IEEE Std C37.20.2a-2020 pdf download – IEEE Standard for Metal-Clad Switchgear Amendment 1: Control and Secondary Circuits and Devices, and All Wiring.
7.3.2 Voltage transformer fusing The following requirements shall be met: a) Primary circuits of all voltage transformers shall include current-limiting fuses. b) Secondary circuits of all voltage transformers shall include fuses or their equivalent. EXCEPTION: Fuses may be omitted from secondary circuits of voltage transformers if the secondary burden includes voltage regulators, protective relays, or other devices considered sufficiently essential to the operation of the installation to make it preferable to incur the hazards associated with the possible destruction of the voltage transformer by a sustained secondary short-circuit rather than to risk interruption of the voltage supply to such devices as a result of a momentary secondary short-circuit. 7.3.3 Control, secondary, and logic-level wiring Flame-retardant, 600 V insulated stranded copper wire shall be used for internal wiring between components of switchgear assemblies, and to terminals for connection to external controls, metering, or instrumentation. Wiring within components is assumed to be covered by standards applicable to those devices and is not covered by this standard. Wiring for the purpose of conveying power to external switchgear loads is not covered by this clause. The switchgear manufacturer is responsible for the performance of the wiring system provided by the manufacturer within the switchgear. This applies to the integrity of internally generated signals in the control wiring, and may require the use of special precautions such as shielded wire, twisted-pair wire, or segregation of certain wires.
7.3.3.4 Wire type Wiring shall be rated for 600 V, 90 °C, be flame-retardant, and shall meet the requirements of NEMA WC70/ICEA S-95-658. Preferred wire is type SIS as listed in NFPA 70 (NEC®) or an equivalent such as XHHW, RFHH-2, RFHH-3, or the like. Other wires which meet the requirements of this clause are also acceptable. Wiring used for connection directly on circuits above 600 V shall be rated for the voltage involved (or higher), 90 °C, and shall be flame-retardant. In addition, wiring used for dc circuits for rated voltages of 48 Vdc or above shall not contain PVC insulation. EXCEPTION: In addition, low-energy signal wiring may be used as required by the manufacturer of a specific component. This exception includes wiring such as the following:   Resistance temperature detectors (RTDs)  Low-energy signal communication cables (e.g., RS232, RS485, Ethernet, fiber optic, and similar networking cables)