11 kV Switchgear Selection Guide: Ratings, Worked Example, Checklist
Start with the system data
Before any equipment is selected you need: system voltage, the fault level declared by the utility at the point of supply, the loads and transformers to be fed, the earthing arrangement, cable type and size, ambient temperature and altitude. Most specification errors come from guessing one of these.
Ratings that matter
| Rating (IEC 62271-1 / 62271-200) | Typical value for an 11 kV system | What to check |
|---|---|---|
| Rated voltage (Ur) | 12 kV | 36 kV equipment for 33 kV systems |
| Power-frequency withstand (Ud) | 28 kV, 1 min | Utility specification may ask for more |
| Lightning impulse withstand (Up) | 75 kV | Overhead-line-fed sites need the full value |
| Rated normal current (Ir) | 630 A, 1250 A, 2000 A | Busbar and each feeder separately; allow for growth |
| Short-time withstand (Ik, tk) | 20, 25, 26.3 or 31.5 kA for 1 s or 3 s | Must be at least the declared fault level |
| Peak withstand (Ip) | 2.5 × Ik at 50 Hz | Also the breaker's rated making current |
Worked example: a 2 × 2,000 kVA substation
An institutional campus takes an 11 kV supply and feeds two 2,000 kVA, 11/0.433 kV transformers. The figures below illustrate the method; the final values come from the utility letter and the project's protection study.
- Fault level: the utility declares 500 MVA at 11 kV. Ik = 500 ÷ (√3 × 11) ≈ 26.2 kA. Specify 26.3 kA for 3 s (or 31.5 kA if the utility plans reinforcement). Peak withstand = 2.5 × 26.3 ≈ 66 kA.
- Transformer feeder current: 2,000 ÷ (√3 × 11) ≈ 105 A. The smallest standard VCB rating, 630 A, is ample.
- Incomer current: both transformers at full load ≈ 210 A. A 630 A incomer and busbar suit; choose 1,250 A only if a third transformer or a large HT motor is planned.
- CTs: about 150/1 A on the transformer feeders and 300/1 A on the incomer, with a protection class (for example 5P20) and a metering class (0.5 or 0.2S for the utility meter, as the utility specifies).
- LT side check: a 2,000 kVA transformer at 6.25% impedance gives a full-load current of about 2,667 A and a prospective LT fault of about 2,667 ÷ 0.0625 ≈ 43 kA, ignoring source impedance. The LT PCC panel incomer ACB and busbar therefore need a short-time rating of at least 50 kA for 1 s.
Loss of service continuity category
IEC 62271-200 classifies how much of the switchgear must be shut down to open one compartment: LSC1, LSC2A or LSC2B. Plants that cannot take a full shutdown should specify LSC2B, where the cable compartment of one panel can be opened with the busbar live. Also state the partition class: PM (metallic partitions) or PI (insulating).
Internal arc classification
Specify the IAC class and the accessible sides, for example IAC AFLR 25 kA 1 s: arc-tested from the front, lateral and rear for authorised personnel. This decides room clearances and the pressure-relief route (into the room, a duct or outside), so agree it before the civil layout is frozen.
VCB panel, RMU or GIS?
| Option | Where it fits | Watch-outs |
|---|---|---|
| Air-insulated VCB panel (metal-clad, withdrawable) | Main receiving stations, many outgoing feeders, motor feeders, frequent switching | Larger room; needs maintenance access front and rear |
| Ring main unit (load-break switches plus VCB or fuse-switch for the transformer) | Compact secondary substations, ring-fed campuses, package substations | Limited feeder types; check the gas or insulation medium and the end-of-life handling |
| Gas-insulated switchgear | Space-limited or polluted sites, higher ratings | Higher cost; repairs depend on the OEM |
See the full VCB panel vs RMU comparison and the RMU guide.
Protection scheme
A typical industrial 11 kV scheme: overcurrent and earth fault on every feeder, with the high-set element set above the transformer inrush current; transformer trips (Buchholz, oil and winding temperature) wired into the VCB trip circuit; differential protection on transformers above about 5 MVA; restricted earth fault where the earthing calls for it; and trip-circuit supervision with a lockout relay. Ask for a coordination study across MV and LT so that a downstream LT fault clears before the MV breaker.
Altitude and ambient
Above 1,000 m, insulation must be corrected for altitude. Above a 40 °C design ambient, current ratings must be derated. Gulf, desert and many North Indian sites need a 45–50 °C design ambient; state it clearly in the enquiry.
Documentation set
Ask for: the switchgear OEM's type-test certificates for the offered design (named as the OEM's), routine test reports, general arrangement and schematic drawings, relay settings, CT and VT data, cable termination details, and an operation and maintenance manual. A panel arriving without this set slows commissioning and utility approval.
How PowerPanelTech supplies 11 kV switchgear
PowerPanelTech is an LV panel builder. We supply 11 kV VCB panels and RMUs through OEM and fabrication partners, and the type-test certificates belong to the switchgear OEM. Our engineering adds the coordination with your LT panels: matched fault ratings, protection settings and one document set for the substation. Send your SLD and utility letter for a quotation, or see the medium voltage switchgear page.
Frequently asked questions
What fault level should I specify for 11 kV switchgear?
Use the fault level the utility declares at your point of supply, converted to kA (MVA ÷ (√3 × 11)), and add margin for planned grid reinforcement. Indian specifications commonly ask for 25 kA or 26.3 kA (500 MVA) for 1 s or 3 s.
What is the difference between LSC2A and LSC2B?
Both let you open one compartment while others stay in service. LSC2B also keeps the cable compartment of a panel accessible with the busbar live, so one feeder can be worked on without shutting the board.
Should I choose a VCB panel or an RMU for a 2 MVA substation?
For a single transformer on a ring-fed supply an RMU is usually compact and economical. For several feeders, motor loads, frequent switching or a main receiving station, a withdrawable VCB panel board is the better fit.
Does PowerPanelTech manufacture 11 kV switchgear?
No. We supply 11 kV VCB panels and RMUs through OEM and fabrication partners, with the OEM's type-test certificates, and coordinate them with the LV panels we build.
Request a quotation · Submit your SLD / BOQ · +91-9654745703 · powerpaneltech@gmail.com