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What is the Purpose of a Guard Terminal?

Megger MIT1025 10kV Insulation Resistance TesterWhen making an insulation test, we tend to focus on the resistance of the actual insulator and we forget the resistance path on the outer surface of the insulating material. This surface resistance path can be very much part of our measurement and can dramatically affect the measured results.

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Keep in mind that the total current that flows during an insulation resistance test is made up of three main components:

  1. The charging current, which is charging up the object’s capacitance.
  2. Absorption current, which is the current that is being drawn into the insulation by the polarizing of the electrons; initially high but drops over time (at a rate slower than the charging current).
  3. The conduction or leakage current, which is the small, steady state current that divides into two parts:

a.    The conduction path through the insulation.
b.    The current flowing over the surface of the insulation.

The current flowing over the surface is the component of current that we do not want to measure if we want to measure the insulation resistance of the material.  Surface leakage introduces errors into the measurement of insulation resistance. Removing the surface leakage from the measurement becomes more critical the higher the expected insulation resistance values.

Some insulation test sets offer two terminals, while others have three.  Since these are dc tester sets, two of the terminals are the “+” and “-” terminals.  The third (if offered) is a guard.  It does not have to be used and many technicians use insulation tester sets satisfactorily without ever employing the guard.  However, it affords the technician an extra function for diagnosis of equipment problems.  The guard is a shunt circuit that diverts surface leakage current around the test set’s measurement function. If parallel leakage paths exist, a guard connection will eliminate those from the measurement, and give a more precise reading of the leakage between the remaining elements.

Surface leakage is essentially a resistance in parallel with the true insulation resistance of the material being tested. When making a two-terminal measurement, this resistance path is very much part of the measurement and can affect the readings dramatically. A three terminal measurement, which includes the use of the guard terminal, ignores the surface leakage. This can be quite important when testing high voltage components like insulators, bushings and cables where high resistance values are expected.

As an example, dirt and moisture on a transformer bushing will promote surface leakage between the “+“and “–“ connections, thereby bringing down the reading and possibly giving a false impression that the bushing is defective. Connecting the guard to a bare wire wrapped around the bushing will intercept this current and yield a measurement based predominantly upon leakage through defects in the ceramic.

Protec Equipment Resources features the Megger MIT 5kV and 10 kV insulation resistance test sets.  We offer affordable daily, weekly and monthly rental rates and as a Megger Authorized Distributor we are also your source to purchase Megger products.

Relalted products available for rent from Protec:

AEMC 1026 1kV Digital Megohmmeter

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This electrical test equipment is available for rent, lease, or purchase from Protec Equipment Resources. We are here to help you with your testing equipment needs.

Protec’s Calibration Lab offers accurate and affordable calibration services for all makes and models of electrical test and measurement equipment. Contact us today for a quote.

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