Disconnect Switch / Isolator Symbol
Definition: The Disconnect Switch / Isolator symbol represents a manually operated switching device in industrial electrical schematics that provides positive, visible isolation of a circuit or apparatus from its power source, depicted as a single- or multi-pole switch blade with a gap, referenced in IEC 60947-3 and ANSI Y32.2 / IEEE 315, and designated QS in IEC-based control diagrams.
Also known as: isolator switch, main switch, switch-disconnector, isolating switch, knife switch isolator, load break switch, QS switch.
What the Disconnect Switch / Isolator symbol means
The Disconnect Switch / Isolator symbol in an industrial wiring or schematic diagram marks the mandatory isolation point between an upstream power source and downstream equipment such as motor starters, control panels, or machine drives. It indicates that an operator can open the switch to create a confirmed, visible air gap in all phase conductors (and neutral if switched) before work begins on the downstream circuit.
The designator QS (from German: Querschnittschalter—literally 'cross-sectional switch') is the IEC standard identifier for this device type. In single-line diagrams the symbol appears as a switch-blade with Line on the supply side and Load on the equipment side; in multi-line control schematics each pole is drawn on its respective conductor line.
How to identify the Disconnect Switch / Isolator symbol
The Disconnect Switch / Isolator glyph appears as a diagonal blade (movable contact) pivoting away from a fixed contact stub, with a short straight line segment on each side representing the conductors. For multi-pole isolators, identical single-pole symbols are drawn on parallel lines and often linked with a dashed mechanical coupling line to indicate gang operation. An enclosing rectangle or box is used when the symbol represents a housed (enclosed) disconnect. The symbol is distinguished from a circuit breaker by the lack of a trip mechanism indicator, and from a contactor by its manual-only operation and positive-isolation duty.
Function in a circuit
A disconnect switch / isolator provides electrical isolation by physically separating the circuit conductors with a visible air gap. Unlike a circuit breaker, it is not designed to interrupt fault current—it is operated only when the circuit is de-energised or at or below its rated load current. The device must remain in the open position under all likely transient conditions (vibration, thermal expansion) without inadvertently closing. Most industrial isolators are padlockable in the open position to comply with lockout/tagout (LOTO) procedures per IEC 60204-1 and OSHA 29 CFR 1910.147.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-3 specifies requirements for switches, disconnectors, and switch-disconnectors; a disconnector (isolator) is defined as a device capable of opening and closing a circuit when negligible current is broken or made, providing visible isolation. IEC 60617 uses the blade-type switch symbol with isolation qualification. Designator: QS. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315-1975 defines the disconnect switch symbol in the switching-device category. NFPA 70 (NEC) Articles 230, 430, 440, and 680 mandate disconnect switches at specific locations. North American drawings use the designator DS, QS, or S. NEMA 250 covers enclosure types for disconnect switches. |
| Key difference | IEC 60947-3 classifies the device as a 'disconnector' with utilisation category AC-21A or AC-22A and mandates positive isolation. ANSI / NEC uses the term 'disconnect switch' or 'safety switch' with enclosure ratings per NEMA 250. The schematic symbols are functionally identical; designator conventions differ (QS for IEC, DS for ANSI). IEC places greater emphasis on the utilisation category specifying switching duty. |
Terminals / pins
| Pin | Name |
|---|---|
| line | Line |
| load | Load |
Typical values
Industrial ratings: 16 A to 1600 A, 400 V to 1000 V AC (IEC) or 240 V to 600 V AC (NEMA). Poles: 2, 3, or 4. Short-circuit withstand current: 10 kA to 100 kA (Ics / Icu per IEC 60947-3). Mechanical endurance: 1000–10 000 operating cycles. IP ratings for enclosed types: IP20 to IP65.
