Field hospital

The term mobile installations refers to the wiring and equipment found in cars or temporary constructions. Even private cars contain considerable amount of wiring and electronic devices, from main user-interface computer to a number of chips in the engine to the simple window-opening mechanism these days.

Special vehicles like ambulances, firefighters’ truck, reporters’ cars and similar contain extra appliances appropriate to their role. Other mobile installations include any kind of temporary construction, from beer tents at festivals to field hospitals.

Vehicles can be constructed with the whole installation from scratch or adapted from an existing general-purpose design. The adaptations are often inadequate in regards to electrical safety, as functionality is the first priority. Any adapted vehicle has to be thoroughly tested on commissioning and, ideally, after every deployment. The user has to be particularly aware of its limitations.

Mobile installation can be powered by either batteries or a generator. Batteries deliver power immediately, but don’t last indefinitely, and only offer a limited DC voltage. A standard car battery, for example, provides output of 12 VDC. Conversion to 230 VAC is, therefore, essential. For special purpose vehicles it can be already built-in, and that is generally the best option. It is then chosen with a specific use in mind and creates the necessarily clean sine signal with the intended power. If not available in the vehicle, the units in the market vary from cheap consumer pieces that only chop the voltage and cause high harmonic content to high-quality powerful units with perfect sine output. Price and performance have to be considered for the application at hand.

Generators usually cannot be started at once, but are more powerful, can work until they run out of fuel, and produce AC voltage at the designed magnitude. They inherently pose more danger to the user with their higher voltage output. They are often designed for specific purpose, like powering a single consumer, and straying from that purpose can be dangerous. In special purpose vehicles, they can be built-in and likely to be adapted to their expected use. For temporary installations, they have to be carefully chosen to be able to keep up with needs.

Regardless of the power supply, it is possible to use the mobile installation when isolated from ground, or create a temporary earthing. The latter should only be attempted if the ground is suitable for an easy stake and its specific resistance low enough. This is a relatively rare occurrence when in the field. The earth has to be measured in advance (specific resistance) or once the stake is set (earth electrode resistance), or preferably both, to establish safety of the system.
If working isolated from ground, there might be higher interference between devices. Insulation and contact voltages become the main safety considerations. It has to be taken into account that a lot of temporary installations are set up outside and exposed to the elements. Moisture, UV, vibrations and other exposure can lower insulation resistance and leads to faster deterioration.

Any temporary installation has to be protected by overcurrent devices and by RCD or PRCD switches. Battery-operated DC installations can work with only overcurrent devices l. In case of a single fault, the supply should be automatically cut off.