Summary Highlights
- Separate roles of the bus, breaker, measurement and control circuits
- Primary and secondary connections in connected, test and disconnected states
- IEEE construction versus IEC LSC, PM/PI and IAC classifications
- Withstand, interruption and internal-arc ratings with their conditions
- Technical documents needed for selection, retrofit and maintenance scope
Article Details
Metal-clad switchgear is a medium-voltage assembly that arranges switching, protection and measurement equipment within grounded metal compartments. Opening a circuit breaker and separating that breaker from its primary connections are different functions. The construction combines those electrical functions with partitions, a drawout arrangement, shutters and interlocks.
The applicable standard and the exact product matter. A metal-clad label alone does not establish an internal-arc rating, outage-free maintenance or a particular IEC classification. This article explains the construction and the documents needed to assess it; it is not a switching or maintenance instruction for an installed cubicle.
What do metal-clad and metal-enclosed describe?
Metal-enclosed is a broad description of switchgear within a metal enclosure, and it also appears in the names of specific standard categories. Metal-clad describes a more particular construction within that broad description. The public scope of IEEE C37.20.2-2025 explicitly covers drawout electrically operated circuit breakers and separation of major components by grounded metal barriers. The public scope is not a substitute for the complete design and test requirements.
Not every metal-enclosed assembly qualifies as metal-clad. Equally, metal-enclosed does not invariably mean fixed breakers or unsegregated bare components. Compact dimensions, breaker withdrawal and compartment arrangement are separate design choices. Compare the actual barriers, primary interfaces and access provisions rather than ranking products by an unsupported “more advanced” label.
The newer metal-enclosed examples in IEM’s comparison presentation include drawout-breaker and insulated-bus options. This historical manufacturer presentation is neither a standard nor proof of universal product superiority; it demonstrates why a simple fixed-versus-drawout comparison is inadequate.
The power path and the role of each compartment
In a typical outgoing breaker cubicle, the primary path runs from the bus connection through the breaker’s primary contacts to the cable connection. An incoming, bus-tie, bus-riser or metering cubicle has a different duty and arrangement. One breaker-feeder diagram cannot represent every cubicle; the actual single-line diagram identifies power paths and possible backfeeds.
| Component or area | Electrical / mechanical role | What it does not prove |
|---|---|---|
| Main bus | Carries primary current between cubicles, with a specified joint, support and insulation arrangement. | Disconnecting one feeder does not prove the entire bus is de-energized. |
| Circuit breaker and drawout truck | The breaker interrupts load and fault current within its ratings; the truck changes the primary connection position. | An open breaker is not the same as disengaged primary disconnects. |
| Cable area | Houses terminations and, as configured, CTs, VTs, arresters or an earthing switch. | Every model does not contain the same equipment, and cable-side backfeed is not excluded. |
| CTs, VTs and sensors | Provide current or voltage information through the interface required by protection and measurement devices. | Correct ratios, accuracy, burden and sensor compatibility must still be established. |
| Low-voltage control area | Contains relaying, control, indication, communications and auxiliary supply circuits. | Low-voltage is not a declaration that all terminals are touch-safe or de-energized. |
| Shutters and metal barriers | Restrict access to designated primary openings and separate areas. | A closed shutter is not a voltage-absence test or an internal-arc test report. |
| Protective bonding and frame contacts | Provide the designed protective connection of exposed metalwork and the truck frame. | Bonding the frame does not earth the primary circuit for work. |
The Aktif SME and SMC catalogue sections show different compartment and equipment arrangements for named product families. Compartment count, sheet thickness and cubicle width are model attributes, not fixed numbers implied by the term metal-clad.
From measurement to actual fault interruption
A CT or compatible sensor supplies current information to the protection system. Functions that need voltage also use a VT or suitable voltage sensor. The relay evaluates its inputs against settings and logic. Its trip output acts through the required auxiliary devices and wiring to the trip coil; the coil releases the mechanism, the main contacts move and the arc is extinguished so current can be interrupted. Measurement, decision, command and interruption are separate events.
The VacClad-W sequence-of-events diagram separates protective-relay time, auxiliary-relay time, shunt-trip operation, mechanism motion and arcing. A breaker’s advertised three-cycle interruption is not the installation’s total fault-clearing time without the other delays. The times in this historical product example are not settings for a different installation.
A relay trip indication alone does not prove that the breaker’s main contacts opened or that current ceased. The trip supply, circuit continuity, coil, mechanism and feedback must be assessed together. MOC contacts relate to the breaker mechanism; TOC contacts relate to the truck position. Their operation in connected and test positions depends on the product and options. Neither signal is proof of voltage absence.
