This test may be used to check the action of devices such as metal oxide varistors (MOV) that are used to protect transformer windings from transient over-voltages that may be present on the ac supply system.

A varistor is a non-linear semiconductor device that has little effect on the circuit until a particular break-down, voltage is reached. When that voltage is reached, the MOV conducts current and limits the applied voltage to its rated break-down voltage. A MOV is limited by the energy (Joules) that it can dissipate under break-down conditions to transient over-voltages only.

It is typically performed between all primary windings connected together, and all secondary windings plus the screen connected together.

SPECIFYING THE MEASUREMENT

During the test, an ac voltage is ramped up across the protection device (MOV) under test and the current is monitored. When the desired current flows, the test stops and the measured result is the break-down voltage. If the voltage is within the desired limits, the test is a PASS. If the voltage is outside the desired limits, or if no breakdown is detected when the maximum voltage is reached, the test is a FAIL. The voltage will ramp to the specified maximum plus 50V.

In programming the AT3600 you may select the voltage (from 100V to 5kVrms), the frequency (50Hz / 60Hz at the full voltage, or up to 1kHz at reduced voltage), the current trip level (10μA to 5mApeak), and the ramp up times (1 to 30sec), all to suit the specification of the transformer under test.

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