2.44 DCVB - DC Voltage Break Down
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, a dc 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 AT you may select the voltage (from 100V to 7kVdc),
the current trip level (10μA to 3mA), and the ramp up times (1 to 2sec),
all to suit the specification of the transformer under test.