Auxiliary power is part of the design. Eaton’s control-equipment description distinguishes a DC battery/charger arrangement from AC capacitor-trip options. There is no single DC control voltage for all metal-clad switchgear, and a capacitor-trip device is not automatically an uninterruptible supply for all relay logic. Conventional CT secondary circuits and stored mechanical energy also require their own safety assessment.
Connected, test and disconnected positions
Truck position and breaker open/closed status are two different pieces of information. A breaker may be open while connected to the primary circuit. In test position, the primary disconnects may be disengaged while control circuits remain connected. Some designs combine test and disconnected at one physical location with different secondary-connection conditions. The table explains common arrangements; the equipment instructions define the actual positions, interlocks and permitted tasks.
| Position / state | Primary connection | Secondary circuits and stored energy | Limit of this information |
|---|---|---|---|
| Connected / service | Mated with the stationary primary contacts; the breaker itself may be open or closed. | Control connections are normally engaged. | Opening the breaker does not establish isolation from every source. |
| Test | Typically disengaged from stationary primary contacts. | Control connections may remain engaged for functional testing. | Control power and spring energy may remain; adjacent bus or cables are not declared dead. |
| Disconnected | Primary disconnects are disengaged. | Secondary disconnection or manual connection depends on the design. | The position name is not work authorization or proof that all sources are isolated. |
| Removed | The breaker has been physically withdrawn from the cubicle’s primary interface. | External test connections and stored energy need separate consideration. | Stationary primary contacts inside the cubicle may remain energized. Removing a breaker does not make the entire cubicle safe to enter. |
The three-position definitions in ABB’s SafeGear guide explicitly retain the secondary connection in test. The Siemens GM-SG instructions describe secondary disconnects, shutters, MOC and TOC functions separately. A position lamp or “open” indication is not a substitute for verifying voltage absence in the defined work area.
IEEE construction and IEC LSC, PM/PI and IAC
These terms answer different questions. IEEE metal-clad construction, IEC service-continuity and partition classifications, and internal-arc performance must each be read against their own documented conditions. Their appearance together in a catalogue does not establish an automatic conversion between them.
| Term / axis | Question it answers | Conclusion that does not follow automatically |
|---|---|---|
| IEEE C37.20.2 metal-clad | What construction and electrical product category is offered under the stated edition? | IEC LSC2B, PM or an IAC rating is not assigned by the name alone. |
| IEC LSC | What service-continuity category applies when a defined accessible HV compartment is opened? | It does not authorize every task without an outage or guarantee continuous service. |
| IEC PM / PI | What partition and shutter arrangement separates an opened compartment from live main-circuit parts under the defined access conditions? | It does not by itself define the insulation medium or internal-arc performance. |
| Air, gas or other insulation arrangement | What is the main insulation medium and construction? | Air insulation alone does not establish metal-clad, PM or an arc classification. |
| IEC IAC and markings such as F/L/R | What accessibility, assessed sides, current and duration apply to a specified internal-arc test? | It is not directly interchangeable with an IEEE arc-resistant accessibility type. |
| IEEE arc-resistant accessibility type | Which access sides and compartment conditions were assessed under the specified test? | It is not IEC LSC2B and is not general permission for energized work. |
The distinction between LSC2A and LSC2B is more specific than having more compartments. In a typical breaker-compartment example, LSC2A access may require the cable circuit to be de-energized while the bus remains in service; an LSC2B arrangement may also permit the cable connection to remain in service under its defined conditions. Read the declaration for the particular accessible compartment and task. It is not permission to enter an energized bus or cable compartment.
PM describes the grounded metal partition and shutter arrangement between an opened compartment and live main-circuit parts under the defined access conditions. PI describes an arrangement with one or more non-metallic parts in that separation. This does not mean that every internal sheet must be metal. Aktif’s access and partition explanations treat these as separate attributes. Confirm the product declaration, access arrangement and applicable standard edition together.
A useful counterexample is the drawout SME-1 option in Aktif’s catalogue, which the manufacturer describes within an LSC2A/PI family. Drawout construction therefore does not automatically establish LSC2B-PM. Confirm each classification for the exact configuration being purchased; a shared product-family label does not prove equivalence.
