Safety precautions in the amateur station

Introduction

Probably the most important safety precaution in the amateur station arises from the use of high voltages. While much amateur equipment uses transistors operating from a 13.5V supply, many high-power HF, VHF and UHF linear amplifiers using valves operating at voltages over 2000V are in use.

It is not possible to define a voltage which is 'safe to touch' - no voltage source should ever be touched deliberately. The effects of an electric shock of a particular voltage depend on so many factors, from the source of the voltage to the state of health of the unfortunate person involved.

It is therefore best to consider any voltage above 50V as dangerous and to act accordingly. Probably the three most important precautions are:

1. Ensure that all equipment is satisfactorily earthed - the integrity of the earthing system should be checked periodically.
2. Switch off and disconnect from the mains supply before attempting the investigation or repair of any equipment.
3. Capacitors of high value, particularly if the dielectric is paper, will hold their charge for long periods (days/ weeks). Bleed resistor chains should therefore be connected across high-voltage smoothing capacitors in order to dissipate this charge in a few seconds. Bleed resistor chains should be conservatively rated as regards dissipation and ideally should consist of two separate chains in parallel. In spite of the presence of bleed resistor chains, high-voltage smoothing capacitors should be shorted as recommended in paragraph 5 of the RSGB Safety Recommendations before any servicing of a power supply is carried out.

Using a low-voltage but high-current power supply must not be allowed to lull one into a state of false security. The input voltage is 240V. and this can be fatal under some circumstances. The high current capability may be sufficient to generate molten metal if the supply is accidentally shorted to earth by, say, a ring on one's finger.

The electromagnetic compatibility aspects of the protective multiple earthing system of house wiring are dealt with in Chapter 9. The installation of PME systems started in the mid 'seventies. It would seem likely therefore that the majority of houses in UK do not use PME. Before basing any grounding or safety precautions in your station on PME, it is essential to check with your local electricity board whether or not your house wiring uses PME.

The 'Safety Recommendations for the Amateur Radio Station' and 'Mobile Safety Recommendations' which follow should be studied carefully.

Photo 12.1. A typically busy and crowded radio amateurs 'shack'