and fuses disconnect when current exceeds a predetermined safe level. However they cannot sense other critical faults, such as unbalanced currents—for example, when a transformer winding contacts ground. By themselves, circuit breakers and fuses cannot distinguish between short circuits and high levels of electrical demand. Merz-Price circulating current scheme Differential protection depends upon Kirchhoff's current law, which states that the sum of currents entering or leaving a circuit node must equal zero. Using this principle to implement differential protection, any section of a conductive path may be considered a node. The conductive path could be a transmission line, a winding of a transformer, a winding in a motor, or a winding in the stator of an alternator. This form of protection works best when both ends of the conductive path are physically close to each other. This scheme was invented in Great Britain by Charles Hesterman Merz and Bernard Price. Two identical current transformers are used for each winding of a transformer, stator, or other device. The current transformers are placed around opposite ends of a winding. The current through both ends should be identical. A protective relay detects any imbalance in currents, and trips circuit breakers to isolate the device. In the case of a transformer, the circuit breakers on both the primary and secondary would open. Distance relays A short circuit at the end of a long transmission line appears similar to a normal load, because the impedance of the transmission line limits the fault current. A distance relay detects a fault by comparing the voltage and current on the transmission line. A large current along with a voltage drop indicates a fault. Classification Several different classifications of switchgear can be made: By the current rating. By interrupting rating (maximum short circuit current kAIC that the device can safely interrupt) Circuit breakers can open and close on fault currents Load-break/Load-make switches can switch normal system load currents Isolators are off load disconnectors which are to be operated after Circuit Breakers, or else if the load current is very small By voltage class: Low voltage (less than 1 kV AC) Medium voltage (1 kV AC through to approximately 75 kV AC) High voltage (75 kV to about 230 kV AC) Extra high voltage, ultra high voltage (more than 230 kV) By insulating medium: Air Gas (SF6 or mixtures) Oil Vacuum Carbon dioxide (CO2) By construction type: Indoor (further classified