Reading ratings as a set of conditions
Selecting a lineup by a kV and kA headline is insufficient. System voltage and frequency, continuous load, prospective fault duty, protection clearing time, insulation level and environmental conditions must be checked for the devices and the complete assembly. A replacement breaker’s higher rating does not automatically uprate the bus or lineup.
| Rating | Conditions to read with it | Common incorrect conclusion |
|---|---|---|
| Rated voltage and insulation withstand | Frequency, power-frequency withstand and impulse withstand, including phase-to-earth and phase-to-phase definitions. | System voltage and every test-voltage value are not the same quantity. |
| Continuous current | Bus and breaker ratings, natural/forced cooling, temperature, altitude and configured equipment. | The highest fan-cooled rating does not apply to every cubicle. |
| Short-time withstand current | The stated rms current paired with its documented duration. | A kA value is not indefinite and should not be expressed as kA per second. |
| Peak withstand / making duty | The specified peak or making value and its test conditions. | Peak current is not the symmetric rms fault-current rating. |
| Short-circuit breaking duty | Breaker interruption at the stated voltage and relevant circuit conditions. | A disconnector or an item with only a withstand rating does not have the same breaking duty. |
| Internal-arc rating | Current, duration, accessibility, assessed sides and installation conditions. | Short-time withstand current is not an IAC rating. |
| Mechanical / electrical endurance | The test duty and the definition of the operation count. | No-load mechanical operations are not an equal number of fault interruptions or a service-life guarantee. |
The Aktif SMC technical table separates short-time, peak and internal-arc entries; its test note also has a stated current limit. Schneider’s SureSeT characteristics separately identify peak withstand and two-second rms withstand. These are product examples. Combining maxima from different models or catalogue editions does not create a new verified assembly rating.
Metal-clad does not automatically mean arc-resistant
The SureSeT technical table explicitly identifies the metal-clad assembly’s internal-arc entry as “Non-Arc Rated.” Grounded barriers, closed-door controls, remote racking or an arc-detection relay alone do not establish an arc-resistant or IAC rating for the assembly.
In an IAC marking, F identifies the front, L the lateral sides and R the rear. Accessibility A concerns restricted access by authorized personnel, while B concerns unrestricted access; neither marking is work authorization. An internal-arc assessment needs more than current and duration. Accessibility, assessed sides, door and cover conditions, the pressure-relief path and the installed room arrangement matter. Plenums, exhaust ducts, anchoring and openings must match the applicable design conditions. A blocked exhaust outlet or altered cover cannot simply be treated as the tested arrangement.
The Type 2B conditions in ABB’s SafeGear guide distinguish the permitted LV instrument-door condition from the requirement for other doors and covers to be secured. This is not permission to enter an HV compartment. Its door and exhaust details also show that door construction changes with the arc-duration configuration. Do not equate AFLR with Type 2B or infer zero hazard, no PPE or unconditional energized access.
What shutters, interlocks and indications establish
Shutters help restrict inadvertent access to stationary primary contact openings. Depending on the design, mechanical, electrical or key interlocks prevent racking a closed breaker, closing in an unsuitable position, or incompatible equipment movements. The interlock matrix should identify which condition blocks which movement; it is not identical for every model.
These are design measures that reduce the likelihood of specified errors, not guarantees that failures or human mistakes cannot occur. A damaged lock, inconsistent position indication or mechanism requiring abnormal force is not a normal operating condition. Forcing a shutter, defeating a lock or bridging a permissive is not a technical remedy.
The GM-SG frame-grounding and shutter descriptions distinguish protective frame bonding from access to the primary contacts. A closed shutter, extinguished lamp or disconnected truck does not replace assessment of every source, auxiliary supply and stored-energy hazard. The work area and permitted task are established through the authorized work plan and equipment-specific isolation and voltage-verification arrangements.
Documents that make switchgear offers comparable
Start with the required duty: incoming, outgoing, bus-tie or metering; present loads, fault conditions and planned extension. A bus tie or double-bus arrangement alone does not provide uninterrupted supply or complete redundancy. Source independence and capacity, transfer and synchronism conditions, fault zones and protection logic require separate engineering.
- The applicable standard and edition, exact model, cubicle function, single-line diagram and primary arrangement.
- Assembly, bus and breaker rating schedules, including cooling and environmental limitations.
- Claimed LSC, PM/PI and internal-arc attributes, with the relevant test report linked to the offered configuration.
- CT/VT or sensor ratios, accuracy/burden, relay logic, trip supply, control drawings and interlock matrix.
- Cable entries, access sides, exhaust arrangement, room layout, anchoring and maintenance space.
