DO4A Issue 3/2001
13
Connections
When making good quality measurements, it is important to ensure that all measuring
leads are in good condition, and less than 0.2 ohms resistance.
It should also be noted that some spade tags and crocodile clips can produce high
thermal emfs when warmed, particularly nickel-plated brass types. This can cause
problems when, for example, connecting too hot motor windings. The solution is to
use plain copper or brass connections keeping them clean and oxide free.
12.
Protection
Every effort has been made to protect the instrument against voltages being applied
to the terminals. A large 6 Amp fuse is fitted internally to the C measuring line and a
gas discharge tube, GDT, is across the C terminals. If voltages above approximately
90V are applied to the measuring terminals, the GDT will strike effectively shorting
the C terminals through the 6 Amp protective fuse, which will interrupt the circuit.
The fuse fitted will interrupt up to 40,000 Amps. The P terminals are not fused and
will withstand up to 460 Volts without damage to the instrument.
WARNING!
To replace the protection fuse, the top instrument panel should be removed,
but only
after the mains supply and all input connections are removed
. The protection
fuse is located on the main printed circuit board. Only replace with the correct fuse
rating.
13.
Calibration Process
WARNING! This should only be carried out by competent and trained personnel.
The calibration is carried out from the front panel and no mechanical adjustments are
necessary.
Equipment required - Milliohmmeter Test Standard type MTS2 or a set of Resistance
standards 4m
Ω
, 40m
Ω
, 400m
Ω
, 4
Ω
, and 400
Ω
and 4k
Ω
. A calibration key is
supplied with the instrument.
The DO4A is designed to eliminate all calibration. The factory adjusted internal
standards are constructed to give long term stability and minimal drift. However, for
those wishing to adjust the ohmmeter, proceed as follows.
