Evershed & Vignoles Series 2 Bridge Megger

Megger/Varley/ Bridge switch on left and ratio switch on right

Meter movement dated October 1955

Evershed & Vignoles Megger series 2. This is a very common and robust instrument housed in a cast metal housing unlike the earlier series 1 range of instruments.  It can be used as a conventional insulaton tester, a conventional resistance bridge and for the "Varley" method of line test. This simplified diagram shows how this is accomplished. In common with other Megger models an RC snubber is connected across the generator terminals. In this model, and I suspect others, this comprises a 4700 ohm resistor in series with a 0.1mFD capacitor.

This instrument is one of a number given to me by Martin Harrison which belonged to his father who was a a Chartered Electrical Engineer. He collected both old electrical test equipment and due to his name (John Harrison) old clocks. Martin thought his father would have been pleased with his items been given a new home and owner.

I found the instrument to be in working order and accurate when tested against a range of standard resistors on both the Bridge and Megger modes of operation. The open circuit output on the Megger switch position was 488 volts and on the Bridge position in the region of 20 to 30 Volts.

A lighter, more modern version (not in my collection) dating from the 1970s is shown below:

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 generator connected to a specially-designed meter. GB patent number 400728 was granted in 1933.

Rob Ford, a retired engineer joined BT in 1968 as an apprentice and worked on overhead lines and subscribers apparatus, Strowger exchanges and TXE2 electronic exchanges came upon this page and forwarded his reminiscences:

"By the time I joined the hand cranked meggers had been phased out. I remember some installers preferred the field telephone with the hand cranked ringing generator as when the handle became stiff the other engineer had picked up their receiver to answer. I knew quite a few engineers who were unable to use an AVO meter and used their field telephone to feel for low resistance lines.

I actually prefer my vintage analogue meters as opposed to digital meters as they don't read spurious signals, though my latest digital meter does have a low Z function. Spurious readings often used to confuse competent electricians. Once in one of my exchanges an electrician who wanted to recover a redundant cable was measuring 180v on a non terminated cable and couldn’t understand why my AVO read zero volts.

I never bothered with a field telephone on my kit, I used an operators headset with the earpiece and microphone wired in series. I could easily tap out a number without a dial. I could monitor by connecting one wire to a block terminal and the other terminal with my damp finger in series. The last time I worked on overhead lines was as a result the two Hurricanes after Michael Fish got the weather forecast wrong. I was on exchange maintenance then but I volunteered for a month of weekends."