of lower amplitude than that of the dielectric tests and gives a result expressed in kW, MW, GW or TW.

This resistance indicates the quality of the insulation between two conductors. Since it is non-destructive, it is particularly useful for monitoring insulation aging over the life of electrical equipment or installations. This measurement is performed with an insulation tester, also known as a megohmmeter The measurement of insulation resistance is based on Ohm`s law. (R=V/I). By injecting a known DC voltage that is lower than the voltage for dielectric tests and then measuring the current flow, it is very easy to determine the value of the resistance. In principle, the insulation resistance value is very high, but not infinite, so by measuring the low current flow, the megohmmeter displays the insulation resistance value and gives a result in kW, MW, GW and also TW (on some models). This resistance characterizes the quality of the insulation between two conductors and gives a good indication of the risk of leakage currents. Developed in the early 20th century, the Insulation Resistance (IR) test is the oldest and most widely used test to assess insulation quality. The insulation resistance test is the second test required by electrical safety testing standards.

The insulation resistance test consists of measuring the insulation resistance of a tested device while the phase and the neutral conductor are short-circuited together. The measured strength shall be higher than the limit value specified in international standards. A megohmmeter (also called insulation resistance tester, teraohmmeter) is used to measure the ohmic value of an insulator under a DC voltage of high stability. Main rule! For every 10 ◦ c increase, halve the resistance. For each decrease of 10 ° C, double the resistance. Each type of insulation has a different degree of resistance change. The dielectric stress test, also known as the “rupture test,” measures an insulator`s ability to withstand a medium-long voltage surge without a flash of spark occurring. In reality, this voltage increase can be due to lightning or induction caused by a fault on an electricity transmission line. The main objective of this audit is to ensure that the design rules of the paths and escape distances have been respected. This test is often performed by applying an AC voltage, but can also be performed with a DC voltage. This type of measurement requires a hipot tester.

The result is a voltage value usually expressed in kilovolts (kV). Dielectric tests can be destructive in the event of a defect, depending on the test benches and the energy available in the device. For this reason, it is reserved for type testing on new or refurbished equipment. With this method, you just need to connect the Megger device via the insulation to be tested and run it for a specific short period of time (60 seconds is usually recommended). As Figure 1 shows schematically, you have simply selected a point on a curve of ascending resistance values. Commonly used DC tests Voltages for routine maintenance are as follows: Vote for this question. Although I think the ideal value is the one indicated on the technical sheet of the equipment, it is often overlooked, so I am also looking for some kind of general standard. Insulation begins to age as soon as it is manufactured. As it ages, its insulating performance deteriorates.

All harsh installation environments, especially those with extreme temperatures and/or chemical contamination, speed up this process. Constraints due to various factors such as: Insulation may not be perfect, just as something may not be smooth. This means that there is always some electricity passing through. This is called a “leakage current.” This is acceptable with good insulation, but if the insulation deteriorates, leaks can cause problems. So, what makes “good” insulation? Well, it needs high resistance to current, and it must be able to maintain high resistance for a long period of insulation resistance must be made to avoid dangers such as electric shock and short circuits, which are caused when insulation in electrical equipment, parts and equipment used in industrial facilities, buildings and other environments, deteriorated over long periods of use. 6 Steps to Establish an Electrically Safe Working Condition Wet electrical equipment is a common hazard to which all maintenance technicians are exposed. If the device is wet with fresh water, it is enough to dry it. Salt water leaves corrosive salt deposits on metal and insulating surfaces, as well as in cracks in insulation. Here we connect our tester via isolation and use it for a short period of specific time. Note that the measurement varies over time. This means that after 30 seconds you read a different value.

Temperature, humidity and insulation status influence the measured value. This testing method is effective if you are testing low- or low-capacity devices. I want to be an AutoCAD programming professional. Can you advise us on that?. This test is independent of the size of the device. Increasing the strength for clean, dry insulation is done in the same way as if the component is large or small. Well, if you look at a high number of IR, you have good insulation. If, on the other hand, it is relatively low, the insulation is poor. A multimeter is capable of measuring different orders of magnitude, including electrical resistance, the unit of which is expressed in ohms. Its operation, especially for measuring resistance, is given by the action of an internal battery (low voltage) that circulates a small current through the measured resistance or, if this is not possible, the conductor or winding. The value obtained in ohms belongs to the electrical resistance, which circulates the current through the conductor and is increased according to its length and cross-section. The following guide will help you determine your actions.

In summary, a multimeter measures the electrical resistance of a conductor (coil), while a megger measures the insulation resistance of an isolated group (two coils relative to the ground), which a multimeter cannot do. Another advantage of this double reading test, as it is sometimes called, is that it gives you a clearer picture, even if a point measurement indicates that the insulation is beautiful. Is there a standard for the recommended insulation resistance values? To measure high-value resistance, techniques for measuring a low-value stream are used. A constant voltage source is applied to the resistor to be measured and the resulting current is read on a highly sensitive ammeter circuit that can display the resistance value. If the insulation contains a lot of moisture or impurities, the absorption effect is masked by a high leakage current, which remains at a fairly constant value and keeps the resistance value low (remember: R = e / I).