Evershed & Vignoles Megger series 3 patent 400728 (also known as the "Wee" megger) complete with brass case and handle (although often they were housed in substantial leather cases). This is a very common instrument, every electrician would have needed one of these well constructed robust insulation testers. I have had this one since 1966 when I bought it second hand for £ 5, sadly today they are still only worth about £ 5 to £ 10 working.
The "Megger" instrument for measuring the insulation resistance of electrical devices was introduced by the British firm of Evershed and Vignoles in 1903. The name comes from the fact that the insulating resistance of a properly-designed appliance is in the range of tens and hundreds of meghoms. The crank on the end powers a DC generator connected to a specially-designed meter. GB patent number 400728 was granted in 1933. Connections are made by depressing the spring loaded black buttons and slipping the bare end of the connecting wires into the holes on the sides.
You will note that I have added two labels indicating the polarity of the terminals because I use it to check electrolytic capacitors.
Angus Jamieson has sent me this picture of the inside of his megger. He discovered that one of the meter deflection coils and one of the wire wound resistors was open circuit. He fortunately had another meter movement and dismantled the dial assembly completely to replace the deflection coil, re-attach springs etc. The faulty wire wound resistor was bypassed with two metal film resistors in series. The capacitor is an aluminium cased Dubilier made 0.1mFd 1500V DC Test. It is connected in series with a 47K ohm resistor across the generator terminals. Both wire wound resistors are 100K ohms. He has kindly scanned a copy of the 1947 edition of the handbook (5Mb PDF) which you can download by clicking on the image showing the diagram of connections (which does not show the above mentioned capacitor and resistor). These were included on later models to improve the output voltage.The pages when printed out can be reassembled into a booklet. Also a scanned a copy of his 1943 edition of the handbook on continuity and polarity testing (4.8Mb PDF)can be downloaded by clicking on the second image.
Note the slipping clutch on the main gear wheel to limit the voltage output.
Selyem Toth Sandor from Hungary has sent me some pictures of his meggers [serial number 963610] one of which I reproduce here.
You will see that it is the same as mine but has been labelled by the Austrian agent for Evershed and Vignoles.He also sent me this copy of a Hungarian leaflet.
I don't know how old this leaflet is but you may be able to discern that this picture was made before the patent was granted. S T Sandor has over 700 radios see here:
This little megohmmeter proved very popular and over the years Evershed and Vignoles made many improvements and used the same format to manufacture related instruments. Until 1948 [I think] the internal design stayed much the same as it did in 1932. About this time the whole range was renamed and the 'Wee megger' became a 'Series 3 megger'. Mine and that of Angus Jamieson are, I believe MK2 versions. The series 3 MK3 version introduced in 1960 incorporates an AC generator and a diode voltage multiplier to provide the test voltage.
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
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.
The control coil is actually in two parts, in series, the outer part being a compensating coil. The two parts are arranged with numbers of turns and radii of action such that, for external magnetic fields of uniform intensity, their torques cancel one another thus giving an astatic combination.
The instrument has a small permanent magnet d.c. generator developing 500 V DC. (Other models have 100, 250, 1,000 or 2,500 V generators). The generator is hand-driven, through gearing and a centrifugally controlled clutch which slips at a predetermined speed so that a steady voltage can be obtained.
The guard terminal (if fitted) acts as a shunt to remove the connected element from the measurement. In other words, it allows you to be selective in evaluating certain specific components in a large piece of electrical equipment. For example consider a two core cable with a sheath. As the diagram below shows there are three resistances to be considered.
If we measure between core B and sheath without a connection to the guard terminal some current will pass from B to A and from A to the sheath. Our measurement would be low. By connecting the guard terminal to A the two cable cores will be at very nearly the same potential and thus the shunting effect is eliminated.
Other insulation testers work on the same principle though the voltage required may be obtained from a battery operated inverter or a motor driven generator. see this example. For more information on Evershed and Vignoles early insulation testing equipment see: here.
Evershed & Vignoles Ltd of Acton Lane Works,Chiswick were taken over by AVO Ltd. Avocet House, 92-96 Vauxhall Bridge Road, London, SW1. AVO was a member of the Metal Industries Group of companies. Though known for their Avometer general purpose multimeter (see below), they made a wide range of test gear including valve testers. The Acton Lane works closed down at about that time - around 1986. The current range of products can bee seen at: http://www.avointl.com/
The firm Megger Ltd is now based at: Archcliffe Road, Dover, Kent, CT17 9EN, United Kingdom.