IEEE C37 Switchgear Standards: A Working Map

How the C37 family is organized

C37 is the designation for the IEEE switchgear standards. The numbering is not strictly hierarchical, but the family falls into recognizable groups: assembly standards (the C37.20 series), circuit breaker standards (C37.04, C37.06 and C37.09 for high-voltage breakers, C37.13 for low-voltage power breakers), switch and fuse standards, relay standards (the C37.90 series), and the foundations C37.100 (definitions) and C37.2 (device function numbers).

One vocabulary point saves confusion: in C37 terminology, high-voltage means above 1,000 V, so the standards for 5 kV to 38 kV class breakers used in what the industry calls medium-voltage switchgear are the high-voltage breaker standards. Low-voltage means 1,000 V and below.

Assembly standards: the C37.20 series

The C37.20 series defines the metal-enclosed assemblies that house breakers and switches. The distinction between C37.20.2 and C37.20.3 is the one most often blurred: metal-clad is a defined construction, not a synonym for medium-voltage switchgear, and an interrupter switch and fuse lineup is metal-enclosed and should be specified to C37.20.3. C37.20.7 is a testing guide applied on top of an assembly standard, so arc-resistant switchgear is C37.20.2 or C37.20.3 gear additionally tested to C37.20.7 for a stated accessibility type.

• IEEE C37.20.1: metal-enclosed low-voltage power circuit breaker switchgear, the basis for UL 1558 listing
• IEEE C37.20.2: metal-clad switchgear, with drawout breakers, grounded metal barriers between compartments, insulated bus and automatic shutters
• IEEE C37.20.3: metal-enclosed interrupter switchgear, using load-interrupter switches with fuses or fixed breakers and fewer barriers
• IEEE C37.20.4: indoor AC switches (1 kV to 38 kV) for use in metal-enclosed switchgear
• IEEE C37.20.7: guide for testing switchgear for internal arcing faults, defining arc-resistant accessibility types
• IEEE C37.20.9: metal-enclosed switchgear incorporating gas insulating systems

Circuit breaker standards

For breakers above 1,000 V, three standards work as a set. IEEE C37.04 defines the rating structure: the quantities a breaker is rated for and how they are defined, including rated maximum voltage, continuous current, short-circuit current and interrupting time. IEEE C37.06 tabulates preferred ratings and related required capabilities, which is where a specifier finds the standard values for a 15 kV class, 1,200 A, 25 kA breaker. IEEE C37.09 defines the test procedures that demonstrate the ratings.

IEEE C37.13 covers low-voltage AC power circuit breakers used in enclosures, the LVPCBs installed in C37.20.1 switchgear, with UL 1066 as the listing counterpart. Application guides accompany several of these standards and are worth reading when a breaker is applied near its limits.

Relays, device numbers and definitions

The C37.90 series covers relays and relay systems associated with electric power apparatus: general requirements in C37.90 and the withstand tests in its companions, including surge withstand capability, radiated electromagnetic interference and electrostatic discharge. Specifying a relay to the C37.90 series is how a project establishes that it will survive the switchgear environment.

IEEE C37.2 defines the device function numbers used on one-line and schematic diagrams: 52 for a circuit breaker, 50/51 for instantaneous and time overcurrent, 27 for undervoltage, 86 for lockout, 87 for differential and so on. IEEE C37.100 defines the terms used across the family, which is where the formal difference between metal-clad and metal-enclosed, or between interrupting and withstand, is settled.

How the standards interlock

The documents reference one another deliberately. An assembly standard such as C37.20.2 requires the breakers within it to be rated and tested to C37.04, C37.06 and C37.09, and its own tests address the assembly (dielectric, temperature rise, momentary and short-time current, mechanical operation) rather than repeating the breaker tests. C37.20.7 assumes a compliant assembly and adds the internal arcing fault test. Relays in the low-voltage compartment are qualified to the C37.90 series, and the drawings that connect it all use C37.2 device numbers.

Third-party listing follows the same structure. For low-voltage switchgear, UL 1558 is the listing standard built on C37.20.1. For medium-voltage assemblies, listing where required is generally performed to the C37.20 series standards themselves.

Citing C37 standards in a specification

Cite by equipment type, and cite the set. A metal-clad specification names IEEE C37.20.2 for the assembly, IEEE C37.04, C37.06 and C37.09 for the breakers, IEEE C37.20.7 with the required accessibility type if arc-resistant construction is wanted, the C37.90 series for relays and C37.2 for device designations. A low-voltage switchgear specification names UL 1558 and IEEE C37.20.1 for the assembly and UL 1066 and IEEE C37.13 for the breakers. An interrupter switchgear specification names IEEE C37.20.3 and C37.20.4.

Avoid citing a standard that does not apply to the equipment, and keep edition control in a single reference clause or the project's standards register rather than in body text. Applicable listings, standards and design requirements depend on equipment type, configuration, project specifications and jurisdiction.