MV Switchgear Datasheet: Withstand and Internal Arc Ratings

Two MV switchgear offers can sit side by side with the same headline line — 12 kV, 630 A, 25 kA — and still describe different cubicles. One quotes 25 kA for one second, the other for three. One carries an internal arc classification with a stated test current and duration, the other leaves that field blank. Nothing looks wrong until the protection study fixes a fault clearing time, and by then the purchase order has been signed.

This is a datasheet-reading problem, not a selection problem. Our guide on how to select MV switchgear covers choosing a lineup and comparing configurations; the MV switchgear overview sets out what the equipment is. Here we take five lines from a manufacturer’s data sheet and ask what each one commits the supplier to, and what happens downstream when the line is read loosely.

Rated short-time withstand current is two numbers, not one

IEC 62271-200 defines the rated short-time withstand current I_k as the r.m.s. value the equipment carries for a stated rated duration t_k. The standard duration is 1 s, but 2 s, 3 s and 4 s are all offered. The thermal stress on a busbar is proportional to I_k² × t_k, so the two offers above are not close: 25 kA for 1 s gives 25² × 1 = 625 kA²s, while 25 kA for 3 s gives 25² × 3 = 1,875 kA²s — three times the heat the conductor and its supports have to survive.

Pick t_k from the upstream protection setting, not from habit. A feeder cleared by a relay in 0.4 s does not need a 3 s rating, but a bus section waiting on a backup clearing time may. If the study is not finished, say so on the enquiry and ask the supplier to state which clearing time the quoted figure assumes. A number without a duration is a marketing number.

Peak withstand current and the first half-cycle

The r.m.s. value sizes the heat; the peak sizes the force. IEC 62271-200 sets the rated peak withstand current I_pk at 2.5 × I_k for 50 Hz systems and 2.6 × I_k for 60 Hz, because the d.c. offset in the first half-cycle depends on the network X/R ratio and the point on wave at which the fault starts. On 25 kA that is 62.5 kA at 50 Hz and 65 kA at 60 Hz. Electrodynamic force between conductors scales with the square of the instantaneous current, so insulator spacing, busbar clamps and the bracing inside the cubicle are chosen against this peak, not against 25 kA.

That is why a 60 Hz project cannot simply inherit a 50 Hz type test report. We build 60 Hz equipment as a normal order rather than an adaptation, but the frequency belongs on the data sheet because it changes the peak factor and the core design of any instrument transformers in the panel. State it at enquiry and the peak line can be checked against the right multiplier instead of the default one.

Internal arc classification: read IAC letter by letter

The internal arc classification, IAC, is a type-test result, not a promise. Under IEC 62271-200 the marking carries the accessibility category, the protected sides, and the test current and duration: a typical string is IAC A FLR 25 kA 1 s. Category A means the enclosure is accessible to authorised personnel only; category B covers areas with unrestricted public access. F, L and R stand for front, lateral and rear — the faces on which the arc was applied and on which the five assessment criteria were met.

Those criteria cover doors and covers staying closed, no dangerous parts being ejected, hot gases not igniting the indicators placed around the cubicle, the earthing circuit remaining effective, and the arc not burning through into an adjacent compartment. Ask which sides were tested. A panel classified FL is not classified on the rear, and a rear-facing cable compartment in a walkable room is exactly where the missing R matters. Certification and type-test documentation are supported per project rather than assumed from a fixed package; send the classifications and reports your specification calls for and we will confirm what can be arranged.

Pressure relief direction belongs to the room, not just the panel

An IAC result is valid for the pressure relief arrangement that was tested. Most metal-clad cubicles vent an arc upwards through flaps on the roof, or rearwards into a dedicated duct. If the switchroom has a low ceiling, a cable tray directly above the vent, or a wall a short distance behind the cubicle, the tested path is no longer the installed path, and the classification has to be re-confirmed for that arrangement.

The same check gets tighter inside a compact substation, where the enclosure is small, the operator may stand close to the door, and the vent path has to be reconciled with the ventilation louvres and the cable entry. Draw the relief direction on the layout drawing, show the clearance above it, and show where personnel stand. That single annotation prevents most of the arguments that otherwise surface at commissioning, when changing a roof flap is no longer a drawing change.

Five questions to close before you approve the data sheet

First, what fault level and clearing time set I_k and t_k? Second, is the system 50 Hz or 60 Hz, and does the quoted I_pk use the right multiplier? Third, which accessibility category and which faces does the IAC marking cover, at what test current and duration? Fourth, where does the pressure relief go in the installed room? Fifth, which of the quoted figures come from a type test on this design and which are measured on each unit? Answered together, they turn a one-page data sheet into a checkable specification.

Get the answers on the same page as the single line diagram and the room layout. Our switchgear and control pages show the usual arrangements, and the MV switchgear range lists the standard configurations. Type-test and certification requirements are handled per project rather than claimed from a fixed package, so send the standards and tests your project requires before any order is placed. Delivery is 30 days from approved drawings and the warranty runs 2 years.

Send Us the Datasheet Questions

Send the fault level, clearing time, frequency, IAC marking and the switchroom layout. Email info@xsdfftransformer.com, call +8615867897761, or message WhatsApp +852 5416 2620. The same list can be sent through our enquiry page.