Evershed & Vignoles Hand Driven Generators, Meggers and other insulation testers

Background

Telegraphy became a commercial success with the advent of the railways and those responsible for the design and upkeep needed to know about electricity. This was one of the factors which led to the birth of the term 'Electrical Engineer' as opposed to Civil or Mechanical engineers. The Society of Telegraph Engineers and of Electricians, which was the forerunner of the Institution of Electrical Engineers (IEE) published some rules in 1882 which became over the years the foundations for the current Wiring Regulations (BS767)

Rules and Regulations for the Prevention of Fire Risks Arising from Electrical Lighting

THESE rules and regulations are drawn up for the reduction to a minimum, in the case of electric lighting, of those risks of fire which are inherent in every system of artificial illumination, and also for the guidance and instruction of those who have, or who contemplate having, electric lighting apparatus installed on their premises.The difficulties that beset the electrical engineer are chiefly internal and invisible, and they can only be effectually guarded against by "testing," or probing with electric currents. They depend chiefly on leakage, undue resistance in the conductor, and bad joints, which lead to waste of energy and the dangerous production of heat.

These requirement, though not obligatory at the time, led to the growth of a new industry, the design and manufacture of portable easily used electrical instruments for the measurement of voltage, current and resistance. It was on the latter (in particular high resistance or 'leakage') that Evershed and Vignoles concentrated and for which they became famous .

For many years, the James G. Biddle Company in Pennsylvania (now solely owned by Megger Ltd.), was the U.S. importer of Megger products. and they also manufactured equipment under their own name , many of which were based upon the Evershed and Vignoles designs.

The " Megger " instrument for measuring the insulation resistance of electrical devices was introduced by the British firm of Evershed and Vignoles in 1905. The name comes from the fact that the insulating resistance of a properly-designed appliance is in the range of tens and hundreds of megohms. The firm had been manufacturing meters and hand powered generators prior to this and some early examples are shown and described here.

Evershed & Vignoles Ltd., Acton Lane Works, Chiswick co-founded by Sydney Evershed (1858 -1939) in 1895 was 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 here, 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 be seen at: http://www.megger.com/

The firm Megger Ltd. is now based at: Archcliffe Road, Dover, Kent, CT17 9EN, United Kingdom.

Evershed's patent. (US patent 639849) a small part of which is reproduced here:

"Be it known that I, SYDNEY EVERSHED, a subject of the Queen of Great Britain and Ireland, residing at London, England, have invented a new and useful Improvement in Portable Hand-Dynamos, (for which I have made application for Letters Patent in Great Britain under No. 1,758, bearing date January 25, 1899,) of which the following is a specification.

This invention relates to portable hand dynamos, such as are used for testing purposes or for firing electric fuses or the like purposes, where a small current of high electromotive force can be utilised; and it consists in an improvement whereby the frictional resistance to rotation (of importance in such small appliances) is reduced to a minimum both in the armature-bearings and in the commutator, the current is collected with highest electromotive force, the hand 'which is made collapsible when not in use and is then in a protected position, and connection to any instrument or object, at a reasonable distance from the generator, is rendered easy and rapid without the use of loose wires, which may be mislaid, and without any binding-screws projecting from the portable box containing the dynamo.

Figure 1 is a plan of the box, with the lid open, containing the dynamo in the body of the box and the winch-handle ready for operation issuing therefrom and the connection device in the inside of the lid."  If you want to read more and to view other patents try this link www.google.com/patents

Jack Davidson.C.Eng. FIEE. says "The generator was originally designed by a Mr Goolden of Messrs. Easton, Anderson & Goolden and the generator was used separately from the instrument itself probably to obviate interference by the permanent magnet fields of the generator. Evershed & Vignoles took over the manufacture, and raised the voltage to 200 d.c. from the original 120V. The lower voltage was probably to test installations at the working voltage, commonly 110V at that time, but someone then thought that twice the working voltage would be a better test, which is what we do today, with 500V. d.c. test for supply voltage around 250 Volts.

From the beginning insulation testers had a meter with a voltage coil and a current coil combined in the movement. This was the most important, and at the time, unique feature of the Evershed and Vignoles insulation tester. Their meter movement relied upon the work of Ayrton and Perry.

