IEEE Std C37.20.1a:2020 pdf download.IEEE Standard for Metal-Enclosed Low-Voltage (1000 V ac and below, 3200 V dc and below) Power Circuit Breaker Switchgear Amendment 1: Control and Secondary Circuits and Devices, and All Wiring.
NOTE—The editing instructions contained in this amendment define how to merge the material contained therein into the existing base standard and its amendments to form the comprehensive standard.
The editing instructions are shown in bold italic. Four editing instructions are used: change, delete. insert, and replace. Change is used to make corrections in existing text or tables. The editing instruction specifies the location of the change and describes what is being changed by using strikethrough (to remove old material) and underscore (to add new material). Delete removes existing material. Insert adds new material without disturbing the existing material. Insertions may require renumbering. If so. renumbering instructions are given in the editing instruction. Replace is used to make changes in figures or equations by removing the existing figure or equation and replacing it with a new one. Editing instructions, change markings, and this NOTE will not be carried over into future editions because the changes will be incorporated into the base standard.
7. Construction
7.1 General requirements
Replace 7.1.3 through 7.1.3.1.5 as follows:
7.1.3 Control and secondary circuits and devices, and all wiring
7.1.3.1 General
All voltage circuits used for control, relaying, or metering shall be protected within the switchgear as follows:
a) All circuits supplied from external sources (ac or dc) shall have short-circuit protection. This may bc provided by a single set of short-circuit protective devices within the control source incoming section.
b) All circuits supplied from internal sources (ac or dc) circuits shall have short-circuit protection within the same section as the supply source. If these circuits arc supplied by a control power transformer, this protection may be in the primary circuit only.
c) Overcurrent protection of voltage circuits may be provided in addition to the required short-circuit protection.
d) Other circuits supplying loads, such as heaters, receptacles, and lights, shall have overload and short-circuit protection.
e) Circuits connected directly to the bus shall be grouped separately from the other components and wiring, and be kept to minimum length.
Overcurrent protection of current transformer secondary circuits shall not be provided. Open circuit protection should be considered where current transformer wiring connects to devices external to the switchgear assembly.
Primary and secondary protective devices may be omitted from voltage-dividing devices such as capacitive and resistive voltage dividers.
7.1.3.2 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.IEEE Std C37.20.1a pdf download.