HT instruments HT7052 - Manual (Page 17)

HT7052
EN - 15
6.1.2 Withstanding voltage test
Some standards allow the use of a DC voltage as an alternative to AC withstanding
voltage testing. For this purpose the test voltage has to be present across the insulation
under test for a specific time. The insulation material only passes if there is no breakdown
or flash over. Standards recommend that the test starts with a low voltage and reaches the
final test voltage with a slope that keeps the charging current under the limit of the current
threshold. The test duration normally takes 1 min
The instruments offers Withstanding Voltage test of insulation material. It covers two types
of tests:

Breakdown voltage testing of high voltage device, e.g. transient suppressors and
 DC withstanding voltage test for insulation coordination purposes
Both functions require breakdown current detection. The test voltage increases step by
step from the Start up to the Stop value over a predefined time and it is kept at the Stop
value for a predefined test time (see Fig. 10 – left part) or it happens the breakdown on
device under test (see Fig. 10 – right part)
Fig. 10: Withstanding test without breakdown (left) and with breakdown (right)
Ut = test voltage
t = time
Ustart = starting voltage
Ustep = voltage step approx. 25 V (fixed value - not modify)
Ustop = end test voltage
Tstep = test voltage duration per step
Tend = constant test voltage duration after reaching End value
Ub = breakdown voltage
Humidity and insulation resistance measurements
When testing outside the reference ambient conditions, the quality of the insulation
resistance measurements can be affected by humidity. Humidity adds leakage paths onto
the surface of the complete measuring system, (i.e. the insulator under test, the test leads,
the measuring instrument etc). The influence of humidity reduces accuracy especially
when testing very high resistances (e.g. T
). The worst conditions arise in environments
containing high condensation, which can also reduce safety. In the case of high humidity, it
is recommended to ventilate the test areas before and during the measurements. In the
case of condensed humidity the measuring system must dry and it can take several hours
or even few days to recover