Any PI value below 1.0 is considered bad insulation because the measured resistance was eventually less than when the test started. Measured values ranging anywhere between 1.0 and 2.0 is questionable insulation, 2.0 to 4.0 is classified as good, and anything above 4.0 is considered excellent.
Insulation resistance can be determined between live, neutral and earth connections with no loads or appliances connected to any sockets or wiring. Any problems would be shown by a lower-than-expected resistance being present between them. In domestic wiring anything above 2 megohms is acceptable.
Readings should be greater than 1MΩ for an Earthed system, or 2MΩ for Class 2 or “floating” systems. Ideally this reading should be as high as possible. PV150 can read up to 19.99MΩ, and the PV200/210 up to 199.99MΩ. A good result would be “>19.99MΩ” on the PV150, and “>199.99MΩ” on the PV200/210.
So, when you've conducted your Megger test, you'll want your device to read between 35 and 100 megohms. If your reading is below that range, there's likely an issue with your cable that will negatively affect its performance. Cable insulation can degrade over time due to: Prolonged use.
The minimum insulation test result of 1.0 Megaohms is for the complete installation, not a circuit, even if all the circuits are above 1.0 Megaohms the entire installation can be below 1.0 Megohm and fail (resistances in parallel across all the circuits).
A failed insulation resistance test indicates that the insulation is breaking down, which could lead to electric shock. Common causes of failure include damage to the cable or equipment, moisture, or a faulty power supply.
For many years, maintenance professionals have used the one-megohm rule to establish the allowable lower limit for insulation resistance. The rule may be stated: Insulation resistance should be approximately one megohm for each 1,000 volts of operating voltage, with a minimum value of one megohm.
Press the test button on megger, the megger will generate current. This current flows through cable, the resistance in scale is noted which ranges between 35 to 100 Mega Ohms. Note to maintain this contact for 30 to 60 seconds. Acceptable IR for electrical cable = 1 Mega Ohm for 1000 V.
The general rule of thumb that is used for acceptable values for safe energisation is 1 MΩ per 1000 V of applied test voltage plus 1 MΩ.
A low resistance between phase and neutral conductors, or from live conductors to earth, will result in a leakage current. This current could cause deterioration of the insulation, as well as involving a waste of energy which would increase the running costs of the installation.
Low resistance fault refers to the occurrence of abnormally low resistance in a cable.
What is the resistance between live and neutral? In theory, zero. It's a pure voltage source. In practice, it depends where you measure it, but at the input to the building in an advanced country you would hope for no more than a few tens of milliohms.
High resistance values of about 1 to 10 megohms typically indicate good insulation integrity whiles Insulation resistance values between 1 to 100 megohms are commonly accepted for low voltage equipment subject to the specific application and operating voltage.
The rule states that insulation resistance should be approximately one megohm for each 1000 volts of operating voltage, with a one megohm minimum. (By following this rule, a motor rated at 2400 volts should have a minimum insulation resistance of 2.4 megohms). Each plant, however, should establish their own standards.
The lower the resistance, the higher the current flow. If the resistance is abnormally low, it could be due to insulators that were damaged by moisture or overheating.
Well, that was unheard of for years; manufacturers typically stopped at 5kV. Until, Megger developed a 10kV model to meet the new testing recommendations outlined by the IEEE. Since then, insulation testing at 5, 10, and even 15kV has become common practice for certain testing situations.
During the test, if your megger reads a resistance under 1 (1,000 ohms) after the initial 60-second interval, it's likely that the cable has failed and should be removed. If the reading falls between 1-1.25 on your meter, the cable passes with flying colors. Any reading above 1.25 is considered excellent.
Typical minimum values range from 100MΩ to 100,000MΩ. For motors, the minimum IR value is (rated voltage in kV + 1) MΩ or higher depending on the motor specifications. Guidelines are also given for tap changers, cables, and other equipment. Maintaining proper IR values is important for ensuring insulation integrity.
Ideally a ground should be of zero ohms resistance. There is not one standard ground resistance threshold that is recognized by all agencies. However, the NFPA and IEEE have recommended a ground resistance value of 5.0 ohms or less.
For a successful test, the insulation resistance measurement must be equal to or greater than 1 Gigaohm (1 Gigaohm = 1 G ohms = 1000 Mega ohms = 1000 M ohms). We recommend that you always check the megohmmeter instruction manual as each manufacturer can use different symbols or values for their testers.
Because it works with only a few milli-Amps of test current, the megohmmeter has limited power and does not damage the insulation. This is important because the most effective use of a megohmmeter involves repetitive, routine testing in order to establish a lifecycle trend and perform preventive maintenance.
For domestic installations, according to IEC 60364, the insulation resistance at low voltage must be greater than 1 MOhm, except for voltages lower than 250V. In this case, it must be greater than 500 kOhm.
The insulation test pass mark will be 1MΩ. This means that the Insulation reading must be over 1MΩ to Pass the test. The PAT tester will measure the resistance between the live parts (Live and Neutral) connected together and earth.
Some of the more common causes of insulation failure include: excessive heat or cold, moisture, dirt, corrosive vapors, oil, vibration, aging and nicked wiring.