A metal-theft detection device comprising: a first sensing terminal configured to be electrically coupled with an earth wire of a mains electricity circuit; a second sensing terminal configured to be electrically coupled with a neutral wire of the mains electricity circuit; a third sensing terminal configured to be electrically coupled with a live wire of the mains electricity circuit; a wireless communications module; and circuitry (11-1, 12, 14, 15, 16, 17) configured: to measure a voltage between the second and third sensing terminals; to cause the wireless communications module to transmit a wireless message based on the measured voltage; to measure a resistance of a conductive path connected between the first and second sensing terminals; to cause the wireless communications module to transmit a wireless message based on the measured resistance; to detect whether the mains electricity circuit has become de-energised; and in response to detecting that the mains electricity circuit has become de-energised, either: to switch from causing transmission of wireless messages based on the measured voltage to causing transmission of wireless messages based on the measured resistance, or to initiate transmission of wireless messages based on the measured resistance. a first sensing terminal configured to be electrically coupled with an earth wire of a mains electricity circuit; a second sensing terminal configured to be electrically coupled with a neutral wire of the mains electricity circuit; a third sensing terminal configured to be electrically coupled with a live wire of the mains electricity circuit; a wireless communications module; and circuitry (11-1, 12, 14, 15, 16, 17) configured: to measure a voltage between the second and third sensing terminals; to cause the wireless communications module to transmit a wireless message based on the measured voltage; to measure a resistance of a conductive path connected between the first and second sensing terminals; to cause the wireless communications module to transmit a wireless message based on the measured resistance; to detect whether the mains electricity circuit has become de-energised; and in response to detecting that the mains electricity circuit has become de-energised, either: to switch from causing transmission of wireless messages based on the measured voltage to causing transmission of wireless messages based on the measured resistance, or to initiate transmission of wireless messages based on the measured resistance. to measure a voltage between the second and third sensing terminals; to cause the wireless communications module to transmit a wireless message based on the measured voltage; to measure a resistance of a conductive path connected between the first and second sensing terminals; to cause the wireless communications module to transmit a wireless message based on the measured resistance; to detect whether the mains electricity circuit has become de-energised; and in response to detecting that the mains electricity circuit has become de-energised, either: to switch from causing transmission of wireless messages based on the measured voltage to causing transmission of wireless messages based on the measured resistance, or to initiate transmission of wireless messages based on the measured resistance. to switch from causing transmission of wireless messages based on the measured voltage to causing transmission of wireless messages based on the measured resistance, or to initiate transmission of wireless messages based on the measured resistance.