- Factory/site verification scope, handover documents, spares and model compatibility.
Matching kA headlines or cubicle counts do not prove equivalent offers. The breaker, compartments, doors and exhaust arrangement in a report must be related to the supplied product. Check whether a document covers another family or an older configuration. An ISO management-system certificate is not an internal-arc test report, and cabinet dimensions are not automatically room or statutory clearances.
What the construction means for maintenance and retrofit
Compartmentalization does not remove maintenance needs. Primary contacts, the truck and shutter mechanisms, protective bonds, terminations, measurement chain and trip circuit must be considered together. The equipment instructions and installation-specific work plan determine which checks are appropriate in each energy state. This definition article does not provide test voltages, numerical acceptance limits or fixed maintenance intervals.
For a retrofit, matching the new breaker’s envelope or continuous current is insufficient. The primary interface, insulation clearances, mechanical travel and interlocks, trip/close energy, secondary connections, protection settings and the documented performance of the complete assembly need reassessment. Adding a relay or remote control does not automatically change an internal-arc classification.
The metal-clad testing and maintenance article covers the inspection topic separately, while the protection-relay explanation focuses on detection and tripping. For an installation review, provide model/nameplate information, the current single-line diagram, rating and test documents, relay/control data, fault and maintenance history, and the proposed outage scope. These inputs help define a useful technical assessment and proposal.
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Frequently Asked Questions
Does every cubicle with a drawout breaker qualify as metal-clad?
Drawout construction alone does not establish the category. The applicable standard, grounded barriers, primary interfaces and complete assembly must support the claim. A drawout product can also be declared LSC2A/PI under an IEC product classification; withdrawal alone proves neither LSC2B-PM nor internal-arc performance.
Does a vacuum or SF6 breaker change the metal-clad classification?
Vacuum or SF6 describes the breaker’s interruption medium; metal-clad describes the assembly construction. A product family may offer different breaker options. Breaking duty, insulation, control interfaces, maintenance and assembly compatibility are checked for the selected model, rather than assigning a compartment classification from the interruption medium alone.
Can the breaker receive commands while in test position?
Yes. Many designs disengage the primary contacts while retaining secondary control connections in test. Control power and stored spring energy may remain. The actual connections and energy sources must be established from the equipment instructions and work plan rather than treating test as a de-energized work state.
Does a relay trip record prove that the fault was cleared?
No. It may record the relay’s decision or output, while the trip source, wiring, coil, mechanism and interruption remain separate stages. Auxiliary contacts and event records must be interpreted for what they actually indicate. A relay message or open indication alone is not proof of voltage absence.
Does adding an arc-detection relay make switchgear arc-resistant?
Arc detection and rapid tripping may be protection functions, but they do not replace an assembly internal-arc test. An arc-resistant or IAC claim is tied to a particular construction, current, duration, door/cover state, access and exhaust arrangement. Adding a relay or remote control does not automatically create a new classification.
Is IAC AFLR equivalent to IEEE Type 2B?
No. They are markings from different assessment frameworks. The IAC accessibility, sides, current and duration and the IEEE type’s access and door conditions must be checked separately. A Type 2B allowance for a specified LV instrument-door condition does not permit access to a high-voltage compartment.
Why specify short-time withstand and breaking current separately?
Withstand concerns carrying a stated current for its documented duration and conditions. Breaking duty concerns the breaker interrupting current under its specified circuit conditions. A withstand rating for a disconnector or bus does not give it the breaker’s interruption duty, and internal-arc performance is a separate rating again.
Are CTs, VTs, arresters and SCADA standard in every cubicle?
No. Equipment depends on cubicle function, model and ordered options. Incoming, metering, bus-tie and cable-connection cubicles do not have identical lists. Check the standard/optional schedule together with primary and control drawings; not everything visible in a catalogue photograph is necessarily included in the supply.
Does a bus tie eliminate maintenance outages?
Not by itself. Source and bus capacity, fault zones, load transfer, protection, transfer logic and the isolation required for work determine the result. Double-bus or tie arrangements can support a continuity plan, but a tie symbol alone proves neither outage-free maintenance nor full load redundancy.
Can a higher-current breaker uprate an existing cubicle?
A higher breaker rating alone is insufficient. Busbars, primary contacts, insulation, cooling, mechanical interfaces, interlocks and control circuits set assembly limits. The proposed change must be assessed against the complete assembly’s ratings and supporting documents before the new breaker rating is treated as the lineup’s capacity.