Electrical Safety - On the Secondary Side

If your electronic equipment is coupled to the mains via an isolating transformer, there is no protection from the mains earth leakage detector or RCD. Here is some useful information which might help to protect you from electrical shock.

by Lloyd Butler VK5BR

(Originally published in the June 1997 Newsletter of The Adelaide Hills Amateur Radio Society and reprinted in Amateur Radio January, 1998)


These days we are fortunate to have Residual Current Devices (RCDs) to provide protection from shock in the event of touching the 240V mains. These devices detect current unbalance between the two mains lines such as might be caused by human contact bypassing some of the current from one line leg to ground.

However, where equipment power circuits are coupled to the mains via a transformer, the RCD provides no protection against electrical shock from high voltage in the equipment itself. Most modem electronic circuitry is solid state, operating from low voltage supplies which present no danger. Working on such equipment tends to make one complacent concerning electrical shock and liable to overlook the possible additional presence of high voltage. However, much equipment still used by radio amateurs contains electron tubes operated from high DC voltage, so we should take care.

Many transceivers. although essentially solid state, use electron tube final RF power amplifiers. Such transceivers could utilise DC potentials in the order of 600V and AC potentials at the transformer secondary even higher. A typical high powered linear amplifier might operate from a DC supply of several thousand volts. Even earlier types of valve radios have their dangers with DC potentials in the order of 250 V and AC potentials of 300 V to 400 V. Not to forget that the cathode ray tube in the computer monitor can be supplied with some rather dangerous potentials.


So what precautions can we take? First of all don't put your fingers into the circuitry when it is turned on. If a test probe is to be inserted, hold the probe by its insulated end and keep the other hand away from the metal chassis. Of course, the whole idea is to prevent current passing through the main part of the body in the event of contact of high voltage by the one hand. In the days I worked on large transmitter racks we adopted the following procedure: stand on a rubber mat, only use one hand to carry out adjustment and keep the other hand well in the trouser pocket. Such transmitters were fitted with gate interlocks which turned off the high tension when any gate of the transmitter was opened. However, the gate switches often had to be bypassed to enable amplifier neutralisation and other active adjustments.

An obvious piece of advice is: if you have to work on the circuitry, make sure you turn off the power to it first. When equipment is connected by a power cord, I personally like to see the power plug removed from its socket before I put my fingers in. However, in spite of removing the power supply source, the equipment might still not be safe as there can be capacitors left charged up to a hazardous voltage.

A bleeder resistor connected across the high tension of a transmitter power supply to discharge capacitors is a good in-built safety measure and is, hopefully, incorporated in the equipment. However, you can never rely on a bleeder being fitted and after first turning off power, it is a wise procedure to short out the high tension line with the metal shaft of an insulated screwdriver. If the energy stored is large enough you might burn a hole in the screwdriver but better this than endangering yourself. Even the more docile 250 V which could be stored in the filter capacitors of a valve radio receiver can make quite a zap when discharged by short circuit. It can give you quite a jolt if you happen to get across the un-discharged DC line yourself.

Now, you may know that the equipment has a bleeder resistor installed or has other load circuitry which discharges the capacitors. Even so, don't be too anxious to get into the equipment. The time constant of the circuit capacitance and the bleeder resistance might be long enough to hold the voltage to a high level for quite some time before it decays to a safe level. The short circuit screwdriver is still a good precaution even if the bleeder is installed.

Check First

If you have the slightest doubt about whether a circuit is electrically dead, then first check it with a voltmeter. However, if you must touch it, make the first touch with the back of the hand rather than grab it with the palm of the hand (of course, the other hand and the rest of the body should also not be in contact with anything else not insulated.) The reason for this procedure is that, if you get a shock, the current flow through the body actuates muscles and if the active conductor is in the palm of the hand, it might clench up so that you can't let go and release yourself from the circuit. This happened to me once, so I can speak from experience - that's another story.

If you play around with the inside workings of electronics and certainly radio transmitters, you should be aware of the latent dangers within. Hopefully, my few words of wisdom will help you stay alive.

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