Medium Voltage Metal Enclosed Switchgear: Types, Ratings & Selection

Medium voltage metal enclosed switchgear is the backbone of modern power distribution systems, providing safe, reliable control and protection for electrical networks operating between 1 kV and 38 kV. Whether you are specifying equipment for a utility substation, industrial plant, or commercial facility, understanding how this switchgear is built, rated, and selected can directly impact system safety, uptime, and total cost of ownership.

The core function of medium voltage metal enclosed switchgear is to isolate, protect, and switch power circuits while keeping all live parts enclosed in a grounded metal housing — dramatically reducing arc flash exposure and enabling safer maintenance. Major standards governing this equipment include IEEE C37.20.2 (metal-clad switchgear) and IEEE C37.20.3 (metal-enclosed bus), along with IEC 62271-200 internationally.

Metal-Clad vs. Metal-Enclosed: A Critical Distinction

Many engineers use "metal enclosed" as a catch-all term, but IEEE standards draw a sharp line between two types:

Metal-Clad Switchgear (IEEE C37.20.2)

This is the highest-tier design. Key characteristics include:

• Draw-out (removable) circuit breakers with mechanical interlocks
• Separate, grounded metal barriers between main bus, breaker, and cable compartments
• Automatic shutters that cover bus stabs when the breaker is withdrawn
• Typically rated from 4.16 kV to 38 kV, with interrupting ratings up to 63 kA

Metal-Enclosed Interrupter Switchgear (IEEE C37.20.3)

A more economical design using stationary (non-draw-out) switches and fuses rather than vacuum or SF₆ breakers. It is common in load-interrupter applications such as ring-main units and pad-mount substations where fault interrupting duty is handled upstream. Typical ratings fall between 4.16 kV and 15 kV with interrupting ratings up to 40 kA symmetrical.

Key Electrical Ratings You Must Specify

Selecting the wrong rating is the most common — and most costly — specification error. Each nameplate value carries real engineering weight:

Voltage Rating

Standard maximum voltage ratings (kV) for North American equipment are 5, 8.25, 15, 27, and 38 kV, corresponding to system voltages of 4.16, 6.9, 12–13.8, 23, and 34.5 kV respectively. Always specify the maximum system voltage, not the nominal voltage.

Short-Circuit (Interrupting) Rating

This is the RMS symmetrical fault current the breaker can safely interrupt. Common values are 25 kA, 31.5 kA, 40 kA, and 50 kA. A 2022 EPRI study found that over 30% of industrial switchgear failures were linked to equipment operating beyond its rated interrupting capacity — undersizing this rating is a life-safety issue.

Continuous Current Rating

Typical bus and breaker continuous current ratings are 1200 A, 2000 A, and 3000 A. Apply a load growth margin of at least 20–25% above the calculated peak demand when specifying this value.

Withstand Ratings

• Momentary (peak) withstand: typically 2.6× the symmetrical interrupting rating (e.g., 65 kA peak for a 25 kA unit)
• Short-time withstand (1 s or 3 s): used for bus-bracing and thermal withstand verification
• Basic Impulse Level (BIL): for 15 kV class equipment, standard BIL is 95 kV; for 38 kV class it is 150 kV

Construction and Internal Compartmentalization

The metal enclosure does far more than contain the equipment — its internal layout directly determines arc flash incident energy and maintenance risk. A well-designed metal-clad unit uses a three-compartment architecture:

1. Bus compartment: houses the main copper or aluminum bus bars, isolated from all other compartments by grounded steel barriers
2. Breaker/switch compartment: contains the draw-out vacuum or SF₆ interrupter unit; shutters automatically close when breaker is withdrawn
3. Cable/load compartment: provides termination space for incoming and outgoing power cables, current transformers (CTs), and potential transformers (PTs)

Arc-resistant (AR) switchgear takes this further by designing the enclosure to safely vent arc flash energy away from personnel through pressure relief flaps or plenums, typically directed upward or to the rear. IEEE C37.20.7 defines four arc-resistant accessibility types (Type 1 through Type 2C). For locations where personnel must work in front of energized gear, specifying arc-resistant construction rated to Type 2B or 2C can reduce incident energy at the worker's position from potentially hundreds of cal/cm² to less than 8 cal/cm².

