That's not the way to do it!

Keith Wilson - Electrical engineer

In the earliest days of electricity, long before measuring instruments of any kind had been invented, Henry Cavendish, the eccentric English physicist for whom the world-famous Cavendish Laboratory at Cambridge University is named, subjected himself to electric shocks as a way of judging the strength of electric currents. His measuring technique was simple: the more severe the shock, the higher the current.

The accuracy of this method is rather questionable and, in addition, it’s unlikely that it would now be condoned by any health and safety body anywhere in the world! This is, of course, an extreme example but measuring techniques that would today be considered distinctly questionable were in everyday use at a much later stage in the development of the electricity supply system.

A little booklet produced by Megger in 1943 provides an excellent example. Entitled “Handbook on Continuity and Polarity Testing” this was produced to help electricians to test electrical installations in line with the 11th Edition of the IEE Wiring Regulations. It contains instructions for carrying out tests on both live and dead circuits using a purpose-designed instrument that was, in effect, a low-range ohmmeter.

In 1943, this instrument was necessarily an analogue electromechanical type and it seems unlikely that it would have been possible to adequately protect it against damage if it was accidentally connected to a live mains supply. So how could tests be carried out safely on live circuits?

The answer was to use a cunning device described as the “Evershed Detector Spike”. For live circuit testing, this was used as one of the test probes and was effectively in series with the instrument. Internally, the spike contained a high value resistance connected in series with a small neon bulb. 

For anyone too young to remember, a neon bulb contains two electrodes a few millimetres apart, and is filled with neon gas at low pressure. When a voltage of more than about 100 volts is applied across the electrodes a discharge is set up in the gas, which produces a characteristic orange glow. 

Only a tiny current is needed to establish and maintain the discharge but, because the resistance of the bulb is low once the discharge has been established, a high value series resistor is used in almost all applications.

The Megger booklet explains that to carry out live circuit continuity tests, one side of the instrument should be connected to the installation main earth, and the other side connected – via the Evershed Detector Spike – to the various points around the installation where it was required to verify earth continuity. The diagram, taken from the original publication, should make this clear.

If the Spike was applied to any part of the circuit that was live, the neon would glow and the booklet contains strict instructions for the test to be terminated at this point. If, however, there was no glow from the neon, the instrument user pushed on the probe. This closed an internal spring-loaded switch that short-circuited the neon and its series resistor, allowing the resistance of the earth conductor to be measured to verify continuity.

All well and good unless the user accidentally pushed on the probe when the neon was lit, when a damaged instrument would be the likely result. Unfortunately, this was made even more likely by the procedure recommended for checking supply polarity on live circuits, where the probe was to be used – without pushing – to check that the live connection of the socket or switch was, in fact, connected to the live side of the supply.

How easy it must have been to forget, especially after a long hard day – do I push on the spike for this test or not? The Evershed Detector Spike was undoubtedly an ingenious and inexpensive solution to a very real problem but it was hardly foolproof, so perhaps we should be grateful that today’s installation test sets shrug off accidental connection to live circuits without so much as hiccup.