Guard terminal in insulation resistance testing — excluding surface leakage current

The [guide on insulation resistance measurement](/guides/nen-3140/isolatieweerstandsmeting) covers the measurement in general. This article covers a specific, often unused third connection point on a megger: the guard terminal, which keeps surface leakage current across a contaminated, damp or greasy surface of the object under test out of the measurement, so the reading reflects only the insulation itself and is not partly determined by a contaminated surface.

The problem: surface leakage current distorts the reading

A standard insulation resistance measurement uses two connections: Line (the applied test voltage) and Earth (the return conductor to the instrument). In theory, the measured current between these two connections is purely the current flowing through the insulation. In practice, however, part of the current also flows across the surface of the object under test — for example along a contaminated, damp or greasy bushing, cable sheath entry, or terminal board — especially at higher test voltages and on objects that have been stored outdoors or in a damp environment. Without a guard terminal, this surface leakage current simply gets added to the measured current, making the calculated insulation resistance come out lower than the actual resistance of the insulation itself — an apparent insulation problem that is in fact a surface contamination problem.

How the guard terminal solves this

The guard terminal is a third measurement lead connected to the surface of the object, between the measurement point and the edge along which the surface current would flow — for example a bare wire wound tightly around the bushing of a cable or transformer, between the core insulation and the earth clamp. The surface leakage current then flows to the guard terminal instead of to the Earth terminal, and is not counted by the instrument as part of the measured current — the guard branch simply diverts the current without it affecting the reading.

Note: it is not uncommon for surface leakage current on a contaminated or damp object to be an order of magnitude higher than the actual current through the insulation itself. Without a guard terminal, an otherwise healthy insulation can therefore show a severely underestimated, alarmingly low insulation resistance value.

When a guard terminal is worthwhile

Bushings, cable sheath entries and feed-through insulators where a significant part of the surface is exposed to moisture, dust or contamination between the high-voltage point and earth.
Large electric motors and generators, particularly in combination with polarisation-index and DAR measurements, where a contaminated terminal board or damp winding-end insulation can otherwise distort the PI or DAR ratio.
Old cables or cables stored outdoors before being put into service, where a damp or contaminated sheath entry can make an otherwise intact core insulation look like an insulation problem.
PV strings on the DC side (see the guide on insulation resistance testing on PV strings), where outdoor connectors and module frames can show significant, temporary surface leakage current after rain.

Practical relevance

When an insulation resistance measurement produces an unexpectedly low value on an object with a visibly contaminated, damp or greasy surface, it is important to repeat the measurement with the guard terminal correctly connected between the measurement point and the edge of the surface, before concluding there is an insulation problem and starting a costly replacement or repair — a cleaned surface or a correctly connected guard terminal can still reveal a perfectly healthy insulation value.

Common mistakes

Not using a guard terminal on a visibly contaminated, damp or greasy object, causing a surface problem to be wrongly interpreted as an insulation problem.
Connecting the guard wire in the wrong place — not between the measurement point and the edge along which the surface current flows — so the guard connection has no effect.
Using a megger without a guard terminal for an object known to be sensitive to surface leakage current, when an instrument with a guard function is more suitable for that type of object.
Concluding immediately that replacement is needed for a low insulation resistance value without first cleaning the surface or trying a guard connection to rule out surface leakage current as the cause.