MV Load-Break Switch and RMU: Operation and Testing

What it is and where it is used

The medium-voltage switch-disconnector (also called a load-break switch) is a switching device with two functions: it makes and breaks service currents and, when open, it guarantees an isolating distance like that of a disconnector. Its standard is IEC 62271-103, which covers switches above 1 kV up to 52 kV. Its disconnecting function is also verified against IEC 62271-102.

You find it in the ring cubicles of distribution networks (RMU, Ring Main Unit), in the incoming cubicles of industrial customers and in the feeders to distribution transformers. Unlike the medium-voltage vacuum circuit breaker, it does not interrupt short circuits. It can close onto one, because it has a rated short-circuit making capacity, but the fault is cleared by a fuse or by an upstream breaker.

Main parts

• Main contacts and, in gas-insulated designs, an arc-extinction system that rotates the arc so it does not stay at one point. Other designs use vacuum interrupters or air break.
• Spring mechanism, which gives an operating speed independent of the operator. In fuse combinations it is a stored-energy mechanism with charged closing and opening springs.
• Earthing switch, separate or built into a three-position switch.
• Current-limiting fuses with a striker, on the transformer feeders.
• Mechanical interlocks, position indicators, auxiliary contacts and, as options, a motor and a shunt release.
• Enclosure, which in gas-insulated RMUs is a sealed tank.

How it works

Load switching. The operator charges the spring with the handle, or the motor does it; when released, the spring moves the contacts at constant speed. On opening, the arc is lengthened and cooled in the extinguishing medium until it goes out at a current zero. Then the isolating distance remains.

Three-position switch. In many RMUs, a shaft carrying the contact blades rotates between three positions: CLOSED (contacts on the busbar), OPEN and EARTHED (contacts on the earth point). Because the busbar and the earth are on opposite sides of the travel, it is physically impossible to close to busbar and to earth at the same time. To go from closed to earthed you have to stop at open: the operating front forces this and has separate access points for the switching function and the earthing function.

Switch-fuse combination (IEC 62271-105). The fuses extend the breaking capacity of the assembly beyond that of the switch. The sequence is:

1. On a high fault current, the fuses limit the current before its first peak and interrupt it.
2. The striker of the first fuse to operate releases the mechanism, and the switch-disconnector opens all three poles.
3. With low fault currents, above the minimum melting current but below the minimum breaking capacity of the fuse, the striker makes the switch complete the interruption.

The transfer current is the value at which fuse and switch share the interruption: above it the fuses clear the fault and below it the switch does, tripped by the striker. Each combination is certified with a list of reference fuses given by its manufacturer.

Variants and differences from other levels

• General-purpose switch-disconnector: handles all the currents that the standard defines for distribution networks (load, closed loop, unloaded cables, earth fault).
• Limited-purpose and special-purpose: only some of those operations, or particular duties such as capacitor banks or motors.
• SF6, vacuum or air, depending on the manufacturer and the installation.

In high and extra-high voltage the switch-disconnector practically does not exist as such. There the high-voltage disconnector is used, which does not break load, together with a power circuit breaker. Transformer protection with fuses is explained in the guide on the medium-voltage fuse, and the cubicle as a whole in medium-voltage switchgear.

Rated values and nameplate

The switch-disconnector nameplate shows, among other things:

• rated voltage (Ur), insulation levels (Ud and Up) and rated current (Ir);
• short-time withstand current (Ik) with its duration (tk) and peak current (Ip);
• short-circuit making capacity (Ima);
• breaking currents: mainly active load (Iload), closed loop (Iloop), cable charging (Icc), earth fault (Ief1) and cable and line charging under earth fault (Ief2);
• endurance classes: mechanical (M1 or M2), electrical (E1, E2 or E3) and capacitive switching (C1 or C2).

For example, a manufacturer declares for its three-position switch the classes M1 (1000 mechanical operations), E3 (100 operations at load current and 5 short-circuit makes) and C2. The earthing switch has its own classes under IEC 62271-102. Combinations add the transfer current and the maximum power of the protected transformer.

Integration with protection and control

The switch-disconnector has no relay of its own. It is tripped by the fuse striker or by a shunt release, for example from the transformer temperature. The auxiliary contacts report the position of the switching function and of the earthing function to the controller, the RTU or SCADA.

In IEC 61850 it is modeled with the logical nodes XSWI (switch position), CSWI (control) and CILO (interlocking). When the RMU is remotely controlled, the motor and the auxiliary contacts must be wired correctly and the interlocking logic must be consistent with the mechanics.

The RMU and its tests

An RMU (Ring Main Unit) is a compact assembly of medium-voltage cubicles, almost always gas-insulated. It has three-position switch-disconnectors for the ring in and out. For the transformer it carries a switch-fuse combination or a circuit breaker with a relay; the choice is explained in fuse or breaker to protect an MV transformer. The enclosure follows IEC 62271-200, the load-break switches IEC 62271-103 and the combinations IEC 62271-105. The tests leave the unit ready to energize, with the cables connected, and are repeated after a fault or after a high number of operations. The acceptance criteria are those of the manufacturer and its standards, not generic values.

Self-powered or auxiliary-powered relay. When the RMU has a relay with auxiliary supply, it is tested with secondary injection of the phase and ground overcurrent protection and a real trip. Many RMUs use self-powered relays: they take their energy from dedicated CTs and trip a low-energy coil. In that case primary injection is the test that covers the whole chain (CT, relay and coil) and confirms ratio and polarity; secondary injection is used only with the adapter that the manufacturer specifies.

Switch-fuse combination. Verify that the fuses are of the type and rating on the manufacturer’s reference list and that the striker of any of them opens all three phases.

Common faults and what to check first

• The switch does not close after changing fuses: a striker was left tripped or the fuse was fitted with the striker on the wrong side.
• Three-phase trip with no apparent fault: a fuse aged by inrush currents. If one or two fuses blow, the manufacturer recommends replacing all three.
• Stiff mechanism or one that does not complete the operation: corrosion, lack of lubrication or an uncharged spring.
• Wrong position signal in SCADA: misadjusted auxiliary contacts.
• Low gas pressure: a leak in the tank; do not operate beyond what the manufacturer indicates.

Testing and commissioning with Tiza Group

At Tiza Group we test and commission switchgear and RMUs with switch-disconnectors in substations and industry, from medium voltage up to the high and extra-high voltage bays that feed them, in Colombia or elsewhere on request. Contact us and we will propose the scope.

Related guides

• High-voltage disconnector: types, parts, classes and tests
• Medium-voltage switchgear
• Medium-voltage fuse
• Fuse or breaker to protect an MV transformer
• Medium-voltage vacuum circuit breaker
• On-site withstand voltage test
• Contact resistance test
• Phase and ground overcurrent protection

References

• IEC 62271-1, High-voltage switchgear and controlgear: common specifications.
• IEC 62271-102, Alternating current disconnectors and earthing switches.
• IEC 62271-103, Switches for rated voltages above 1 kV up to and including 52 kV.
• IEC 62271-105, Alternating current switch-fuse combinations.
• IEC 62271-200, AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV.
• IEC 60282-1, High-voltage fuses: current-limiting fuses.
• IEC 61850-7-4, Logical nodes XSWI, CSWI and CILO.

Frequently asked questions