Using The Guard Terminal; Measurements Above 100 Gω - Megger S1-5001 Guia Del Usuario

value is displayed sequentially with the corresponding leak
age 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
is allowed to discharge before touching any connections. If
necessary, switch the instrument off, and then on again to reset
the display.

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.
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 S1-5001 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' on page 15.
Leakage Path
To -ve Terminal
11