The South and Mid Wales CRT have used single wire earth return or twin wire hand held sets for years (49 in my experience). There are many designs available on sites like CREG. They are simple cheap and ideal for semi skilled home construction. The only operating downside of single and twin wire is that both ends have to be manned although some designs have a simple buzzer alert. They are almost bomb proof as long as the single wire is in good condition and the operators remember to ground the sets using gloveless hands (yes it has happened more than once). They will work over kilometers and have no problems with mineralization or radio interference. The only practical downside we ever found was relying on fixed lines. Unless the wire is high quality, corrosion resist and regularly tested they will always let you down. There are twin wire designs which have the advantage of duplex speak (no press to talk) and having the power supplied from the base station (single wire normally have a small 9v cell in each set)
Big advantage of wired sets is that rescuers who are not familiar with the system can follow the wire
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Might be worth approaching the UK teams via the British Cave Rescue Council to see if there are any redundant single sets out there.
Jopo
