Earthing (Grounding) Switch: E0, E1, E2 and Interlocking

Short answer

The earthing (grounding) switch connects an already de-energized section to ground so that work can be done on it safely. IEC 62271-102 classifies it by its ability to close onto a short circuit (E0, E1 and E2) and, on lines, by its ability to switch induced currents (classes A and B). It is always interlocked with the disconnector or the circuit breaker of the same section.

What it is and where it sits

The earthing switch (grounding switch in North American usage), called an earthing disconnector in the standard, is a mechanical device that connects the conductors of a circuit section to ground. It is not for clearing faults: it is closed after opening the circuit breaker and the disconnectors, so that a voltage that reappears by mistake, or one induced by another line, discharges to ground and not through people.

In an outdoor bay it is usually mounted on the disconnector itself, on the line side or the equipment side, with an arm that swings to a fixed contact on the terminal; it also exists as a stand-alone device. In medium-voltage switchgear it is in the cable compartment, and in RMUs and GIS it is part of the three-position disconnector. Its standard is IEC 62271-102, and for metal-enclosed switchgear the requirements of IEC 62271-200 are added. The guides on the high-voltage disconnector and medium-voltage switchgear describe the complete equipment.

Parts

Arm or moving blade, with its contact, which in outdoor designs usually has a counterweight to balance the swing.
Fixed contact on the disconnector terminal or on the busbar of the section.
Flexible connection or braid between the moving part and the structure, and a conductor to the ground grid.
Operating mechanism of its own, manual or motorized, with auxiliary contacts; the motor operator of the disconnector is explained separately.
Mechanical interlock with the disconnector and provision for a padlock.

How it works

Closing the earthing switch is the last step in isolating a section. The full order of a switching operation is described in substation switching operations. The sequence is always the same: open the circuit breaker, open the disconnectors on both sides, verify absence of voltage and close the earthing switch. To return the section to service, the sequence is run in reverse.

Isolating a section

Opening the circuit breaker. Interrupts the current in the section
Opening the disconnectors. On both sides of the section
Verifying absence of voltage. Before grounding
Closing the earthing switch. To return the section to service, run the sequence in reverse

Closing onto a fault. If the section is still energized by mistake, the switch closes against a bolted short circuit. IEC 62271-102 classifies that capability:

E0: no short-circuit making capacity.
E1: with short-circuit making capacity; the first edition of the standard assigned it two closing operations.
E2: with more rated closing operations; the first edition gave it five. One manufacturer declares, for example, E2 with 5 closing operations for the earthing switch of its 36 kV switchgear.

Since the 2018 edition (current with its 2022 amendment 1) that clause is called classification by short-circuit making capacity, and the same edition added a mechanical endurance class M1 for earthing switches.

The reverse case, energizing the line with the earthing switch left closed by mistake, is cleared by the line protection with switch-onto-fault (SOTF) logic, which trips without delay during a window after closing.

Induced current. When the grounded line runs parallel to an energized one, two phenomena appear. With both ends grounded, magnetic coupling drives a current around the line-to-ground loop (electromagnetically induced current). With only one end grounded, capacitive coupling leaves an electrostatically induced current and voltage. The switch must make and break those currents without damage. The standard defines two classes for voltages of 52 kV and above: A, for relatively short lines or lines with little coupling, and B, for long lines or lines with strong coupling, with separate rated values for each type of coupling. A switch can have a combined class, such as A+E1.

Differences by voltage level

Medium voltage: the earthing switch of a switchgear panel almost always has making capacity (E1 or E2), because the risk of closing onto an energized cable is real. It is interlocked with the circuit breaker or the withdrawable truck and with the cable compartment door, and is complemented with voltage presence indicators.
High voltage: earthing switches on line and equipment disconnectors. Induced current starts to matter on parallel lines.
Extra high voltage: long lines and double circuits make class B, or higher values agreed with the manufacturer, decisive. On lines of 550 kV or more, high-speed earthing switches (IEC 62271-112) are also used, which ground the open phase during single-pole reclosing to extinguish the secondary arc.

Interlocking and wiring

IEC 62271-200 states that an earthing switch whose making capacity is lower than the peak current of the main circuit must be interlocked with its disconnectors. In practice:

Mechanical: the disconnector and its earthing switch cannot be closed at the same time. In three-position units it is physically impossible.
Electrical or logic: the earthing switch closes only with the adjacent disconnectors open; many schemes also require absence of voltage, measured by the line VT.
Signals: its auxiliary contacts go to the bay controller as a double point, to SCADA and to the interlocking of neighboring bays.
IEC 61850: position in XSWI, control in CSWI and interlocking in CILO, the same as the disconnector.

The full conditions for the line earthing switch and the busbar earthing switch, with their differences between bay and station level, are in interlocking of line and busbar earthing switches.

Common faults and what to check first

Incomplete closing: the operator reaches the end of its stroke, but the arm does not enter the fixed contact. Check the adjustment and the counterweight.
Arc on opening: induced current higher than the rated class, or a slow operation.
Corroded or broken braid or ground connection: the switch closes, but does not ground properly.
Interlock bypassed and never restored.
Wrong position signal: a misadjusted auxiliary contact that blocks operations in neighboring bays.

Testing and maintenance

The disconnector testing guide describes the procedure. For the earthing switch the following are verified:

contact resistance of the circuit to ground, including the braid;
alignment, penetration and contact pressure;
every interlock condition, mechanical and electrical, in local, in remote and with the hand crank;
auxiliary contacts and the signal at the controller;
inspection after closing onto a fault, according to the manufacturer’s manual.

During testing, portable grounds are kept on both sides: induction from neighboring lines can be high.

Testing and configuration with Tiza Group

At Tiza Group we test earthing switches and their interlocking on site, in Colombia or elsewhere on request, in medium-voltage switchgear and in high and extra-high-voltage bays of substations and industry. Write to us to coordinate the tests.

References

IEC 62271-102:2018+AMD1:2022, Alternating current disconnectors and earthing switches.
IEC 62271-112:2021, High-speed earthing switches for secondary arc extinction on transmission lines.
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 XSWI, CSWI and CILO.

Frequently asked questions

What is the difference between an E0, E1 and E2 earthing switch?

The E0 has no short-circuit making capacity. The E1 and E2 do, and they differ in the number of closing operations they withstand: the original edition of the standard assigned two to the E1 and five to the E2. The E2 is common in medium-voltage switchgear.

Why does an arc appear when opening the earthing switch of a de-energized line?

Because an energized parallel line induces current in the grounded line, by magnetic or capacitive coupling. The switch must have the right induced current class (A or B) to open that current without damage.

Why can't I close the earthing switch if the disconnector is closed?

This is the normal interlock: the earthing switch is blocked while the disconnector is closed and can only be closed with everything open. Open the disconnector first; if the control still does not let you close it, a position signal or a voltage-absence signal is usually missing.