Description and Instructions for EVERSHED'S PATENT MEGGER [1904 pattern]

[The following has been reproduced from a copy of E&V List No.2 dated December 1904 provided by C M Deavin who gave me copies of a number of original Evershed and Vignoles documentation provided by Marion Heard, the publicity assistant at Megger back in 2003.]

EVERSHED & VIGNOLES, Ltd.

EVERSHED'S PATENT MEGGER [1904 pattern]

DESCRIPTION

Fifteen years ago the Evershed Testing Set was invented to enable insulation to be tested at or above, working voltage.At that time the inventor was almost alone in realising the necessity for such tests. Today every electrical man believes in testing the insulation at full working pressure and many thousands of Evershed's Testing sets are in daily use all over the world, testing insulation resistance at a pressure at lest equal to the working voltage. Such tests are regarded as essential to economic installation and as the best insurance against breakdown, while Government Departments insist upon them as essential to public safety.

The Megger is the latest form of the Evershed Testing Set. It is a radical departure from the old familiar two box patterns, and, in addition to the combination of the Ohmmeter and Generator in one box, the Megger embodies a number of novel features greatly enhancing the utility, scope, and general convenience of the apparatus.

Externally, and so far as the user is concerned, the Megger is simply a substantial teak box with a window in the top through which the ohmmeter dial is seen. The generator winch handle is at one end of the box. On the side of the box are two terminals clearly marked LINE and EARTH. To test the insulation of a circuit it is only necessary to set the Megger down on a fairly level base, connect the circuit wires to the line and earth, terminals and give the winch handle half-a-dozen rapid turns. The Ohmmeter is dead beat and the index promptly comes to rest and points to the insulation resistance in four or five seconds from the start. That is all the user has to do. There: are no other connections to be made. There is no levelling to be done except in high-range Meggers. There are no switches, no plugs, no adjustments, no key to be tapped, no galvanometer to watch, no rheostat to adjust. There is no multiplying or dividing by ten or by a hundred: the scale is always read direct without any calculation whatever, and the Ohmmeter index points to where the value of the insulation resistance is plainly written in so many hundred thousand ohms or so many megohms.

A strong leather handle is provided for carrying the Megger; it is made detachable at one end -to enable it to be thrown hack clear of the dial. When not in use the generator handle is folded back into a recess in the case, and the window is covered by a stout metal lid.

THE GENERATOR
Inside the Megger box are a hand dynamo, or generator, and an Ohmmeter. The dynamo needs no description; it is the machine we have supplied with our Testing Sets over the past few years and is as perfect as human ingenuity and good workmanship call make it.

THE CONSTANT VOLTAGE GENERATOR
A novel feature has been introduced enabling an absolutely steady and constant voltage to be obtained from the dynamo independently of the speed or rotation. The constant voltage generator is well adapted for cable testing where, owing to large electrostatic capacity, a perfectly constant electromotive force is essential. It should always be employed if the circuits to be tested have a capacity exceeding half a microfarad, or say one mile of sheathed rubber cable. For smaller capacities our ordinary generator may be used. All high-range Meggers are fitted with the constant-voltage generator.

THE MOVING COIL OHMMETER
The Ohmmeter is entirely novel. It is a dead-beat, moving-coil instrument with a current-coil moving in a powerful magnetic field, and an astatic pressure coil controlled by the same field. As compared with our older types of Ohmmeter the new instrument has many striking advantages. The working forces are enormously greater and the range or the instrument reaches 2,000 megohms, as against a maximum of 50 megohms in our old patterns. The movement is absolutely dead-beat. The indications are entirely unaffected by stray magnetic fields. The coils arc carried upon an axle fitted at each end with a large and' very strong pivot working in a sapphire bearing yet notwithstanding the size of these bearings the movement is without a trace of friction. And lastly, a greatly improved scale enables the readings to be extended down to zero without the use of a shunt and its attendant switch. The new scale, being approximately logarithmic, can be read to a nearly equal degree of accuracy throughout the greater part of the range.

GUARDED INSULATION
The Ohmmeter is fitted throughout with a novel system of guarded insulators which entirely prevents the possibility of any leakage, either in the Ohmmeter or across the insulators of the line and earth terminals, affecting the accuracy of the instrument. It being common practice to use a guard wire to eliminate surface leakage at the exposed ends or a cable under test, we fit all high range Meggers with a third, or guard, terminal to which the guard wire should be attached.

These were very expensive instruments. To put this in perspective, in 1900 male teachers earned around � 150-� 200 per annum and a three bedroom terrace house would have cost between � 200 and � 300.

The following are the instructions for use inserted in the lid of the Megger:-

INSTRUCTIONS.

POSITION---Stand the Megger on a firm and fairly level base and hold it firmly. Turn the winch handle smoothly and steadily in a clockwise or right handed direction. The Generator when running at 100 revolutions of the handle per minute applies the rated voltage to the insulation under test.