William Edward Ayrton (1847 - 1908) English engineer and inventor, with his colleague John Perry (1850 - 1920) invented many electrical measuring instruments. In these, use was made of an ingeniously devised flat spiral spring (as used in modern analogue meters) which yields a relatively large rotation for a small axial elongation.

They also invented the principle of moving a pointer by the combined fields of separate voltage and current coils thus enabling power or resistance measurements to be made with only one instrument. They used fixed coils at right angles to each other and there was a magnetised needle at the pivot end of the pointer. There was no problem with coil connections since they were fixed and then as now, there was no control spring. The needle doesn't come to zero until the test terminals are shorted and the generator operated.

Evershed & Vignoles tidied things up. They made a double moving coil, one for voltage and one for current. They were fixed together, but not at right angles. The pointer was fixed to the coil assembly and the coil connections were ligaments, no hairsprings since the only influence on the coils was that of a powerful permanent magnet. This got over the gradual loss of magnetisation which the Ayrton and Perry instrument suffered from. It also enabled the generator and meter to be combined into one instrument as it was no longer necessary to obviate interference from the permanent magnet field of the generator.

More complete descriptions of some of the instruments mentioned below may be found by clicking on the illustrations on this page

Evershed and Vignoles Hand operated Generator s/n 24798 housed in a polished two part wooden box 5.5" x 8.5" x 6" high with leather carrying handle and fold away crank handle. The handle is geared to 8 pole generator comprising horse shoe magnets, armature and brass ring commutator. Interior coiled spring return for 2 core output lead.The 200 volt output is obtained from the two lower outside brass wheel collectors shown in the picture below. The reel mechanism needs attention. 

Jon Clafton who has collected Evershed & Vignoles stuff for several years  has sent me a picture of the cable reeling mechanism on his generator and the cable connector which is designed to work with the ohmmeter he also owns.

Clive Jackson has provided a description and pictures of a similar item which measures 7" X 6 1/2" X 6 1/2" in plan and also is housed in a polished wooden box with leather handle. Inside the door there is a fixed piece of card giving technical details and instructions which indicate that it was used to test cable insulation.The generator appears to be very much the same as mine although the design of the handle on his appears to be more elementary and the serial number would seem to indicate that it is an earlier model. Perhaps '247' is the actual number and '98' signifies the date, 1898? It isn't likely that production quantities could have reached a five figure number in those early days, but you never know what coded information serial numbers contain.

The following picture (from Instruments of Science: An Historical Encyclopaedia by Robert Bud and Deborah Warner (December, 1997) - Garland) shows an early Evershed and Vignoles insulation tester but has no indication of the date. On p.89 of Sir David Salomans' book, 'Electric Light Installations, Part 3' published in 1894 there is a picture of such an instrument and its hand generator connected by 'curly' single conductors rather than the self-coiling cable reel which my item has. This picture shows a well finished handle, with an ebonite, (presumably), hand-grip Perhaps Clive's one has had a handle replacement or repair, the same seems likely with the simple leather strap in place of the proper leather handle with metal fittings.

Jon Clafton who has the generator and an accompanying ohmmeter shown below thinks the generator was made in the late 1890s and the ohmmeter the early 1900s.

These are the instructions printed on the lid of the above meter.

PORTABLE TESTING SET EVERSHED'S PATENT
INSTRUCTIONS FOR USE.

Adjust the Ohmmeter until the bubble is in the centre of the spirit level.

Place the generator not less than 18 inches away from the Ohmmeter and couple its terminals to the terminals G1 G on the Ohmmeter. Couple the mains to be tested to the "LINE" and "EARTH" terminals of the Ohmmeter. Turn the Generator handle steadily in either direction at any speed above 110 revolutions per minute, and the Ohmmeter index will point to the resistance under test.

When making tests in the neighbourhood of dynamos and generally when very accurate results are required two observations should be made, first when the generator is revolving in one direction and then in the other. The mean of the two readings so obtained gives the value of the resistance under test.

Francois  has a generator which is very similar to mine which has no reeling mechanism. The label on his reads: "Evershed & Vignoles Ltd., London, No. 12867, Patent, 200 volts, 100 revs".

I have a copy of "Direct Current Electrical Engineering" by JR Barr published by Pitman in 1908 which describes "..........Sidney Evershed's ohmmeter or 'Megger'...." This is a combined instrument, the generator and the meter sharing the same magnetic field. Although there is no picture of the construction it would seem to be the same principle to that used on modern hand driven insulation testers. This seems to me to indicate that all three generators predate 1908 and they were intended to be used for cable testing in conjunction with an ohmmeter.

