Specifying MV Switchgear: Key Ratings Explained

A switchgear panel can carry its normal load current for decades without incident, yet the moment that matters most is the fraction of a second when a fault occurs. Will the busbars hold together under electromagnetic forces? Will the circuit-breaker interrupt the current? If an internal arc occurs, will people standing nearby be protected? A datasheet answers these questions through a set of ratings, and each rating covers a different part of the problem.

This article answers a practical question: which ratings define medium-voltage (MV) switchgear, what does each one protect against, and why must they be evaluated together rather than one at a time?

Understanding the key ratings

Standards framework: the IEC 62271 series

Most MV switchgear supplied internationally is designed and tested to the IEC 62271 series. At the level of what each part governs:

Standard	What it covers	Why it matters
IEC 62271-1	Common specifications for alternating current switchgear and controlgear — defines rated characteristics, standard values and common test requirements used by the other parts.	Sets the rating values and tests referred to throughout a switchgear specification
IEC 62271-100	Alternating current circuit-breakers.	Applies to circuit-breaker functions, such as incomers and feeders that must interrupt fault current
IEC 62271-102	Alternating current disconnectors and earthing switches.	Covers the isolating and earthing functions that make maintenance safe
IEC 62271-103	Switches for rated voltages above 1 kV up to and including 52 kV.	Applies to load-break switches, such as the ring switches in RMUs
IEC 62271-105	Alternating current switch-fuse combinations.	Applies to fused switch functions, commonly used to protect distribution transformers
IEC 62271-200	AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV.	The assembly-level standard for the switchboard as a whole, including internal arc classification
IEC 62271-202	High-voltage/low-voltage prefabricated substations.	Relevant when the switchgear forms part of a compact / package substation

A switchgear assembly is therefore assessed at two levels: the assembly (for example to IEC 62271-200) and the devices inside it (circuit-breakers, switches, earthing switches, fuse combinations, each to its own part). A specification should refer to both. Edition years matter; the applicable editions should be confirmed for each project.

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Rated voltage (Ur) vs system voltage

The nominal system voltage is the voltage by which a network is identified (for example 11 kV, 20 kV or 33 kV). The rated voltage (Ur) of switchgear is the maximum system voltage for which the equipment is designed. IEC standard rated voltages within the scope of AdepaTrade’s supply include values such as 7.2, 12, 17.5, 24 and 36 kV.

Standard values from IEC 60038, Table 3, Series I (50 Hz and 60 Hz systems). Confirm the rated voltage against the network operator’s requirements:

Nominal system voltage	Highest voltage for equipment (typical switchgear rated voltage)
6.6 kV or 6 kV	7.2 kV
11 kV or 10 kV	12 kV
15 kV (non-preferred)	17.5 kV
22 kV or 20 kV	24 kV
33 kV or 30 kV	36 kV

The two nominal values in a row are alternatives; IEC 60038 recommends that only one of them is used in any one country, and it shows 15 kV and 17.5 kV as non-preferred values. 13.8 kV is not a Series I value: IEC 60038 lists it in Series II (60 Hz systems, mainly North American practice), with a highest voltage for equipment of 14.52 kV. Because equipment may only be used on systems whose highest voltage does not exceed its highest voltage for equipment, 17.5 kV IEC-rated switchgear meets this condition on a 13.8 kV network. The rated voltage is still a selection to agree with the network operator.

The common error is to specify “11 kV switchgear” without stating the rated voltage, or to assume the next rating up automatically covers local practice. Some networks operate close to the upper limit of a rating band, and some utilities mandate a specific rated voltage regardless of nominal voltage.

Rated normal current (Ir)

The rated normal current is the r.m.s. current the equipment can carry continuously under specified service conditions without exceeding permitted temperature rises. It applies separately to busbars and to each functional unit (incomer, feeder, transformer feeder, bus coupler).

