Section 17 Circuit Breakers,Reclosers, Switches, and Fuses 170. ARRANGEMENT This rule requires circuit breakers, reclosers, switches, and fuses be accessible only to qualified persons. Section 17 is part of Part 1, Electric Supply Stations ; therefore, the rules of this section apply only to circuit breakers, reclosers, switches, and fuses located in the electric supply substation. Circuit breakers, reclosers, switches, and fuses in the electric supply station are accessible only to qualified persons when Rule 110A is met. To protect persons from energized parts or arcing, this rule requires the following: Walls Barriers Latched doors Location Isolation Other means The requirements of Rule 124, guarding of live parts, also applies. Conspicuous and unique markings must be provided at the switching device or at any remote operating points to identify the equipment (e.g., circuit breaker, recloser, switch) that is being controlled. When the switch contacts are not normally visible (e.g., under oil, contained in a vacuum bottle, etc.), the switching device must be equipped with an operating 101
102 ELECTRIC SUPPLY STATIONS Rule 171 position indicator. The work rules in Part 4 provide switching control procedures and rules for de-energizing equipment or lines to protect employees. An example of a switch position indicator is shown in Fig. 170-1. 171. APPLICATION The following ratings must be considered when applying a circuit breaker, circuit switcher, recloser, switch, or fuse: Voltage Continuous current See Photo Fig. 170-1. Example of a switch position indicator (Rule 170-1).
Rule 172 CIRCUIT BREAKERS, RECLOSERS, SWITCHES, AND FUSES 103 Momentary current Short-circuit current interrupt rating If the device is used to interrupt fault current, the maximum short-circuit current interrupt rating must be considered. When to apply ratings for momentary currents and interrupt currents is dependent on how the switch is used. If a switch is used as a disconnect only (i.e., it does not open under a fault condition), then it must be able to withstand a fault current flowing through it but it does not need to be rated to interrupt the fault current. The fault current it must withstand is termed the momentary fault current rating. If a switch is used to interrupt a fault, it must be rated to withstand the momentary fault current and interrupt the fault current without damage to the switch itself. A device that interrupts fault current must have a fault current interrupt rating. The interrupting capacity should be reviewed prior to each significant system change. For example, if a substation recloser is rated to interrupt 1250 A of short-circuit current and the substation transformer is replaced, the available fault current may increase to a value larger than 1250 A, which could cause damage to the recloser and personnel when the recloser operates to interrupt a fault. An example is given in Fig. 171-1. 172. CIRCUIT BREAKERS, RECLOSERS, AND SWITCHES CONTAINING OIL Circuit breakers, reclosers, and switches containing oil receive special attention due to the flammability of the oil. Similar requirements are outlined in Rule 152 for oil-filled power transformers and regulators. Fig. 171-1. Example of checking interrupting capacity prior to a significant system change (Rule 171).
104 ELECTRIC SUPPLY STATIONS Rule 173 Segregation of oil-filled circuit-interrupting devices is required in the electric supply station. Segregation of oil-filled equipment minimizes fire damage to adjacent equipment or buildings. Segregation can be provided by the following methods: Spacing Fire-resistant barrier walls Metal cubicles Gas release vents are required, and means to control oil discharges from vents or tank rupture are required. Methods to contain oil are outlined below: Absorption beds Pits Drains Combination of the above This list is slightly different from the requirements for oil-filled transformers and regulators in Rule 152 but the general intent is the same. IEEE Standard 979, IEEE Guide for Substation Fire Protection, is an excellent reference for fire protection requirements. Buildings or rooms housing circuit breakers, reclosers, and switches containing flammable oil must be of fire-resistant construction. Not all circuit breakers, reclosers, and switches contain oil. Some are air break, some are vacuum break, and some are SF 6 gas insulated. This rule applies only to circuit breakers, reclosers, and switches that contain oil and are located inside the substation fence. 173. SWITCHES AND DISCONNECTING DEVICES Switches and disconnecting devices must have capacity for the following system ratings: Voltage Current Load break current (if required) Switches can be used to break load currents or open under no-load conditions. If required to break load current, the load current they are rated to interrupt must be marked on the switch. This value should not be confused with the short-circuit (fault) current-interrupt rating discussed in Rule 171. Switches and disconnectors must be able to be locked open and locked closed, or plainly tagged where locks are not practical. Part 4 of the NESC specifies the work rules applicable to operating, locking, and tagging switches. Switches that are operated remotely and automatically must have a disconnecting means for the control circuit near the disconnecting apparatus to limit the likelihood of accidental operation of the switch. Rule 173C has been deleted by the Tentative Interim Amendment (TIA) located at the end of the NESC.
Rule 174 CIRCUIT BREAKERS, RECLOSERS, SWITCHES, AND FUSES 105 174. DISCONNECTION OF FUSES Disconnecting an energized fuse can be dangerous at any voltage. This rule requires fuses in circuits of more than 150 V to ground or more than 60 A (at any voltage) to be classified as disconnecting, or arranged to be disconnected from the source of power, or removed with insulating handles. In most cases, fused cutout-type switches are configured such that the switchblade or fuse is dead when the switch is in the open position. Loop feeds can create energized blades when the switch is in the open position. The NESC does not have a requirement for how the switch blades or fuses are energized but does require the proper handling of the fuse. Proper consideration must be given to clearance requirements when fuse and switch blades are in the open position.