Intrinsically safe communications circuit

An electrical circuit comprising a power supply, a load, a pair of parallel positive cables between the power supply and the load, a pair of parallel negative cables between the load and the power supply, and incendive arc prevention means,

• in which the incendive arc prevention means comprises monitoring means adapted to monitor the pair of positive cables and the pair of negative cables, and to detect if the current and/or voltage in one of the pair of positive cables deviates from the other, and if the current and/or voltage in one of the pair of negative cables deviates from the other,
• and in which the incendive arc prevention means comprises a control means adapted to fully or partially isolate the power supply from the pair of positive cables and the pair of negative cables if the monitoring means detects that the current and/or voltage of one of the pair of positive cables has deviated from the other, or if the current and/or voltage of one of the pair of negative cables has deviated from the other, characterised in which:
• the electrical circuit comprises a common positive section between the power supply and the pair of positive cables, and a common negative section between the pair of negative cables and the power supply, in which a first modulation means is provided in the common positive section or the common negative section, in which impedance means is provided between the power supply and the first modulation means, and in which the first modulation means is adapted to generate a modulated communications signal.