Evershed & Vignoles Series 3 MeggerSer no.1379564

Evershed & Vignoles Megger series 3 patent 400728 (also known as the "Wee" megger) complete with brass caseand handle (although often they were housed in substantial leather cases). This is a very common instrument, everyelectrician 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.

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The "Megger" instrument for measuring the insulation resistance of electrical devices was introduced by the Britishfirm 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 generatorconnected to a specially-designed meter. GB patent number 400728 was granted in 1933. Connections are made bydepressing the spring loaded black buttons and slipping the bare end of the connecting wires into the holes on thesides.

You will note that I have added two labels indicating the polarity of the terminals because I use it to check electrolyticcapacitors.

Angus Jamieson has sent me this picture of the inside of his megger. He discovered that one of the meter deflectioncoils and one of the wire wound resistors was open circuit. He fortunately had another meter movement anddismantled the dial assembly completely to replace the deflection coil, re-attach springs etc. The faulty wirewoundresistor was bypassed with two metal film resistors in series. The capacitor is an aluminium cased Dubilier made0.1MFD 1500vDC Test. It is connected in series with a 47K ohm resistor across the generator terminals. Both wirewound resistors are 100K ohms. He has kindly scanned a copy of the 1947 edition of the handbook (5Mb PDF) whichyou can download by clicking on the image showing the diagram of connections (which does not show the abovementioned capacitor and resistor). These were included on later models to improve the output voltage.The pageswhen printed out can be reassembled into a booklet. Also a scanned a copy of his 1943 edition of the handbook oncontinuity and polarity testing (4.8Mb PDF)can be downloaded by clicking on the scond image.

Note the slipping clutch on the main gear wheel to limit the voltage output.

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Selyem Toth Sandor from Hungary has sent me some pictures of his meggers [serial number 963610] one of whichI 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 alsosent me this copy of a Hungarian leaflet.

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I don't know how old this leaflet is but you may be able to discern that this picture was made before the patent wasgranted. S T Sandor has over 700 radios see here:

Versions This little megohmmeter proved very popular and over the years Evershed and Vignoles made manyimprovements and used the same format to manufacture related instruments. Until 1948 [I think] the internal designstayed much the same as it did in 1932. About this time the whole range was renamed and the 'Wee megger' becamea 'Series 3 megger'. Mine and that of Angus Jamieson are, I believe MK2 versions. The series 3 MK3 versionintroduced 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 testinsulation between circuit and earth, connect one terminal to the circuit and the other to a good earth. To testbetween a winding and its frame connect one terminal to the winding and the other to the frame. For a test betweenconductors connect one to each terminal. Having made the connections turn the handle at about 160 r.p.m. Theresistance 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 checkingis deenergised, discharged and isolated before using the megger. Normal insulations should read infinity. Any smallresistance reading indicates the insulation is breaking down. The circuit or item you are testing may have considerablecapacitance and retain an electrical charge after testing. After you make your connections, you apply the test voltagefor 1 min. This is a standard practice to enable relatively accurate comparisons of readings from previous tests. Theinsulation resistance reading should drop or remain relatively steady. This is because electrical insulation materialsexhibit capacitance and will charge up during the course of the test. After 1 min, you should read and record theresistance value.

What affects insulation resistance readings? Apart from dirt and damp insulation resistance is temperature-

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sensitive. When temperature increases, insulation resistance decreases, and vice versa. A common rule of thumb isinsulation resistance changes by a factor of two for each 10 degree C change. So, to compare new readings withprevious ones, you'll have to correct your readings to some base temperature. For example, suppose you measured100 megohms with an insulation temperature of 30C. A corrected measurement at 20C would be 200 megohms (100megohms times two). It is also worth bearing in mind that as time goes by the quality of insulating materialsgradually 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 asmall generator, with polarities such that the torques produced by them are in opposition. The coils move in the airgap of a permanent magnet. The control coil is in series with a fixed control circuit; the deflecting coil is connected inseries with a fixed deflecting circuit resistance and the resistance under test. If this last is infinitely high no currentflows 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. Thecontrol torque produces a restoring torque which progressively increases with the angular deflection, and theequilibrium position of the movement is attained when the two opposing torques balance.

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The control coil is actually in two parts, in series, the outer part being a compensating coil. The two parts are arrangedwith numbers of turns and radii of action such that, for external magnetic fields of uniform intensity, their torquescancel 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 clutchwhich 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 otherwords, 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 beconsidered.

If we measure between core B and sheath without a connection to the guard terminal some current will pass from Bto A and from A to the sheath. Our measurement would be low. By connecting the guard terminal to A the two cablecores 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 batteryoperated inverter or a motor driven generator. see this example. For more information on Evershed and Vignoles earlyinsulation testing equipment see: here.

Evershed & Vignoles Ltd, Acton Lane Works,Chiswick. were taken over by AVO Ltd. Avocet House, 92-96 VauxhallBridge Road, London, SW1. AVO was a member of the Metal Industries Group of companies. Though known for theirAvometer general purpose multimeter (see below), they made a wide range of test gear including valve testers. TheActon 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.

For hand driven generators, Meggers and other insulationtesters click here

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