Grounding Switch Symbol
Definition: The Grounding Switch symbol represents a mechanically-operated isolating switch used in high-voltage and industrial electrical systems to connect a de-energised line conductor to Earth ground for personnel safety during maintenance, as standardised in IEC 62271-102 (AC disconnectors and earthing switches) and IEEE C37.30 (high-voltage air switches).
Also known as: earthing switch, earth switch, safety earthing switch, maintenance grounding switch, ground and test switch, temporary earth switch.
What the Grounding Switch symbol means
The Grounding Switch symbol denotes a switch whose sole purpose is to connect the Line terminal of a circuit to Earth (ground) after the main isolator has been opened, ensuring that any stored charge on the conductors or capacitively coupled voltages are safely discharged and the conductors are held at earth potential during maintenance work. The switch is operated only when the circuit is de-energised and the upstream isolator is open-locked; it is not designed to interrupt load current.
In schematic diagrams the Grounding Switch symbol is drawn as a standard switch symbol with a ground (earth) connection at one end and the Line connection at the other. It is typically shown on one-line diagrams (single-line diagrams, SLDs) and panel schematics for substations, switchgear, and industrial motor control centres (MCCs) to indicate where maintenance earthing provisions are installed.
How to identify the Grounding Switch symbol
The Grounding Switch symbol is drawn as a single-pole switch blade (a diagonal line or arc representing the moving contact) with the stationary contact terminating at an Earth Ground symbol (three horizontal bars of decreasing length). The switching action is shown as the blade connecting the line conductor (top, Line pin) to the earth terminal. In IEC single-line diagram notation the earthing switch is a distinct symbol from the disconnector (isolator): the earthing switch blade is shown in the closed (earthed) position pointing downward toward the ground symbol, or in the open position as a raised blade.
Function in a circuit
A Grounding Switch performs two functions: it discharges stored electrical energy (capacitive charge, induced voltage) from de-energised conductors to earth, and it maintains those conductors at earth potential throughout maintenance operations, protecting workers from accidental re-energisation and induced voltages from adjacent live circuits. In high-voltage switchgear the grounding switch is interlocked with the main isolator so it cannot be closed unless the isolator is fully open, and the isolator cannot be closed unless the grounding switch is fully open. Some grounding switches include a make-before-break function (fast-acting earthing switch) capable of conducting fault-current levels if inadvertent re-energisation occurs.
Standards: IEC vs ANSI
| IEC 60617 | IEC 62271-102 (High-voltage switchgear — AC disconnectors and earthing switches) defines the design, testing, and ratings of earthing switches. IEC 60617 defines the earthing switch symbol as a single-pole switch blade connected to an earth symbol (S00196), used on single-line diagrams of substations and switchgear panels. |
|---|---|
| ANSI/IEEE 315 | IEEE C37.30 (IEEE Standard Definitions and Requirements for High-Voltage Air Switches, Bus Supports, and Switch Accessories) and IEEE C37.20 (Switchgear Assemblies) govern grounding switches in North American practice. In ANSI one-line diagrams the grounding switch is shown as a switch contact with a ground symbol, consistent with IEEE 315-1975 (ANSI Y32.2) notation for maintenance earthing devices. |
| Key difference | IEC 62271-102 and IEEE C37.30 use virtually identical schematic symbols for the earthing/grounding switch — a switch blade connected to an earth symbol. The terminology differs: IEC uses 'earthing switch', North American ANSI/IEEE uses 'grounding switch'. IEC 62271-102 defines additional classifications (ES1, ES2, ES3) based on electromagnetic and electrostatic induction withstand capability, which are not part of the IEEE C37.30 classification. |
Terminals / pins
| Pin | Name |
|---|---|
| line | Line |
Typical values
Voltage ratings: 1 kV to 1200 kV (system voltage dependent). Short-circuit making capacity: up to 63 kA peak for fault-making earthing switches (IEC 62271-102). Rated short-time withstand current: 20–63 kA (1 s or 3 s). The grounding switch is rated for zero interrupting duty — it must never be operated under load current.
Where the Grounding Switch symbol is used
- High-voltage substations — installed on bus bars and feeder lines to earth de-energised conductors before maintenance on transformers, cables, or busbars
- Medium-voltage switchgear cubicles — integrated into withdrawable circuit breaker cells to earth the cable compartment when the breaker is withdrawn for inspection
- Motor control centres (MCCs) — grounding switches in the cable compartment allow safe access to motor terminals and cable lugs during motor maintenance
- Power transmission lines — portable or permanently installed line grounding switches at maintenance access points protect linemen from induced voltages on long transmission circuits
- Wind turbine and solar farm collection systems — earthing switches isolate medium-voltage cable sections for inverter or transformer maintenance in the field
- Industrial high-voltage process equipment — furnaces, drives, and reactors use grounding switches in their MV supply switchgear for maintenance isolation
- Test laboratories — high-voltage test sets include grounding switches (often called ground and test devices) to safely discharge test objects before handling
Example
In a 33 kV substation single-line diagram, the Grounding Switch symbol is shown connected between the line side of the 33 kV disconnect isolator and the earth busbar. During normal operation the grounding switch is open. When the transformer bay is taken out of service for maintenance, the sequence is: open the circuit breaker, open the isolator (disconnect), verify dead, then close the grounding switch. The symbol shows the switch blade closed downward against the earth symbol, and an interlock symbol indicates it is mechanically prevented from closing unless the isolator is fully open.