Vacuum vs. SF₆ Interrupting Technology

The two dominant interrupting technologies in medium voltage metal enclosed switchgear each have proven track records, but differ in application suitability:

For most new medium voltage metal enclosed switchgear projects below 38 kV, vacuum technology is now the preferred choice, driven by lower lifecycle maintenance costs, zero gas handling requirements, and tightening regulatory restrictions on SF₆ in several jurisdictions.

Protective Relaying and Metering Integration

Modern medium voltage metal enclosed switchgear is rarely just a mechanical switching device — it integrates digital protection and metering as standard. Key considerations include:

Relay Selection

• Overcurrent (51/50): the baseline protection function for feeder and bus applications
• Differential (87): applied on transformer or bus protection where fast, high-security tripping is required
• Distance (21): used on transmission tie feeders
• Multifunction numerical relays (e.g., SEL-751, GE D60) consolidate multiple functions into a single device, reducing panel wiring by up to 40% compared to electromechanical designs

CT and PT Sizing

Current transformers must be sized to avoid saturation during maximum fault conditions. A C200 or C400 accuracy class CT is typical for feeder protection at 15 kV. Undersized CTs that saturate during faults are a leading cause of relay misoperation. Always verify CT saturation curves using the relay manufacturer's guidelines or IEEE C57.13 criteria.

Installation, Clearance, and Environmental Requirements

Proper installation is as important as proper specification. Common installation requirements include:

• Front access clearance: minimum 1.0 m (39 in) for standard switchgear; arc-resistant designs with rear/top venting may require 1.5–2.0 m clearance at vent outlets
• Ambient temperature: standard ratings apply up to 40°C; derating is required above this, typically 1% per °C above 40°C for continuous current capacity
• Altitude: above 1000 m (3300 ft), dielectric and current ratings must be derated per IEC 60694 or IEEE standards
• Humidity and contamination: for outdoor or coastal environments, specify IP54 or higher enclosures and consider anti-condensation heaters to prevent tracking on insulating surfaces
• Seismic requirements: facilities in seismic zones must specify equipment tested to IEEE 693 or IBC requirements; many utilities require qualification to 0.5g or 1.0g PGA levels

Lifecycle Maintenance and Common Failure Modes

Medium voltage metal enclosed switchgear is designed for a service life of 30–40 years, but only if maintained properly. NETA MTS (Maintenance Testing Specifications) recommends routine intervals of 1–3 years depending on operating conditions. Key maintenance tasks and failure modes include:

Infrared thermography alone has been shown to reduce unplanned outages by up to 50% when performed annually, making it one of the highest-return maintenance investments for this class of equipment.

Specification Checklist for Procurement

When writing a specification or purchase order for medium voltage metal enclosed switchgear, ensure you have defined all of the following:

1. Maximum system voltage class (5, 8.25, 15, 27, or 38 kV)
2. Short-circuit interrupting rating (kA symmetrical)
3. Continuous current rating (bus and breaker, in amperes)
4. BIL (Basic Impulse Level) in kV
5. Applicable standard (IEEE C37.20.2, C37.20.3, or IEC 62271-200)
6. Arc-resistant construction type (if required per IEEE C37.20.7)
7. Interrupting technology (vacuum or SF₆)
8. Protective relay type and functions
9. Metering requirements (revenue-grade vs. indication-grade)
10. Environmental conditions (temperature, altitude, humidity, seismic zone)
11. Communication protocol (SCADA, IEC 61850, DNP3, Modbus)
12. Factory Acceptance Test (FAT) requirements