Line and bus earthing switch interlocking

What the earthing switch is for and why it is interlocked

The earthing switch (device 57 in IEEE C37.2 nomenclature, a grounding switch in North American usage) connects an already de-energized section to the earthing grid, so that whoever works on it is protected against accidental re-energization, induced voltage or a capacitive discharge. It is the fourth of the five rules of EN 50110-1: disconnect completely, secure against reconnection, verify absence of voltage, earth and short-circuit, and protect against adjacent live parts.

The order of those rules is exactly what the interlocking turns into logic. An earthing switch that closes onto an energized section causes a bolted fault to earth right where the operator stands. Even when the switch has short-circuit making capability (classes E1 and E2 of IEC 62271-102), that capability is a margin for error, not a way of operating. This guide covers the logic, not the construction of the device.

Line earthing switch

The line earthing switch (57L) earths the section between the line disconnector and the bay outlet. Its usual conditions are:

1. Line disconnector (89L) open. It is what separates the line from the local substation. In breaker-and-a-half and ring bus, where the line depends on two breakers, the isolation must exist toward both sides.
2. Absence of voltage on the line. The local disconnector does not separate the line from the remote end. Absence of voltage is checked with the line voltage transformer, through an undervoltage function or a voltage presence detector, or with a detector at the work location.
3. Confirmation from the remote end. The other end must be open and isolated. In most systems this is operational coordination between control centers, not an interlocking signal. Some utilities complement it with a remote permissive over the communication channel.
4. Valid position of every device involved.

The reverse relationship closes the logic: with the 57L closed, the 89L cannot close. And before re-energizing, every earth on the section must be removed, both fixed and portable. Portable grounds have no auxiliary contact, so the interlocking cannot see them: their control is procedural.

Induced voltage. A de-energized line running parallel to another one in service can carry induced voltage and, with the earthing switch closed, current can flow through it. IEC 62271-102 classifies line earthing switches by their capability to switch those currents (classes A and B). The dead-line threshold of the interlocking must tell an energized line from one with induced voltage. If the threshold is set too low, the earthing switch is never enabled; if it is set too high, it is enabled on a live line.

Bus earthing switch

The bus earthing switch earths a whole bus for maintenance. Its condition is easy to write and hard to implement: every disconnector that connects anything to that bus must be open. That includes the bus disconnectors of each bay, those of the bus coupler and those of the bus sectionalizer, if any. Absence of voltage measured by the bus voltage transformer is usually added.

Because the conditions are spread over all the bays, the bus earthing switch is a station interlock. In a hardwired substation it requires running contacts from every bay; in a numerical one, every controller must publish the position of its disconnectors and the one handling the earthing switch must receive them by GOOSE. If a single bay does not report, the permissive must drop.

The reverse relationship is also station-wide: with the bus earthing switch closed, no disconnector toward that bus may close. In a double bus this includes the bays that are on the other bus at that moment, because they could try to transfer to the earthed bus.

Bay and breaker earthing switches

Many bays have earthing switches on both sides of the breaker, between it and its disconnectors. They allow work on the breaker or on the current transformers. Their condition is that the disconnectors bounding that section are open. Since the section lies between disconnectors of the same bay, the logic is local. To close those disconnectors again, those earthing switches must be open.

Enclosed equipment

Medium-voltage switchgear and GIS often use a three-position disconnector (closed, open and earthed), in which the mechanism itself prevents going from closed to earthed without passing through open. That solves the relationship with its own disconnector, but not the absence of voltage or the coordination with other sources, which still depend on electrical logic or on procedure. In medium-voltage switchgear, IEC 62271-200 requires the earthing switch to be interlocked with its associated switching device when its making capacity is lower than the peak current of the circuit. Switchgear interlocks are covered in the guide on MV switchgear and RMU interlocking.

How it is verified

Each condition is tested positively and negatively. For the line earthing switch, the dead-line threshold is checked by injection into the voltage transformer circuit. For the bus earthing switch, the position of each disconnector on the bus is negated one at a time, and the loss of each bay’s message is simulated. Then the reverse relationship is checked: with the earthing switch closed, no associated disconnector may accept a command. The full test plan is in the guide on interlocking tests.

We configure and test your logic

Our team designs, configures and tests the logic of line and bus earthing switches, including station conditions by GOOSE, in medium, high and extra-high voltage substations and in industry. Field testing is done in Colombia, or elsewhere on request. We load and change the logic, interlocks and signals remotely through our remote relay and controller configuration service, without waiting for a site visit. See secondary engineering or contact us.

Related guides

• Bus disconnector interlocking: conditions
• Interlocking tests: what is verified at FAT and SAT
• Interlock bypass: release, risks and control

References

• EN 50110-1, Operation of electrical installations, 6.2 (five rules for dead working).
• IEC 61936-1:2021, Power installations exceeding 1 kV AC, 6.2.16 (interlocking) and 8.4 (measures for working de-energized).
• IEC 62271-102:2018, Alternating current disconnectors and earthing switches (classes E0, E1 and E2; induced current classes A and B).
• IEC 62271-200, AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV.
• IEC 61850-7-4, logical nodes CILO and XSWI.
• IEEE C37.2, Standard for Electrical Power System Device Function Numbers.

Frequently asked questions