“SafeLink”™ is a complete radio link concept which, by means of repetitive self-check signals and secure information coding, can alert the carer not only to the alarm but also to the loss or degradation of the radio signal. Most importantly it warns of interference from other systems using the same frequency range (e.g. door-bells, car locks, burglar alarms). As many as 15 systems can be used in the same area without interfering with each other. The radio signal will penetrate wallsso that the system issuitable for linking bedrooms, between floors or even between adjacent houses, up to a clear sight range of 450 metres.