In medium-voltage switchgear the interlocks are inside the equipment and mostly mechanical: the switching device and the earthing switch cannot both be closed, the truck only moves with the breaker open, and interlock-controlled compartments only open when de-energized and earthed. A ring main unit adds the three-position switch and the fuses. Between panels, as between two incomers and a tie, the interlock is defined by the project.
Where the interlocks are in medium voltage
In high voltage, each device of a bay is a separate piece of equipment and the interlocking is solved in the control system. In medium voltage, the breaker, the disconnector, the earthing switch and the compartments share a metal enclosure, and the manufacturer interlocks them inside the equipment itself. The reference standard is IEC 62271-200, for metal-enclosed switchgear above 1 kV up to 52 kV. In North America the IEEE C37.20 series applies.
What the manufacturer solves are the relationships within one panel. What is left to the project are the relationships between panels and with other equipment: two incomers and a tie, the panel and the transformer, the panel and the low-voltage switchboard.
Interlocks within a panel
IEC 62271-200 sets mandatory interlocks. In general terms:
- Withdrawable part. The breaker can only be withdrawn or inserted when open, and can only operate in the service, test or disconnected position, never in an intermediate one.
- Earthing switch. When its making capacity is lower than the peak current of the circuit, it must be interlocked with the associated devices. In truck-type switchgear, manufacturers interlock it with the truck position: it does not close with the truck in service.
- Compartment access. Interlock-controlled compartments can only be opened with the internal parts de-energized and earthed, or with the truck disconnected and the shutters closed.
- Safety without auxiliary supply. If an interlock is not mechanical, the design must be safe even when the control supply is lost.
On top of these are the locks each manufacturer declares, such as the cable compartment door with the earthing switch. The complete list is the interlocking matrix of each panel, which is what gets tested at acceptance.
Service continuity. The LSC category of IEC 62271-200 indicates what stays energized when a compartment is opened. LSC1 requires de-energizing more than the panel itself; LSC2 allows a compartment to be opened with the busbars energized. The category is not an interlock, but it defines what the interlock protects and what remains live next to it.
RMU: three-position switch and fuses
A typical ring main unit (RMU) has two cable functions, to enter and leave the ring, and one transformer protection function. The cable functions use a load-break switch (IEC 62271-103), and the transformer function a switch-fuse combination (IEC 62271-105) or a circuit breaker with a relay.
Three-position switch. In many RMUs the same device has three positions: closed, open and earthed. The mechanism prevents going from closed to earthed without passing through open, so the interlock between device and earthing switch is inherent to the design. What it does not solve is whether the cable is energized from the other end of the ring: that is checked with the panel’s voltage presence indicators before moving to earth.
- Is the function's switch in the earthed position?
- NoThe door stays locked
- YesWas absence of voltage checked on the indicators before moving to earth?
- NoProcedural error: moving to earth may have closed onto a cable energized from the other end
- YesThe door can be opened and the cable stays earthed while it is open
Transformer function with fuses. A blown fuse releases a striker that opens the load-break switch, so the transformer is not left fed on two phases. To replace fuses, compartment access is interlocked with the switch open and the earthing switch closed; in many designs the earthing acts on both sides of the fuse.
Earthing switch making. The earthing switch of an RMU usually has short-circuit making capability, class E1 or E2 of IEC 62271-102. That capability protects the operator against an error, but it does not authorize closing onto voltage.
Interlocks between panels
The most frequent cases in a project are:
- Two incomers and a tie. To prevent paralleling two sources, only two of the three breakers may be closed. It is solved with trapped keys, with auxiliary contacts wired between panels or with the relay logic. If the transfer is automatic, the logic must also respect that rule. The low-voltage equivalent is covered in main-tie-main interlocking.
- Panel and transformer. Access to the transformer is interlocked with the panel that feeds it and with the low-voltage breaker if the transformer can be energized from that side. The usual method is a Kirk or Castell trapped key.
- Metering and main protection panel. In some service entrance designs, access to the metering compartment is interlocked with the main protection.
These interlocks do not come from the factory, and they are the ones most often left out of extensions: a panel or a standby generator is added and nobody checks whether the original sequence still covers every source.
Typical failures
- A mechanical interlock forced to rack out a truck in an emergency and never repaired.
- A cable door that opens without the earthing switch closed because of a worn latch.
- Auxiliary contacts that do not reflect the real position and let through a close that should have been blocked.
- An interlock between panels from different manufacturers that was never wired.
How it is verified
At acceptance and during maintenance the manufacturer’s interlocking matrix is worked through: every forbidden operation is attempted and must be rejected. The voltage presence indicators, the fuse striker and the project’s panel-to-panel interlocks are also checked.
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Related guides
- Interlocking at EHV, HV, MV and LV: what changes
- Primary selective and MV ring interlocking
- Main-tie-main interlocking: secondary selective
- MV automatic transfer: logic and blocking
References
- IEC 62271-200, AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV.
- IEC 62271-103, Switches for rated voltages above 1 kV up to and including 52 kV.
- IEC 62271-105, Alternating current switch-fuse combinations for rated voltages above 1 kV up to and including 52 kV.
- IEC 62271-102:2018, Alternating current disconnectors and earthing switches.
- IEEE C37.20 (series), metal-enclosed switchgear.
- Schneider Electric, Electrical Installation Guide, “Interlocks and conditioned operations”.
Frequently asked questions
Which interlocks does IEC 62271-200 require in a panel?
Among others: the withdrawable part may only be removed with the switching device open, the switching device may only operate in the service or test positions, an earthing switch with a making capacity lower than the peak current of the circuit must be interlocked with its associated device, and interlock-controlled compartments may only be opened with the internal parts de-energized and earthed. Interlocks that are not mechanical must remain safe without auxiliary supply.
Why does the cable door of an RMU open only with the earthing switch closed?
Because the cable compartment is the one opened to connect, test or repair cables, and the cable can be energized from the other end of the ring. Requiring the earthing switch closed ensures the cable stays earthed while the door is open.
Do two-incomer and tie switchgear lineups come with the interlock between panels?
Not necessarily. Each panel's internal interlocks come from the factory, but the one preventing both incomers and the tie from closing at the same time depends on the project. It is solved with trapped keys, with auxiliary contacts wired between panels or with the relay logic.