Important points:

• It is defined at stated ambient conditions. Higher ambient temperature, poor ventilation of the switchroom or enclosure, or installation inside a kiosk can reduce the current the equipment can actually carry. Derating information is manufacturer-specific.
• Busbar rating should reflect the total current that can flow along the busbar, including future feeders and possible reconfigurations — not just the largest single feeder.
• Normal current says nothing about fault capability. A 630 A panel and a 1250 A panel may have the same, or different, short-circuit ratings.

Short-time withstand current (Ik) and rated duration (tk)

The rated short-time withstand current (Ik) is the r.m.s. current the equipment can carry in the closed position for a specified rated duration of short circuit (tk) without damage. It is a thermal rating: it tests whether conductors, joints and contacts survive the heating caused by fault current until protection clears the fault.

Ik must always be stated together with its duration, for example “20 kA for 1 s” or “25 kA for 3 s”. A rating of “25 kA” without a duration is incomplete. The duration selected must be consistent with the total fault clearance time of the upstream protection, including relay time, breaker operating time and any intentional grading delays.

Peak withstand current (Ip)

The rated peak withstand current (Ip) is the highest instantaneous current the equipment must withstand in the closed position — typically occurring in the first major loop of an asymmetrical fault. It is a mechanical rating: it tests whether busbars, supports and contacts survive the electromagnetic forces.

IEC 62271-1 relates Ip to Ik through a factor that depends on the system frequency and the DC time constant of the network. For the standard time constant, the commonly used factors are 2.5 × Ik at 50 Hz and 2.6 × Ik at 60 Hz (for example, 25 kA Ik corresponds to 62.5 kA Ip at 50 Hz). Networks with a higher X/R ratio — for example close to large generators or transformers — may require a higher factor; this must be confirmed by the network study.

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Short-circuit breaking and making duties

These apply to switching devices, and the device type matters:

• Circuit-breakers have a rated short-circuit breaking current (Isc), the highest fault current they can interrupt, and a rated short-circuit making current, the peak current they can close onto (normally equal in value to Ip). Breakers also have a defined operating sequence (for example open – delay – close/open – delay – close/open) that matters where auto-reclosing is used.
• Load-break switches are designed to make and break normal load currents. They usually have a short-circuit making capability (so they can be safely closed onto a fault) but are not designed to break short-circuit current.
• Switch-fuse combinations rely on fuses to interrupt fault current; the switch, striker and fuse together define the combination’s performance.
• Earthing switches may have a short-circuit making capability so that accidental closing onto a live circuit does not cause failure.

A common gap is to specify “25 kA” for a ring main unit without distinguishing which functions carry, make or break that current.

Insulation levels: rated withstand voltages

Insulation level is expressed as two values:

• Rated short-duration power-frequency withstand voltage (Ud) — represents temporary overvoltages at system frequency.
• Rated lightning impulse withstand voltage (Up) — represents fast transient overvoltages caused by lightning.

IEC 62271-1 associates standard insulation levels with each rated voltage. Typical values (illustrative, common-value list; confirm the applicable table and any national alternatives):

Two practical points:

• Altitude. Air is a weaker insulator at higher altitude. Standard ratings normally assume installation up to 1000 m above sea level; above that, air-insulated parts may require correction or a higher insulation level. Gas-insulated sections are affected differently. The correction method and its applicability must be confirmed with the manufacturer.
• Exposure. Equipment connected to overhead lines is more exposed to lightning surges; surge arresters and insulation coordination (IEC 60071 series) should be considered at the system level.

Internal arc classification (IAC)

An internal arc is a fault inside the switchgear enclosure that releases large amounts of energy, hot gases and pressure. Internal arc classification, tested under IEC 62271-200, demonstrates that the enclosure protects persons in the vicinity under defined conditions. It does not mean the equipment remains undamaged or in service.