Simon Eastwood sent me pictures of his very similar generator generator which is made by Everett Edgecombe. This is also a 200 volt model serial number 8145. Everett joined Edgecumbe in 1900 so this generator would have been manufactured after this date. see: http://www.museum of technology.com/cic.html and the firm still exists see: http:// www.edgcumbe.co.uk/metrohm/about.html

                   

Jon Clafton has a 200V generator (serial number 17465) identical to mine. The rotary flex 'dispenser' is in perfect condition and the entire device works perfectly. His still has the original cable and fitted connectors. He also has another generator (serial number 18550), similar to the 200V one, but that one simply has two output terminals, and has a 20V output.
He also has four Evershed & Vignoles megger insulation testers similar to the one featured below. These all have the original date on the underside of the lid. The oldest one he has is dated 1912, and the newest is dated 1920. He notes that the generator part of the meggers is very similar to the stand alone generators. This method of construction was used by Evershed and Vignoles for maybe 60 years so it is often difficult to date such instruments.

The right hand picture has been taken from the 'Megger pocket book on insulation and continuity testers' dated 1960. They are described as Series 1, 2 and 3 as indicated on the illustration.

The earlier instruments had two massive bar magnets with pole pieces at both ends, so that one end did for the moving coils and the other provided the generator field.These two pictures from Jon Clafton show the general construction and the second the generator without magnets and meter movement.

Craig Douglas from New Zealand sent me pictures of his megger instrument wondering how old it was (see below). Fortunately he has the instruction manual with the patent number on the title page. Looking at UK Patent Numbers leads to the conclusion that it was manufactured after 1926. Connections to the current and pressure coils from the meter movement are brought out to terminals to enable the instrument to be used with a bridge box.
Tony West has advised that this appears to be a DUCTER by Evershed and Vignoles. It was part of a range of BONDING TESTERS. Typical uses included earth bonding and he remembers a small self contained unit with integrated leads that was used to check aircraft fuselages. (You will see that I have a more modern version of such an instrument Bonding Tester)

Power for the unit came from a bank of nife cells giving 1.3 volts at about 10 amps. These are connected to the terminals at the opposite end to those shown. The test probes comprise of 2 hand held plastic probes each with 2 spring loaded prods. The probes are approx 8 inches long.
PROBE 1----red lead to P1 and black lead to C1
PROBE 2----red lead to P2 and black lead to C1
Press the probes down firmly across the joint under test (ideally the P connection would be closer than the C connections)
Current flows through the "C" circuit ,powers the control coil in the movement and produces a potential difference (p/d) across the resistance under test.
The deflecting coil is fed by the p/d developed by the external resistance and is detected by the "P" prods.
This method of testing removes the need to compensate for volt drop caused by the resistance in the test leads which could be similar to the item under test! Highly accurate and repeatable figures can be obtained.

This is the principle of analogue insulation testers,(see:Megger , Safety Ohmmeter and Metrohm) but the voltage and auxiliary coils are combined into one but still move over the horn shaped projection. For more information see Electrical Measurements & Measuring Instruments by Golding published by Pitman 1935

In Electrical Engineering by TC Baillie MA DSc AMIEE published by Cambridge University Press in 1915 there are descriptions and illustrations of several portable testing sets including the familiar wooden cased Evershed and Vignoles instrument described above. The other instruments described are the Omega manufactured by RW Paul which has a moving coil meter and separate generator mounted in the same enclosure. The Megohmmeter of Kelvin Bottomley and Baird which resembles the Omega but includes a divide by ten key to change the range of the instrument. The Ohmer made by Nalder Bros and Thompson is also a self contained instrument but unlike others has an electrostatic meter movement which made it the lightest instrument on the market at the time.

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. Sadly today they are only worth about £ 5 to £ 10 working. These instruments and their modern larger units are equipped with separate magnetic circuits for the meter and generator.

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 wirewound resistor was bypassed with two metal film resistors in series. The capacitor is an aluminium cased Dubilier made 0.1 MFD 1500 V 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 curiously does not show the above mentioned capacitor and resistor). 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.

  

Selyem Toth Sandor from Hungary has sent me some pictures of his meggers 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.

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 de-energised, 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.