Key facts
- The Grounding Switch symbol represents a maintenance safety device that connects a de-energised line conductor to Earth ground to discharge stored energy and prevent re-energisation during maintenance, per IEC 62271-102 and IEEE C37.30.
- The Grounding Switch has a single electrical pin: Line (id: line), which is the circuit conductor terminal that is connected to earth when the switch is closed.
- A Grounding Switch is NOT designed to interrupt load current — it is operated only after the circuit is confirmed de-energised and the upstream isolator is open-locked; closing it on a live circuit would cause a bolted short-circuit.
- Grounding switches are interlocked with the main isolator (disconnector) by a mechanical or key-exchange system: the earthing switch cannot be closed unless the isolator is open, and the isolator cannot be closed unless the earthing switch is open (IEC 62271-102 requirement).
- Fast-acting earthing switches (fault-making earthing switches, IEC ES2/ES3 class) are rated to close onto and carry short-circuit currents up to 63 kA, protecting against the consequence of inadvertent re-energisation of an earthed conductor.
- IEC 62271-102 classifies earthing switches by their electromagnetic induction (ES1: no requirement, ES2: rated making current, ES3: rated making + breaking current) and electrostatic induction capability.
- In schematic notation the Grounding Switch symbol is a switch blade with its stationary contact terminating at an Earth Ground symbol (⏚, IEC S00196), distinguishing it from a regular isolator or circuit breaker symbol.
Frequently asked questions
What does the grounding switch symbol look like?
The Grounding Switch symbol is drawn as a single-pole switch blade (a diagonal line or arc) with the stationary contact connected to an Earth Ground symbol (three horizontal bars of decreasing length, ⏚) and the moving contact connected to the Line terminal. In single-line diagrams it is shown in the open position (blade raised) or closed position (blade touching the earth contact).
What does a grounding switch do?
A grounding switch connects de-energised electrical conductors to Earth ground during maintenance operations. This discharges stored capacitive and inductive energy, dissipates induced voltages from adjacent live circuits, and holds the conductors at 0 V (earth potential) to protect maintenance personnel from electric shock. It is always operated after the main circuit breaker and isolator are opened.
What is the difference between a grounding switch and a circuit breaker?
A circuit breaker is designed to interrupt normal load current and fault current under live conditions. A grounding switch is designed only for maintenance earthing — it has no current interrupting rating and must only be closed onto de-energised conductors. Closing a grounding switch on a live circuit causes a bolted fault. Some fault-making earthing switches (IEC ES2/ES3) can close onto a live circuit and withstand the resulting fault current, but cannot break it.
What standard governs grounding switches?
IEC 62271-102 (High-voltage switchgear and controlgear — Part 102: AC disconnectors and earthing switches) is the primary international standard for earthing switches. IEEE C37.30 (High-Voltage Air Switches, Bus Supports, and Switch Accessories) governs grounding switches in North American practice. The schematic symbol is defined in IEC 60617 and IEEE 315-1975 (ANSI Y32.2).
Can a grounding switch be closed on an energised circuit?
Standard grounding switches (IEC Class ES1) cannot be closed onto an energised circuit — doing so would create a bolted short-circuit. Fault-making earthing switches (IEC Class ES2 and ES3) are specially designed and tested to close onto energised circuits and carry the resulting fault current, providing protection if a circuit is accidentally re-energised while maintenance earthing is in place.
What is the interlock requirement for a grounding switch?
IEC 62271-102 requires that a grounding switch be mechanically or electrically interlocked with its associated isolator (disconnector): the grounding switch cannot be closed unless the isolator is fully open, and the isolator cannot be closed unless the grounding switch is fully open. This interlock is typically implemented as a mechanical key-exchange system or a direct physical coupling between the two devices.
What is the difference between an earthing switch and a grounding switch?
Earthing switch (IEC terminology, IEC 62271-102) and grounding switch (North American/IEEE terminology, IEEE C37.30) refer to the same device and concept: a switch that connects de-energised conductors to Earth for maintenance safety. The functional requirements, ratings, and schematic symbol are equivalent in both standards; only the terminology differs.
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