Electrical hazards

As with any electrical equipment, electrical power has to be supplied to make the equipment function. It would be preferable if this power could be provided from low power batteries, hence minimizing hazards associated with electrical power. However, in general, microwave equipment needs more power than can be provided from batteries. Some small ENG trucks can be powered from batteries topped up from the vehicle alternator when the vehicle is driving, but it is more usual to use a generator.

In particular, the demand for primary supply power for an SNG uplink and all the associated equipment is quite high, even for a small system. Typically, an SNG uplink requires at least 3 kW, and large SNG systems require at least 6 kW. This can only be derived from AC power, and therefore there are hazards in operating SNG equipment related not only to the power source itself, but also to the condition of the equipment.

Rugged extremes

It has to be borne in mind the type of rugged life the equipment leads, as well as the exposure to widely varying weather conditions, which over time can cause a deterioration in the electrical integrity of the equipment itself unless it is regularly checked by competent technicians. SNG uplinks can be operated in desert conditions, with temperatures reaching over 50 °C; in tropical conditions, with relative humidity over 90%; or in very cold conditions, with the temperature reaching −30 °C.

There is an added hazard in that the amplifying element typically used in most SNG HPAs – the travelling wave tube (TWT) – requires a very high voltage supply, which although generated internally in the HPA is nevertheless a potential hazard. Some of the monitoring equipment also have high voltage supplies, so all the equipment in a system is vulnerable.

Taking care

Regular care of the equipment, as well as respect in its use, is vital to minimize the risks. But care also has to be taken in where you get the supply of power. Generally, the uplink operator can either obtain power from a local supply, or more likely, transport a generator (usually petrol) along with the system. With a truck-based operation, using vehicles built for the purpose, this is relatively straightforward and very professional, and safe power systems are installed in these vehicles. They usually incorporate various safety features to ensure minimal risk to people and equipment.

With a flyaway operation, the situations are usually much more difficult. It is common to transport a system with its own generator, as one never knows if there is an adequate power supply at the destination – if there is one at all. Having arrived at the location, assuming the local supply is not reliable or safe enough (often the case in areas where flyaways are typically deployed), the generator has to be placed in a convenient position. Some thought also has to be given to re-fuelling arrangements (including the safe storage of fuel) as well as the safe routing of cable from the generator to the uplink equipment.

The residual current device (RCD)

Finally, it is worth considering the use of a residual current device (RCD) in the supply to the uplink equipment. An RCD is a safety device that monitors the power supply constantly checking for a fault – we discussed these earlier. It does not act as an over-current circuit breaker or fuse, which protects the circuit from drawing too much current beyond the capacity of the cable and the equipment, but is an additional safety device.

The advantage of the RCD is that it does not require a good earth connection, or any earth connection at all for that matter, to operate. It will protect people even with the most basic electrical supply being available, and is compact, cheap and easy to use.

Residual current device (RCD)