“A Stitch in Time...”

AC Testing vs. DC
Up to now, we’ve talked about testing with DC voltage, but you will hear
of AC testing and need to know the difference. Remember that we spoke
of the kinds of current produced in insulation by DC? (The initial surge of
charging current, the drop with time to absorption current, and then, after
more time, the steady conduction current.) We saw that in insulation testing,
the conduction or leakage current is the one that gives us the information
we need.
In contrast, testing with AC gives a charging current that is extremely
large compared to the other kinds; leakage current is relatively minor. AC
frequently is used for high-potential testing; voltage is increased to some
specified point to see if the insulation can stand that particular voltage. It
is a GO/NO-GO type of test and can cause deterioration of the insulation, in
contrast to the DC test which is basically non-destructive.
If an AC test voltage has been used and you want to use DC tests as
an alternative, you will need to increase the maximum DC test voltage
somewhat to obtain equivalent results.
In some cases, AC testing may be more suitable for proof testing of
equipment (that is, seeing that the equipment meets prescribed standards).
You run the voltage up to the selected value and the equipment either
passes or doesn’t pass the test. With the DC test, you get a more qualitative
picture; you can meter the leakage current as you increase the voltage and
obtain specific values of insulation resistance.
As the size of your equipment increases, there are also marked economic
advantages in DC over AC testing. As the test voltage increases, both the
cost and weight of AC equipment go up much faster than with comparable
DC test equipment. This is because the AC test set must supply the charging
current which becomes and remains very high in the larger machines. As
explained previously, in DC testing, this current drops rapidly after the initial
charging period.
A STITCH IN TIME 17