An IAC designation contains:

• Accessibility type: type A — accessible to authorised personnel only; type B — unrestricted accessibility, including the general public; type C — accessibility restricted by installation out of reach (for example pole-mounted).
• Accessible sides: F (front), L (lateral), R (rear).
• Arc fault current and duration, for example 16 kA for 1 s.

A designation such as IAC AFLR 20 kA 1 s therefore means: tested for authorised-personnel access, with front, lateral and rear sides accessible, at 20 kA for 1 s.

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Interpretation points:

• The classification is valid only when installed as tested — including room dimensions, ceiling height and arc gas exhaust arrangements (ducts, absorbers or venting routes). A wall-mounted installation of a panel classified for rear access, or a missing exhaust duct, can invalidate the assumption.
• The arc fault current is often lower than, or equal to, the short-time withstand current. It should be compared with the actual prospective fault current.
• Other classifications under IEC 62271-200, such as loss of service continuity (LSC) and partition class, describe maintenance and operational continuity rather than arc safety. They are different properties and should be specified separately.

Service conditions

All ratings are valid only under the service conditions defined by the standard or agreed with the manufacturer. IEC 62271-1 defines “normal” indoor and outdoor service conditions, covering ambient temperature range, altitude, humidity, pollution and condensation. Typical normal indoor assumptions include an upper ambient air temperature of 40 °C, a limited 24-hour average, and altitude up to 1000 m — the exact limits must be checked in the applicable edition.

Projects in hot climates, at high altitude, in coastal or desert environments, or with high humidity and condensation risk should state special service conditions explicitly so the manufacturer can confirm derating, insulation correction or design adjustments.

Why ratings must be evaluated together

These ratings interact:

• A high Ik is ineffective if the rated duration is shorter than the actual fault clearance time.
• Ik and Ip must be consistent with each other and with the network X/R ratio.
• A breaker’s breaking capacity must match or exceed the busbar fault level — but load-break switches in the same line-up are not expected to break faults, so protection philosophy must assign that duty correctly.
• IAC ratings are linked to the fault current and the room design; a high Ik with a low arc rating may leave an operator safety gap.
• Every rating is conditional on service conditions; a derating for temperature or altitude can change the outcome of the comparison.

Practical selection guidance

1. Start with the network study. Obtain the maximum prospective fault current (r.m.s. and peak) at the installation point, including future network reinforcement and any embedded generation.
2. Define the rated voltage and insulation level from the network operator’s requirements, adjusting for altitude and exposure where relevant.
3. Size normal currents for busbars and each functional unit, including planned expansion and ambient derating.
4. Match Ik and tk to the protection scheme — confirm total clearance time and any backup protection time.
5. Assign switching duties per function: which units need circuit-breakers, which can use switches or switch-fuse combinations.
6. Specify IAC fully: accessibility type, sides, current and duration — and describe the room or enclosure so the manufacturer can confirm arc gas management.
7. State service conditions explicitly where they depart from normal conditions.
8. Request evidence: type test reports for the specific design and ratings, and routine test documentation for the supplied units.

Common mistakes and specification gaps

• Specifying nominal system voltage instead of rated voltage.
• Giving a short-circuit rating without the rated duration, or without the peak value.
• Assuming a load-break switch can interrupt short-circuit current.
• Selecting fault ratings from typical values rather than a project-specific fault study.
• Ignoring future fault level increases when the network is reinforced or generation is added.
• Specifying IAC without accessibility type and sides, or installing the switchgear in a way that does not match the tested arrangement (wall clearance, ceiling height, exhaust route).
• Omitting altitude, high ambient temperature or pollution conditions.
• Accepting type test evidence for a different rating, configuration or variant than the one being supplied.
• Referring to “IEC 62271” generally without stating which parts and editions apply.

Related AdepaTrade solution

AdepaTrade sources MV switchgear, ring main units and protection equipment up to 36 kV from manufacturers in Türkiye and internationally, matched to the ratings and standards in your project specification. Medium Voltage Distribution & Protection