ET Online
October 2008
The low down on insulation testing

The low down on insulation testing

28 October 2008

Tony Wills - Applications engineer

High voltages are usually the order of the day in insulation testing, but that’s not always the case. There are now many applications, such as the testing of telecommunications installations, aircraft wiring looms, and electrostatic discharge (ESD) equipment where insulation testing has to be performed at a relatively low voltage - typically 100 V or less.

Megger’s insulation testers for high voltages are very well known, but not everyone realises that the company offers no fewer than four models of testers that are specifically designed for lowvoltage applications. Testers in the MIT480 series are optimised for use on telecommunications systems, while the MIT40X is a configurable low voltage insulation tester for general use.

All of these testers are rugged, compact handheld devices and all feature a large clear display which combines a conventional digital readout with a simulated analogue arc that replicates the response of a moving coil instrument. Calibration certificates are provided as standard with all MIT480 series and MIT40X instruments, eliminating the inconvenience of having to request and pay for them separately.

The MIT480 telecommunications insulation tester offers test voltages of 50 V and 100 V, while the MIT481, in addition to these voltages, allows testing at 250 V, 500 V and 1,000 V. The MIT481 can also store test results for later on-screen review. 

The test capabilities of the MIT485 are the same as those of the MIT481. This instrument however, has the ability not only to store in excess of 1,000 test results, but also to make them available for later downloading via a wired or Bluetooth® wireless connection.

MIT480 series testers have a test inhibit feature that terminates the test if a live circuit above 75 V is detected, and they also have a default voltmeter function that is activated on any test function if a circuit voltage is detected. In addition, they have a capacitance measuring facility and can automatically calculate and display cable length if the capacitance per unit length of cable is known.

The MIT40X is an excellent choice for use where a precise insulation testing voltage is needed, as it allows the user to select the test voltage in 1 V steps from 10 V to 100 V. Like the MIT480 series instruments, the MIT40X has a test inhibit function, but this operates at 50 V rather than 75 V.

The instrument can be configured to use pass/fail limit alarms, making routine testing fast and simple and, like the MIT480 series instruments, it incorporates a secondary display that shows the actual voltage being applied to the equipment under test.

In many types of modern electrical and electronic installation, conventional high-voltage insulation testing is not possible. Nevertheless, it is still essential in these cases to be able to check the condition of the insulation. The solution is low voltage testing with Megger instruments that have been specifically designed for this type of work.