Insulation Tester / Insulation Meter

An insulation meter is an important measuring instrument for testing electrical supply lines and consumers to ensure their safety and proper functioning. Without electrical systems and devices, our normal everyday life is almost unimaginable. In order to limit the dangers posed by electric current, there are a number of regulations. Some of these concern the insulation of electrical lines. This serves on the one hand to protect against health hazards caused by electric shock or by malfunctions of electrical systems and devices, and on the other hand – to protect against fires. In order for current to flow as planned through connecting wires, switches, relays and other components, they must be insulated in such a way as to prevent accidental current leakage.

Whether the existing insulation provides sufficient protection against leakage and fault currents can be tested with a suitable insulation measuring device. To do this, a test voltage is generated with the insulation measuring device and then the electrical resistance between various electrically conductive components is measured. If the measured resistance at least meets the requirements specified in the applicable standard, the device or system is considered safe. Since the insulation properties can deteriorate due to aging, wear or damage, insulation measurements are not only performed during the final inspection of equipment and during the commissioning of machinery and equipment. For businesses and public areas, it is mandatory that the electrical consumers are tested regularly to prevent electrical accidents and other damage caused by poor insulation. Depending on which current-carrying parts have to be checked, the requirements for the insulation measuring device sometimes differ considerably. In the following text, we briefly review some of the specifications that should be considered.

Important selection criteria for the insulation measuring device
The most important features in the insulation measuring device include the test voltages that can be generated and the maximum value of the measurable insulation resistance. In addition, the following points should be considered when comparing devices:

- safety functions
- additional measuring functions
- storage options and interfaces
- operating conditions and IP degree of protection

The test voltages for the insulation measuring device
An important criterion for the insulation measuring device is the level of the test voltage. This test voltage must be higher than the operating voltage of the respective tested devices and systems. However, since excessively high voltages can damage or destroy the tested object, it is important to measure with the correct test voltage. Depending on the test object, this can be a few hundred volts or even several thousand volts. For this reason, and because of special test procedures with increasing test voltage, almost every insulation measuring device can generate different test voltages. Common test voltage values are 100 V for telecommunications and extra-low voltage equipment, 500 V for building installation and household appliances, and 1,000 V for low-voltage motors and transformers. When selecting a model, make sure the insulation meter offers the required voltage ratings for the intended areas. Most models can generate three or four different test voltages. Here are some examples of common values:

- 250 V / 500 V / 1,000 V

- 125 V / 250 V / 500 V / 1,000 V

- 500 V / 1,000 V / 2,500 V / 5,000 V

- 1,000 V /2,500 V / 5,000 V / 10,000 V

Safety functions and measuring ranges for insulation resistances Insulation measurements may only be made on voltage-free circuits. Therefore, the absence of voltage must be ensured before the test leads are connected from the insulation measuring instrument to the measuring point. Some instrument models automatically check whether a voltage is already present before generating the test voltage. If so, the insulation measuring device issues a warning and does not start the measuring process. Other instrument types do not automatically check the voltage before determining the insulation resistance, but provide a voltage measuring function so that the user can ensure that no voltage is present. This minimizes both the risk of damage and measurement errors due to the additional voltage.

The insulation resistance measuring range for the insulation tester generally increases with the magnitude of the test voltage that can be generated. The maximum measurable insulation resistance should always be significantly higher than the resistances to be verified. Common values range from 200 megohms at test voltages of 250 and 500 volts to 2,000, 4,000 or 8,000 megohms and up to 60,000 megohms at 1,000 volts and up to 600,000 megohms at 10,000 volts test voltage.

Additional measurement functions in the insulation tester
The insulation meter can often measure and display more than just the current insulation resistance. Even simple instrument types often offer a voltage measuring function for DC and AC voltage. This function is very handy for checking whether voltage is present at the test point before measuring insulation. Many models also offer resistance measurements and continuity tests, and some also offer frequency, current, capacitance, and other multimeter functions. If the insulation tester has a connector for thermocouples, temperatures can also be determined.

PI and DAR Value: For easier evaluation of the insulation resistance, some insulation meters can also automatically determine the polarity index PI and dielectric absorption value DAR. These indicate the ratio of insulation resistances at defined times after the test voltage is applied. Since with good insulation the resistance increases over a longer period of time, the insulation quality can be concluded from these values. The PI value is determined by dividing the resistance measured after ten minutes by the resistance measured after one minute. PI values from 2 to 4 are acceptable, values above 4 indicate good insulation performance. For the DAR value, the resistance measured after one minute is divided by the resistance measured after 15 or 30 seconds. DAR values from 1 to 1.25 are acceptable; values higher than 1.4 occur with very good insulation.

Guard circuit: The insulation measuring device that is also to measure the insulation resistances with values of more than one gigaohm should be equipped with a guard socket for this purpose. The guard circuit is intended to prevent falsification of the measurement result due to leakage currents, and capacitance effects. For this purpose, the Guard line is connected to the surfaces on which the leakage currents can occur due to dust or moisture.

Memory and interfaces Often, the values determined with the insulation measuring device should also be documented. The more connections and devices are to be tested and, if necessary, documented individually, the more a device is recommended that has convenient memory options and interfaces for data transfer. The measurement data can then be easily transferred to a computer, where they can be included into lists or test protocols.