12
Recording to a PC
While carrying out a test, the instrument will output the test voltage,
test current and resistance every second. Refer to section ‘RS232 / USB
connection’ for setup details. Connect the instrument to the RS232 / USB
port of the PC. The data may be captured with Microsoft
®
HyperTerminal
or another suitable programme.
High voltage warning LED
This is a red LED next to the TEST button on the front panel. The LED
flashes when the voltage on the test inputs exceeds 50 V.
Line input present LED
This is a green LED next to the power On/Off button on the front panel.
It is illuminated whenever the mains power is connected.
Test terminals
There are three test terminals , - and G. These terminals are
designed to accept only the test leads supplied. Shutters across the
terminals prevent accidental ingress of dirt and other objects. Test lead
plugs interlock with the shutters and are released by rotating the test lead
plug a quarter turn.
The Guard terminal is explained below and is only used in cases where
surface leakage currents need to be eliminated. Most measurements use
just the + and – terminals. The instrument’s internal voltage generator
drives the + terminal with respect to the – terminal, current being
measured in the – terminal.
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 for example, 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’.
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.
The display will show ‘FUS’ if the internal guard terminal fuse is found to
have blown. The instrument must be switched off to clear the message
before further testing is permitted. The fuse should be replaced by an
authorised repairer. The instrument may be used in the mean time if the
guard terminal not used. Refer to notes regarding measurements above
100 G
Ω
on page 17.
