15
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, MIT1020/2, S1-552/2 and
S1-1052/2 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.