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. Another refinement was an auxiliary coil in series with the voltage coil and moving over a horn shaped projection on one of the pole-pieces which enabled the scale to be more evenly calibrated.

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 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. For more information on Evershed and Vignoles early insulation testing equipment see: here.

Combined Continuity and Insulation Testers Evershed & Vignoles also made combined units housed in the same casings as the Series 1, 2 and 3 housings. The Series 3 Mk111 tester incorporated an AC generator and a rectifier.

The insulation testers detailed below use a conventional moving coil meter. It is therefore necessary for the instrument to have a means for ensuring that the applied voltage remains constant so that the reading on the scale will be an accurate representation of the resistance of the circuit or item being tested.

Everett and Edgecombe Metrohm s/n 988853

"Megometer" DC Voltmeter and Insulation Tester

"Megometer" This is a combined DC Voltmeter and Insulation Tester housed in a wooden case. Dating from the 1930's or possibly even earlier, the meter and generator are housed in a wooden dovetail joint box, with a hinged protective lid which opens to reveal the meter.faulty

The generator produced 450 volts as measured by an AVO but fell to zero when ant load was applied.The meter was completely unserviceable and both wire wound resistors were open circuit.The meter was replaced with a !mA movement and the generator rotor shown below was removed and it was found that the soldered connection to the end of the winding had corroded and was making a poor connection. The wire is very fine and randomly wound and impregnated with varnish so it was difficult to locate the end. I made a soldered connection to two or three adjacent turns on the outside layer and reassembled the generator. It worked well but only produced just in excess of 300 volts.

Rotor with new connections.

Hartmann & Braun Insulation Tester

(Voigt & Haeffner or Union Electric )

Serial number 450941

The terminals and the black knob to the right of them are replacements.To the left of the meter is a small compartment to stow the generator handle when not in use. Unlike other insulation testers that I have, this one is intended to be operated whilst hanging from the operator's neck, the right hand turning the generator handle and the left depressing the button to unclamp the meter needle.

This is a close up view of the meter and the unusual mushroom shaped pivoting mechanism surrounding the meter movement. At the left hand side of the mechanism can be seen a small horizontal and a similar vertical metal plunger. The former is operated by the push button knob adjacent to the output terminals and serves to unclamp the meter needle. The other is operated by an internal lever which serves the same purpose but will permanently lift the clamp and render the push button knob ineffective.

Here are views of the generator showing the steel gearing, the commutator and the centrifugal governor. Note how the unit is completely self contained and equipped with contacts which mate with those on the remainder of the unit shown on the picture below.The series resistor is made up of three banks of three wire wound wooden spools. Two of the banks are labelled 335,000w. As two of these banks were open circuit I have left these in place but shunted them three high stability resistors totalling 335 000 ohms. The lever for unclamping the meter can be seen to the right of the resistor spools. The meter movement has a full scale deflection of 1.5mA and a resistance of 215 ohms.

The Union Electric Company Ltd. was, until the outbreak of World War One, a subsidiary of Voigt and Haeffner which had a factory in Frankfurt, where they made electrical apparatus. They also had works in Birmingham. The name then changed to " The Switchgear Construction Company, Ltd." which incorporated both the British based firms. The majority shareholders were still German. The Union Electric Company installed electric lighting in the public galleries of the Natural History Museum in London as well as lighting for London theatres. I believe that this instrument was made in Germany prior to 1914, the trade mark device on the card either side of the meter scale is H&B which indicates that it was made by Hartmann & Braun, also of Frankfurt.

Pye High Resistance test set  model 11800

 

PYE High Resistance test set housed in wooden box.

This two valve instrument requires two 1.5 volt cells (one for each valve filament) and 3 HT supplies (67.5 112.5 and 22.5 volts). The latter would have been provided by 9 type B115 batteries. Fortunately the previous owner has made a battery replacement. A pencil sketch of the circuit was found in the battery compartment.

            

This instrument can measure insulation resistances in excess of 200 000 megohms! It incorporates a very high input impedance two valve voltmeter. Hidden inside the box was a copy of the circuit diagram indicating that it must have been manufactured by W G Pye at their Granta Works in Cambridge sometime after 1957, though confusingly the copy has been rubber stamped 1 NOV 1967. On the Internal view note wire ended valve mounted through the Bakelite tag strip, the second valve is hidden by the switch.

More complete descriptions of some of the instruments mentioned here may be found by clicking on the illustrations on this page