Rockwell Automation 1502 400 Amp Medium Voltage Contactor (Series D) - Manual (Page 17)

Receiving and Handling 2-5
1502-UM050D-EN-P – June 2013
DC Hi-pot testing is unreliable because of a phenomenon known as Cathode
Ray Tube Effect. This occurs when one contact of the vacuum bottle has a
deformity, such as a burr or deposit, while the other contact remains flat and
true. This sets up leakage currents which flow from a small surface to a large
surface in one direction and vice versa when the polarity of the tester is
changed. The resultant current is large in one direction which would
incorrectly indicate a faulty vacuum bottle.
At best, DC testing will verify only some degree of vacuum integrity. It will
not give any indication of the degree of vacuum since the contact surface can
change with each operation of the vacuum contactor. AC testing, on the other
hand, will provide reliable vacuum integrity indication. As well, the degree of
vacuum within the bottle can be determined by comparing initial test results to
the present readings. Increases in leakage current indicates a reduction in
vacuum within the vacuum bottle.
For these reasons, Rockwell Automation recommends AC testing as the best
and most reliable method of testing vacuum bottles.
A suitable GO–NO GO DC test unit is:
MANUFACTURER ADDRESS
Programma, Model VIDAR Santa Rosa, CA, USA
Insulation Resistance Test
Use a 1000 volt Megger to verify that the resistance from phase to phase or
phase to ground is greater than 500 megohms.