23
Terminals
There are three test terminals , - and GUARD. These
terminals are designed to accept only genuine Megger test
leads. 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 by a
quarter turn.
The GUARD terminal, as explained below, 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, screened leads
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 a warning and fuse terminal symbol if
the internal guard terminal fuse has 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
meantime if the guard terminal not used. Refer to notes
regarding measurements above 100 G
Ω
above.
This diagram illustrates GUARD terminal used to prevent
surface leakage on cable insulation from affecting a high
resistance measurement.
Screened leads are available for the complete range of
insulation testers. They are useful in HV switchyards where
induced currents are an issue. The screen connects to
GUARD and prevents induced currents in the lead.
