The ‘G’ terminal…and it doesn’t mean ground

The ‘G’ terminal…and it doesn’t mean ground

27 October 2017

You know that terminal on your tester that says ‘G’?  The one that you may think means ‘ground’? Well, it actually doesn’t.  It means guard.  And if you connect this terminal to ground, it may in fact short-circuit the test.  (So please avoid doing that!)

Now that you know that the G stands for guard, you may be scratching your head thinking… well, that’s great, but what does guard mean? What does it do?

The Guard terminal acts as a shunt.  Although not necessary to use while performing a test, guard is an added convenience that gives the operator greater flexibility and diagnostic capability when analyzing a problem. 

Removes leakage

Basically the guard acts as a shunt circuit to remove whatever leakage current that is connected from the measurement circuit.  It returns the low side of the tester’s output transformer while bypassing the measurement function.  If the item under test exhibits more than one leakage path (or the operator suspects that there may be more than one), in parallel, the guard can be connected to one (or all but one that are bunched together), to show a clearer picture of the condition of the one remaining leakage path that is connected in the test circuit. Guard terminal


With voltage outputs available up to 15 kV to suit all industrial and commercial applications, there are long surface leakage areas of insulation. During insulation testing, we are often preoccupied with the resistance of the actual insulator and we forget the resistance path on the outer surface of the insulation material.  However, the resistance path is very much a part of our measurement and can dramatically affect the measurement.

The surface leakage is basically a resistance that is in parallel with the true insulation resistance of the material being tested.  By using the guard terminal, and performing the ‘three terminal test’, the surface leakage current is ignored.  This may be important when high values of resistance are expected such as when testing high voltage components like insulators, bushings and cable.  These components generally have large surface areas that get exposed to contamination, resulting in high surface leakage currents across them.

By understanding the electrical layout of the test item, and where to apply the connections of the three terminals – negative or ‘line’, positive or ‘earth’ and guard, an experienced technician can reduce the test item to a three-terminal network and measure the resistance of specific elements, while guarding out parallel leakage through other components.

The Guard terminal helps to improve the reliability of insulation condition diagnosis and predictive maintenance.  It also serves as an important diagnostic tool.  There have been many instances of poor insulation resistance measurements leading to bushings, insulators and cables needing to be replaced needlessly at a high cost.  Only later to find out, by using the Guard terminal, that all that was needed was a good cleaning.

By using the Guard terminal on your instrument, you can save lots of time and money.

Tags: ground, guard, insulation, testing