Knife Disconnect Switch Symbol
Definition: The Knife Disconnect Switch symbol represents a manually-operated visible-blade isolating switch in circuit diagrams, drawn as a single-pole switch with a hinged blade that pivots away from a fixed contact jaw, standardised under IEC 60617-07 and ANSI Y32.2 / IEEE 315, used to provide visible isolation of electrical circuits for safe maintenance, with designator QS (IEC) or DS (ANSI).
Also known as: knife switch, isolator switch, disconnect switch, blade disconnect, QS isolator, knife isolating switch, open-blade switch.
What the Knife Disconnect Switch symbol means
The Knife Disconnect Switch symbol denotes a manually-operated isolating device in which a conducting blade is physically visible in the open (isolated) position, providing unambiguous confirmation that a circuit section is de-energised. In power distribution and industrial control schematics it marks the upstream isolation point — the device that must be opened and locked out before any maintenance work on downstream equipment.
The two pins In and Out represent the line-side (supply) and load-side (downstream circuit) connections respectively. Unlike a circuit breaker, a knife disconnect does not interrupt fault current — it is operated only after fault current has been cleared or loads have been switched off. The visible open blade provides a verifiable air gap that satisfies safety lockout-tagout (LOTO) requirements per NFPA 70E and IEC 60364.
How to identify the Knife Disconnect Switch symbol
The Knife Disconnect Switch symbol is drawn as a single-pole switch with a distinctive hinged blade line rotating away from a fixed jaw or contact, shown in the open (rotated) position. The blade is depicted as a straight line pivoting from the In pin (line side) away from the Out pin (load side) jaw, creating a visible air gap. A padlock symbol may be superimposed to indicate lockout capability. The symbol closely resembles the general SPST switch symbol but the blade pivot is drawn with greater angular separation to emphasise the visible isolation gap.
Function in a circuit
A knife disconnect switch provides safe, visible isolation of a circuit section from its upstream supply. By physically rotating the conducting blade out of the jaw contacts, the operator creates an air gap that is directly visible, confirming isolation without instruments. The switch carries load current in the closed position with low contact resistance, and in the open position withstands the system voltage across the air gap. It is opened after load current has been interrupted by another device (circuit breaker or contactor) upstream.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 defines the isolating switch symbol. IEC 60947-3 (low-voltage switchgear: switches, disconnectors, switch-disconnectors, and fuse combination units) governs knife disconnect switches. The IEC designator is QS. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2-1975 / IEEE 315-1975 use an analogous open-blade switch symbol. NFPA 70 NEC Article 230 and 404 govern service disconnects. UL 508A covers panel disconnects. The ANSI/NEC informal designator is DS (disconnect switch). |
| Key difference | IEC 60617-07 and ANSI Y32.2 use very similar open-blade switch symbols. IEC practice uses the designator QS; North American/NEC practice uses DS or the label 'DISCONNECT'. IEC 60947-3 categorises utilisation categories (AC-23A, DC-23B); NEC refers to horsepower ratings. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Voltage rating: 240 V AC to 600 V AC (low voltage). Current ratings: 30 A, 60 A, 100 A, 200 A, 400 A, 600 A standard fused/unfused ratings. Short-circuit withstand (Icw): 5 kA–50 kA depending on construction. IP rating: IP20 (enclosed panel) to IP65 (outdoor weatherproof). Operating temperature: −40 °C to +40 °C ambient.
Where the Knife Disconnect Switch symbol is used
- Service entrance disconnects at the utility meter-to-panel interface for visible isolation during panel maintenance (NEC 230.70)
- Motor control centre (MCC) feeder disconnects upstream of motor starters
- Industrial machine tool isolation points for lockout-tagout (LOTO) per NFPA 70E and OSHA 29 CFR 1910.147
- Fuse box disconnects that isolate fused circuits for fuse replacement without live-work
- PV solar array DC disconnects at the output of string combiners
- Generator transfer switch main disconnect in standby power systems
Example
In a machine tool panel schematic, the Knife Disconnect Switch symbol shows its In pin connected to the three-phase 480 V bus incoming from the plant distribution, and its Out pin connected to the main contactor coil circuit and motor feeder. During maintenance, the operator opens the disconnect blade, visually confirms the air gap, installs a lockout hasp with a personal padlock, and verifies absence of voltage with a test instrument before beginning work on downstream components.
