In short
For MV switchgear, specify the rated voltage, rated normal current for busbars and feeders, short-time withstand current and duration, insulation level, internal arc classification, loss of service continuity category, breaker type, protection and auxiliary voltage, and the standard, usually IEC 62271-200.
Rated voltage
Rated voltage is the highest system voltage the switchgear is designed for, not the nominal network voltage. IEC standard values include 7.2, 12, 17.5, 24 and 36 kV, so an 11 kV network typically uses 12 kV switchgear and a 33 kV network uses 36 kV switchgear. State your nominal system voltage and the rated voltage you require.
Rated normal current
Give the continuous current for the busbars and for each feeder type, for example 1,250 A busbars with 630 A outgoing feeders. Incomer and bus-section panels usually need higher ratings than feeders.
Short-time withstand current
The switchgear must carry the prospective fault current until protection clears it. This is stated as a current and a duration, such as 25 kA for 3 s. Use the fault level from your network study, and allow for future growth where possible.
Insulation level
The insulation level combines a power-frequency withstand voltage and a lightning impulse withstand voltage, for example 75 kV impulse for 12 kV equipment. Higher altitude sites may need additional insulation margin, so mention site altitude.
Internal arc classification
Internal arc classification (IAC) shows that the switchgear protects people if an arc fault occurs inside it. It is written as accessibility type, accessible sides, arc current and duration, for example IAC AFLR 25 kA 1 s. Specify it whenever operators work near the switchgear.
Service continuity and breaker type
Loss of service continuity categories such as LSC2 describe which compartments can stay energised while another is opened for maintenance. Vacuum circuit breakers are the usual choice at medium voltage. Some markets, including the EU, are restricting SF6 gas in new MV switchgear, so check the rules in the destination country.
Protection, control and auxiliaries
List the protection functions per feeder, the current and voltage transformer ratios, the auxiliary supply (for example 110 V DC) and the communication protocol, such as IEC 61850, if the switchgear connects to a SCADA system.
Checklist
| Rating | Example |
|---|---|
| Rated voltage | 12 kV for an 11 kV system |
| Busbar / feeder current | 1,250 A / 630 A |
| Short-time withstand | 25 kA, 3 s |
| Insulation level | 28 kV power frequency, 75 kV impulse |
| Internal arc | IAC AFLR 25 kA 1 s |
| Service continuity | LSC2 |
| Breaker | Vacuum circuit breaker |
| Standard | IEC 62271-200 |
Illustrative values only. Use your project's actual ratings.