Using The Guard Terminal; Measurements Above 100 Gω; Fault Conditioning (Burn) - AVO MEGGER BM25 Guide De L'utilisateur

Using the Guard terminal

For basic insulation tests and where there is little possibility of
surface leakage affecting the measurement, it is unnecessary to
use the guard terminal. i.e. if the insulator is clean and there are
unlikely to be any adverse current paths.
However in cable testing, there may be surface leakage paths
across the insulation between the bare cable and the external
sheathing due to the presence of moisture or dirt. Where it is
required to remove the effect of this leakage, particularly at high
testing voltages, a bare wire may be bound tightly around the
insulation and connected via the third test lead to the guard
terminal 'G'.
To +ve Terminal
The guard terminal is at the same potential as the negative
terminal. Since the leakage resistance is effectively in parallel
with the resistance to be measured, the use of the guard causes
the current flowing through surface leakage to be diverted from
the measuring circuit. The instrument therefore reads the
leakage of the insulator, ignoring leakage across its surface.
Measurements above 100 GΩ
Measurements up to 100 GΩ can be made without any special
precautions, assuming that the test leads are reasonably clean
and dry. The guard lead can be used to remove the effects of
surface leakage if necessary. The BM25 is capable of
measuring up to 5 TΩ, and down to 0,01 nA (equivalent to 500 TΩ
at 5000 V). When measuring resistances this high, the test leads
should not be allowed to touch each other or any other object
since this may introduce leakage paths. Sharp points at the test
lead connections should also be avoided since this will encourage
corona discharge. For further information see 'Stress
considerations'.
Burn mode disables the 'Breakdown' detector and warning
indicator and therefore allows continuous testing under
breakdown conditions with a 2 mA (nominal) current.
Leakage Path
1. Using the
'BURN'.
2. Using the
required test voltage.
To -ve Terminal
3. Start the test by pressing the red test button for at least one
second.
4. At the end of a test, if complete breakdown does not occur
the final measured value is displayed sequentially with the
corresponding leakage current and capacitance value. This
sequential display repeats until a key is pressed or Auto
switch-off operates.
Note: In this mode, break-down of the circuit and the related
noise interference may cause the instrument to terminate the
operation, possibly causing a loss of display while the circuit
remains energised. In this event, it is important that the circuit

Fault Conditioning (BURN)

Range keys move the right hand cursor to
Selector keys move the left hand cursor to the
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