MEGGER WM4 Insulation and Continuity Tester Hand 500 volt with Leather bag

Description

WM 4/3 500 volt wee megger insulation and continuity tester with built-in hand-operated generator, made by Evershed and Vignoles Ltd; Megger instruments division in 1982, serial no. 67865
Desctription:
The series 3 version was discontinued sometime during the late 1960s to be replaced by the WM4 WM5 and WM6 all of which are powered by an AC generator and include a continuity range
How to use
These instructions were originally included with the megger (I do not have the original):
"To test insulation between circuit and earth, connect one terminal to the circuit and the other to a good earth. To test between a winding and its frame connect one terminal to the winding and the other to the frame. For a test between conductors connect one to each terminal. Having made the connections turn the handle at about 160 r.p.m. The resistance is then indicated on the scale. Further instructions are given in publication No. 200."
NEVER TOUCH THE TEST LEADS WHILE THE MEGGER IS BEING USED
and make sure that the item you are checking is deenergised, discharged and isolated before using the megger. Normal insulations should read infinity. Any small resistance reading indicates the insulation is breaking down. The circuit or item you are testing may have considerable capacitance and retain an electrical charge after testing. After you make your connections, you apply the test voltage for 1 min. This is a standard practice to enable relatively accurate comparisons of readings from previous tests. The insulation resistance reading should drop or remain relatively steady. This is because electrical insulation materials exhibit capacitance and will charge up during the course of the test. After 1 min, you should read and record the resistance value.
What affects insulation resistance readings?
Apart from dirt and damp insulation resistance is temperature-sensitive. When temperature increases, insulation resistance decreases, and vice versa. A common rule of thumb is insulation resistance changes by a factor of two for each 10 degree C change. So, to compare new readings with previous ones, you'll have to correct your readings to some base temperature. For example, suppose you measured 100 megohms with an insulation temperature of 30C. A corrected measurement at 20C would be 200 megohms (100 megohms times two). It is also worth bearing in mind that as time goes by the quality of insulating materials gradually deteriorates (especially at elevated temperatures).
How does it work?
The construction and connections are shown below. The moving system consists of two coils, the "control coil" and the "deflecting coil"-rigidly mounted at an angle to one another and connected, in parallel across a small generator, with polarities such that the torques produced by them are in opposition. The coils move in the air gap of a permanent magnet. The control coil is in series with a fixed control circuit; the deflecting coil is connected in series with a fixed deflecting circuit resistance and the resistance under test. If this last is infinitely high no current flows in the deflecting coil and the control coil sets itself perpendicular to the magnetic axis, the pointer indicating "Infinity." A lower test resistance allows current to flow in the deflecting coil and turns the movement clockwise. The control torque produces a restoring torque which progressively increases with the angular deflection, and the equilibrium position of the movement is attained when the two opposing torques balance. A resistor capacitor snubber circuit is connected across the generator teminals to smooth the DC output.
Note
that these two items are not shown on the typical iMegger instruction leaflets.
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