Using The Guard Terminal; Measurements Above 100 Gω; Fault Conditioning (Burn) - Megger BM25 Guia Del Usuario

T3 Values are independent for PI and DD tests.
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The new test times are retained when the instrument is
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switched Off.
If any two values are the same, the PI result is not calculated,
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and the result displayed as - - -.
End of test Results display
After time T3 is displayed, the test stops and the total test results
displayed in a constantly repeating sequence. For details of display
information, see Appendix 1.

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
To Guard 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'.

Fault Conditioning (BURN)

Burn mode disables the 'Breakdown' detector and warning indicator and
therefore allows continuous testing under breakdown conditions with a 2
mA (nominal) current.
1. Using the L M Range keys move the right hand cursor to 'BURN'.
Leakage Path
2. Using the L M Selector keys move the left hand cursor to the
required test voltage.
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.
To -ve Terminal
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