Key facts
- The Knife Disconnect Switch symbol represents a manually-operated visible-blade isolating switch, drawn as a hinged blade pivoting away from a fixed jaw contact to show a visible air gap in the open position.
- Its two pins are In (line/supply side) and Out (load side); in the open position no electrical connection exists between the two pins.
- The IEC designator is QS (disconnect switch / isolator) per IEC 60947-3 and IEC 60617-07; the North American informal designator is DS per NEC and ANSI practice.
- Knife disconnect switches are isolating devices only — they are not rated to interrupt fault current and must be opened only after load current has been cleared by a circuit breaker or contactor.
- The visible open blade satisfies lockout-tagout (LOTO) visibility requirements under NFPA 70E, IEC 60364-6, and OSHA 29 CFR 1910.147, providing unambiguous visual confirmation of circuit isolation.
- Current ratings for low-voltage knife disconnects are standardised at 30 A, 60 A, 100 A, 200 A, 400 A, and 600 A; voltage ratings cover up to 600 V AC for North American systems and 1000 V AC for IEC equipment.
- NEC Article 230.70 requires a readily accessible main service disconnect at the service entrance point; a knife disconnect switch is one of the approved device types.
Frequently asked questions
What does the Knife Disconnect Switch symbol mean in a circuit diagram?
The Knife Disconnect Switch symbol means a manually-operated isolating switch with a visible blade that swings open to create a visible air gap, confirming that the downstream circuit is de-energised. It marks a safety isolation point where the circuit can be disconnected and locked out before maintenance on downstream equipment.
What does the Knife Disconnect Switch symbol look like?
The Knife Disconnect Switch symbol shows a hinged blade line pivoting away from a fixed jaw contact, with an open gap between them to represent the isolated position. The In pin (line side) connects to the pivot end of the blade; the Out pin (load side) connects to the jaw. The symbol resembles the standard SPST switch but with a more pronounced angular opening to emphasise the visible air gap.
What is the difference between a knife disconnect and a circuit breaker?
A circuit breaker automatically interrupts fault current and can safely open under load and short-circuit conditions. A knife disconnect is an isolating switch that is not rated to interrupt fault current; it must only be operated after current has been interrupted by a breaker or contactor. The knife disconnect provides the visible open-blade isolation confirmation required for lockout-tagout safety procedures.
What standard defines the knife disconnect switch symbol?
IEC 60617-07 defines the isolating switch symbol used for knife disconnects in IEC schematics. IEC 60947-3 governs the device requirements. ANSI Y32.2-1975 / IEEE 315-1975 define equivalent symbols for North American schematics, and NEC Article 230.70 specifies service disconnect requirements in the USA.
What is the designator for a knife disconnect switch?
The IEC reference designator for a disconnect switch / isolator is QS per IEC 60617 and IEC 60947-3. In North American schematics the informal designator DS is common. Both QS and DS are numbered sequentially, e.g., QS1 or DS1.
Can a knife disconnect be used as a main service disconnect?
Yes. NEC Article 230.70 permits a listed knife-blade switch as a service entrance disconnect provided it is rated for the service current and is readily accessible. Fused knife switches (where fuses are mounted in the blade jaws) additionally provide overcurrent protection. However, most modern installations use molded-case circuit breakers (MCCBs) which offer combined overcurrent protection and isolation.
How does a knife disconnect support lockout-tagout?
A knife disconnect's open blade is directly visible, providing unambiguous confirmation that the circuit is isolated without requiring instruments. The paddle or handle of an open knife switch can accept a lockout hasp, allowing multiple workers to attach personal padlocks so the switch cannot be closed until all locks are removed — meeting NFPA 70E, OSHA 29 CFR 1910.147, and IEC 60364-6 lockout-tagout (LOTO) requirements.
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