Where the Disconnect Switch / Isolator symbol is used
- Main incomer isolator in industrial motor control centres (MCCs) to de-energise the entire panel for maintenance
- Machine tool and industrial machinery supply inlets providing operator-accessible isolation per IEC 60204-1
- PLC and automation panel main supply switches ensuring safe access to control wiring
- Sub-distribution board incomers within factory and commercial buildings for zone isolation
- Renewable energy systems (solar inverter DC isolators and AC isolators) per IEC 62548 and local grid codes
- Railway rolling-stock high-voltage battery isolators providing crew-accessible emergency disconnection
- Process plant motor feeder isolation points for permit-to-work electrical isolation procedures
Example
In a three-phase motor control centre one-line diagram, a Disconnect Switch / Isolator symbol (QS1) is drawn on all three phase bars ahead of the main busbar; the Line pins (L1, L2, L3) connect to the incomer cable from the upstream transformer, and the Load pins (T1, T2, T3) connect to the busbar supplying the feeder breakers. The symbol is shown with a padlock indicator and an enclosing box denoting a housed 400 A, 4-pole switch-disconnector in an IP54 enclosure.
Key facts
- The Disconnect Switch / Isolator symbol (designator QS per IEC 60947-3) represents a manual isolation device that creates a confirmed, visible air gap in all phase conductors; it does not provide automatic overcurrent protection.
- The device has two terminal groups: Line (supply side: L1, L2, L3) and Load (equipment side); it is wired in series with the main circuit conductors and interrupts all ungrounded poles simultaneously.
- IEC 60947-3 defines utilisation categories for disconnect switches: AC-21A (switching of resistive loads), AC-22A (switching of mixed loads), and AC-23A (switching of motor loads and other highly inductive loads).
- Disconnect switches / isolators must be padlockable in the open position per IEC 60204-1 (machinery safety) and OSHA 29 CFR 1910.147 (lockout/tagout) to prevent accidental re-energisation during maintenance.
- Unlike contactors (which are designed for frequent operation), disconnect switches are rated for infrequent operation (typically 1000–10 000 total cycles) and must not be used as a regular operating switch.
- A switch-disconnector (IEC 60947-3) combines the functions of a switch (capable of making and breaking load current) and a disconnector (providing positive isolation with visible air gap) in a single device.
- The short-circuit withstand current (Ics, Icu) of the disconnect switch must be coordinated with the upstream overcurrent protective device (fuse or circuit breaker) that provides the fault-current interruption for the isolator.
Frequently asked questions
What does the disconnect switch / isolator symbol mean in an industrial schematic?
The Disconnect Switch / Isolator symbol (designated QS) means that a manually operated isolation device is present at that point, capable of opening all phase conductors to create a visible, lockable air gap for safe maintenance. It is a positive-isolation switching device, not an automatic protective device.
What does the disconnect switch symbol look like?
The Disconnect Switch / Isolator symbol looks like a diagonal blade (movable contact) pivoting away from a fixed contact stub on each conductor line. For three-phase installations, three identical single-pole symbols appear on parallel lines, often linked with a dashed coupling bar indicating they operate simultaneously from one handle.
What is the difference between a disconnect switch and a circuit breaker?
A disconnect switch (isolator) provides visible, lockable isolation of a de-energised or lightly loaded circuit—it is not designed to interrupt fault current. A circuit breaker is an automatic overcurrent protective device capable of interrupting fault currents many times its rated load current. In industrial practice, a disconnect switch is always protected by an upstream fuse or circuit breaker.
What designator is used for a disconnect switch in IEC schematics?
The IEC designator for a disconnect switch / isolator is QS, as specified in IEC 60947-3 and the IEC reference designation standard IEC 81346. In North American ANSI / NEC drawings the designator DS or S is commonly used.
What is the IEC standard for disconnect switches?
IEC 60947-3 governs switches, disconnectors, switch-disconnectors, and fuse-combination units for low-voltage equipment. It defines performance requirements including utilisation categories (AC-21A, AC-22A, AC-23A), making and breaking capacity, mechanical endurance, and dielectric withstand. IEC 60204-1 mandates the use of a main supply disconnecting device on machinery.
Can a disconnect switch interrupt a short-circuit current?
No. A disconnect switch / isolator is not rated to interrupt fault (short-circuit) currents. It has a short-circuit withstand current rating (Ics, Icu) specifying how much fault current it can survive without damage when an upstream protective device clears the fault, but the disconnector itself must never be opened under fault conditions.
What is the difference between an isolator and a switch-disconnector?
An isolator (pure disconnector per IEC 60947-3) may only be operated when the circuit current is negligible—typically only after the load has been switched off by another device. A switch-disconnector can make and break rated load current AND provide positive isolation, combining both functions. Switch-disconnectors are more versatile and are the more common industrial choice.
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