Condition monitoring is playing an increasingly important role within electrical power networks, where its use can help to reduce maintenance costs, improve supply reliability and permit increased utilization of equipment capacity by providing a measure of actual operating conditions as an alternative to relying on more stringent ‘worst case scenario’ assumptions. In this context, energy harvesting may have a role to play in that it offers the possibility of realizing autonomous, self-powering sensors that communicate their data wirelessly. In the vicinity of electrical transmission and distribution equipment, alternating magnetic fields at the power frequency offer a potential source of energy that does not require hard-wiring or batteries. There are many potentially useful locations for sensors where the level of magnetic flux density may be sufficient to provide enough power for a low-power wireless sensor node. This paper describes a ‘free-standing’ inductive harvester for use in positions where there is an ambient magnetic field due to conductors that are remote and/or inaccessible. Using data from surveys of magnetic flux density levels at two substations, optimum core and coil designs for the harvester were obtained through theoretical analysis and experiment. A demonstrator was then constructed in which a wireless sensor becomes self-powering when immersed in a 50 Hz magnetic field. Laboratory results show that this system can deliver a useful average power of 300 μW when placed in a magnetic flux density of 18 μTrms.