Megger INGVAR, User Manual

Page: 44 / 72

44 ZP-BH05E BH0654BE
7 HOW TO USE INGVAR
4]
If necessary readjust the current and inject
again. Repeat this until you have achieved
the desired current value. Deactivate the I/30
function.
5]
Inject until the test object operates and the
operating time is presented on the display.
7.17 Test circuit impedance
Introduction
Different applications have various requirements on
current and load time and the test objects have differ-
ent impedance.
Examples of different applications:
▪
Up to 5 kA through 0.5 m current cables. Applications:
Test of MCCB.
▪
Continuous current generation.
Up to 900 A through 5 meters current cables.
Test circuit impedance limits the
current
INGVAR
The test circuit impedance Z consists of current cable
impedance and impedance of the test object. Accord-
ing to Ohms law I x Z volt is required to push current I
through the impedance. If voltage at INGVAR terminals
is less, current will be lower than desired.
Note Voltage is required to get current through the
impedance and you need to know the imped-
ance of the test circuit.
The current capability is dependent on the
load time. You must also check in the load
time.
Impedance of the test object
Knowledge saves you from surprises
▪
Test object impedance has most impact at higher currents
and when current cables are short.
▪
Test object impedance is less important when cable set is
5 meters or longer and only consists of one or two cables
each side. Then cable set impedance is the major part of
the test circuit impedance.
▪
Test object impedance can be measured. Inject an AC
current, measure voltage drop directly across the object.
Divide voltage by the current to get the impedance
(mΩ = V / kA).
If possible, current magnitude should be the same as
when testing the object. Some test objects saturate at
high currents and measurement at low currents will give a
too high impedance value.