TO TEST INSULATION between a circuit and earth, connect the LINE terminal to the circuit, and the EARTH terminal to a good earth. For insulation between two wires, connect one wire to each terminal.

PRECAUTION.---'The Megger is accurately astatic, and stray magnetic fields no matter how strong, do not cause errors in the scale reading. But a powerful magnetic field may partially demagnetise the generator field magnets and cause the machine to give less than its rated voltage. To guard against this, do not use or carry the Megger within three feet of any pole piece or field coil of a dynamo at work.

REVIEW

Reproduced from a copy of The Electrical Review December 9th 1904 [figures 1&2 omitted] also provided by C M Deavin.

THE "MEGGER"

We recently inspected Evershed's Patent Megger, the new self contained ohmmeter and Generator for testing insulation resistance which has been put on the market this month by Messrs, Evershed and Vignoles, Lt.d.,of Acton. Lane Works, Chiswick. This instrument, which has been in the course at evolution for the last four years, is a natural development from the well known testing set of world-wide reputation, and must be admitted to be a credit to it's parentage, for it is the most ingenious and perfect practical testing instrument that we have seen.

The apparatus is entirely contained in one box; the hand dynamo, which is very easy-running, gives pressures up to 1,000 volts, and the ohmmeter reads up to no less than 2,000 megohms.

The scale is direct reading and approximately logarithmic having , nearly uniform accuracy over a very wide range, so that it has been possible to dispense with the use of shunts and the second scale for low resistances; the dial is covered with a hinged lid, inside which are the printed directions for use. The movement is perfectly dead-beat and astatic, so that the reading is obtained directly the handle at the generator is turned at full speed-that is, in a few seconds at most and is unaffected by stray fields.

There are only two terminals, respectively marked LINE and EARTH, and the former is provided with a guard ring so that internal leakage is entirely harmless, the instrument does not need levelling, except in the highest ranges. A leather handle is provided for carrying the instrument, which weighs in all 18 lbs; the handle is detachable by means of a novel patent latch at "one end, to expose the dial. The generator handle folds back when not In use, 'the dimensions of the box are 61/4 X 61/4 X 12 in.

The dynamo is of the same type as of old, and is noteworthy for its tiny roller bearings and the ingenious mode of current collection, the commutator taking the shape of divided disks, running contact with pivoted spring disks, with the minimum of friction and the maximum of reliability and durability, The EMF, in the high-range , instruments bas to be exceedingly uniform, on account of the presence of capacity in the cables tested; although fluctuations of voltage have no effect on pure resistance tests, the surging set up thereby when capacity is present would vitiate the readings, and to avoid this defect the generator in this case has eight coils, and is provided with an absurdly simple constant speed device, consisting of a centrifugally operated clutch. So effectual is this, that we can vouch for the fact that, after the normal speed of rotation of the handle is attained the voltage remains constant, no matter how much faster the handle is turned. This is a most striking and unexpected feature of the apparatus, and renders it applicable to the testing of cables of large capacity. The high range instruments are also provided with a terminal, to which the guard wire, essential to accuracy in cable testing, can be attached, this is, we believe, the only self' contained instrument in existence which enables one to test the insulation resistance of a few hundred yards of cable at pressures up to 1,000 volts.

So far we heave not referred to the fundamental novelty in the construction of the ohmmeter namely, the fact that the movement is of the moving coil type, To this must be ascribed the enormous increase in the sensibility and aperiodicity at the instrument.

The permanent magnet system is common to both the generator and the ohmmeter. The movement consists a current coil, passing twice through the field in the annular space between the pole-pieces and the cylindrical core, and a pressure coil, of which one leg 1s axial with regard to the movement, and the other leg moves in the field, To the latter is attached a little compensating coil in series with the pressure coil, which ensures immunity from interference due to stray fields. The coils are wound on copper formers, which give the dead beat quality, and carried on steel pivots with strong spherical ends working in sapphire hearings, Thanks to the vibration due to the gearing of the generator, frictional errors are entirely eliminated, whilst the pivots are so strong that there is absolutely no risk of breaking them, The currents are led into the movement by means of patent phosphor-bronze strips a fraction of a mil in thickness, which exercise a control less than 1/400 of that of the pressure coil, and are so guided by means of light bobbins that they cannot become entangled or displaced, the movement is balanced, and it is only in the instruments of the highest range that levelling screw have to be provided.

The internal mechanism requires no attention, so the cover of the box is sealed dust-tight (with a dust-proof washer on the spindle which carries the handle) and is screwed down.

We may note that the principle of the ohmmeter is identical with that of the older types; the only essential difference Is that it has been turned inside out---the magnetic needle, so to speak, is fixed and the coils are free to move. But this change has resulted in an immense improvement. The foregoing is but a brief description, but may serve to indicate the remarkable progress that bas: been made by Messrs Evershed & Vignoles who have not been content " to let well alone," but have laboured incessantly to surpass their own high standard of excellence.