Rockwell Automation 1502 Medium Voltage 400A Contactor - Series E - Manual (Page 16)

16 Rockwell Automation Publication 1502-UM052H-EN-P - June 2013
Chapter 2 Receiving and Handling
Note: Rockwell Automation does not recommend DC Hi-pot testing because
the values obtained during the test may not be a reliable indication of vacuum
bottle integrity. Some specific DC “GO-NO GO” testers may provide suitable
“defective” readings.
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 on 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 indicate 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:
Insulation Resistance Test Use a 1000V Megger to verify that the resistance from phase-to-phase or phase-
to-ground is greater than 500 megohms.
Manufacturer Address
Programma, Model VIDAR Santa Rosa, CA, USA