Read the text. Distribution circuits generally consist of two parts: the primary circuit operating at a relatively high pressure or voltage that carries the electric supply

The primary circuit

Distribution circuits generally consist of two parts: the primary circuit operating at a relatively high pressure or voltage that carries the electric supply to the area here it is to be used. The secondary circuit receives the supply from the primary through transformers that reduce its pressure or voltage to values low enough to deliver the product safely to consumers Typical primary circuit voltages are 2400, 4160, 7620, 13800 and 23000 volts; secondary circuit voltages approximate 120 and 240 volts.

Should a fault occur on the circuit, a large current will flow to the fault that, if permitted to flow, will eventually burn the conductor apart, ‘clearing’ the fault, but with hazards to the public. Relays at the substation, however, will sense this large ‘fault’ current and operate to open the circuit breaker deenergizing the entire circuit leaving consumers without power until repairs are made. This is a basic circuit known as a ‘radial’ circuit.

To shorten the time of outage, the circuit may have a number of sectionalizing switches allowing parts to be connected to other unfaulted circuits, restoring service to some consumers. If the circuit is made into a loop with both ends connected to its source through one or two circuit breakers, the fault can be confined to the one section on which the fault occurred by opening the sectionalizing switches on both ends of the faulted section and closing the one or two breakers at the substation. In the closed loop type of circuit, the fault current flowing to the fault is divided into two parts, flowing in each side of the fault. This may sometime be too small to achieve positive operation of the relays to open the breaker or breakers. If the loop is deliberately opened at some point, the entire fault current will flow through the branch between the fault and the substation breaker, assuring its positive operation. The two ends of the circuit at the open point are connected together through a circuit breaker set to close automatically when one end is deenergized (or this may be accomplished manually with only a sectionalizing switch). All of the loop circuit will now be restored to service except the faulted section.

The sectionalizing switches may all be circuit breakers opening and closing automatically. Since circuit breakers are expensive compared to line switches, arrangements employing fewer breakers may be employed with a lessened degree of reliability for some consumers.