TP 60 - 127/100/220/36/IB power transformer is widely used to power operative circuits, remote control circuits, protection (including intrinsically safe systems), and automatics. TP 60 transformer has secondary winding with an intrinsically safe voltage of 18V. It is located on the same coil as the other windings and separated from power winding with a 0.1mm thick copper grounded shield. Intrinsic safety is provided via a barrier, which consists of 4 series-connected resistors with a total resistance of 400Ω, as well as three pairs of opposite connected 1SNB5931BT3 Zener diodes (maximum Zener current of 83mA, rated voltage of 18V). Control cables connected to intrinsically safe circuits must be no longer than 2.5km and possess resistance of less than 47Ω.