15
TEST MODE SUMMARY
On screen warning codes
Breakdown detection
If insulation breakdown is detected,
the unit aborts the test and flashes
“brd” and the final test result
alternately in the main digital display.
This display is disabled when in burn
mode as the unit will continue the
test without aborting. The instrument
detects breakdown in terms of a rapid
drop in test voltage commensurate
with that caused by an arc across or though the insulation under test.
Noise detection
Excessive noise during test
In environments such as HV switch yards or substations electrical noise
levels can be high enough to effect insulation test measurements. This
noise takes the form of induced currents in the measurement circuit.
The noise current is either picked up by test leads or induced directly
in the item under test. The MIT510/2, MIT520/2 and MIT1020/2 are all
capable of accurate measurements with up to 2mA of induced noise
current. The S1-554/2 and S1-1054/4 are design for very high noise
environments and can provide accurate measurements with up to 4mA of
noise current present.
A level of noise exceeding the instruments specified capability will be
indicated by an alternating sine wave on the display. The test continues
while displaying the noise. If the noise drops back down the normal
display is resumed.
The effects of noise can be reduces significantly with the use of high
quality Megger screened test leads.
The picture displayed on the right
shows the expected screen result
of an IR test on with a 5000V test
voltage after 2 minutes 9 seconds.
The black and grey waveforms
are displayed alternately while the
noise is present. When the noise is
removed the wave form is replaced
by the correct reading.
Test aborted due to extremely high noise
Should an extremely high level of noise
be present, particularly at the start of
a test, the instrument will indicate nSE
(Noise). The instrument will require
switching off and on again to reset.
In certain rare circumstances
breakdown of the circuit under test
may interfere with the sensitive current
measuring circuit. In this event the unit stops and also displays nSE. The
instrument will again require switching off and on again to reset.
