General Description - Megger Model 8 Mk7 Guide De L'utilisateur

GENERAL DESCRIPTION

DESIGN AND CONSTRUCTION
The Model 8mk7 instrument comprises a moulded
front panel on which are mounted the whole of
the switching apparatus, the movement and other
components. A carrying strap is fixed to moulded
lugs on the rear case. The front panel is fitted
into this case with a dust proof joint.
When the instrument is set for operation on d.c.
the moving coil is associated with a universal
shunt and series multipliers, whilst on a.c. diodes
and a transformer are also introduced.
RANGE CONTROLS
The left-hand switch provides all the d.c. current
and voltage ranges and the right-hand switch the
a.c. current and voltage ranges and also the
resistance ranges. These switches are electrically
interlocked so that readings can only be made
after a.c. or d.c. measurement and range has been
selected. Resistance tests require the left-hand
switch to be set to 'Ω' and the right-hand one to
the desired range.
The ranges are marked on the front panel and
arrow heads indicate the actual range selected.
The current ranges are indicated by a red arc and
4
the voltage ranges by a grey arc. Resistance
ranges have a black arc. Wide coverage in
resistance has been achieved by having a
fundamental range as marked on the scale,
together with ranges of 'x 100' and 'x 10 k' to
supplement it. Each resistance range has its own
zero adjustment control.
In addition a 200 megohm range, marked 'ins' is
available, using an external d.c. voltage source.
THE MOVEMENT
The meter movement in the Model 8mk7 is a
robust AVO centre pole movement type ACP1,
®
fitted with spring mounted jewelled bearings. The
meter has a full scale deflection of 37,5 µA. A
knife edge pointer enables very fine readings to
be taken, whilst the whole movement is balanced
and damped so that the pointer quickly comes to
rest.
SCALING
The scale plate has three calibrated scales. One is
for resistance measurement and is marked 0 to
2000 ohms, the second is for current and voltage
(both a.c. and d.c.) and is marked 0 to 10 with
100 divisions whilst the third scale calibrated 0 to