Relay NC Contact Symbol
Definition: The Relay NC Contact symbol represents a Normally Closed (NC) relay or contactor contact — defined in IEC 60617-07 and ANSI Y32.2 / IEEE 315 — a two-terminal switching element (In, Out) that is closed (conducting) when its controlling coil or actuator is de-energised, and opens (interrupts the circuit) when the coil is energised.
Also known as: NC contact, normally closed contact, break contact, b-contact, Form B contact, NC auxiliary contact.
What the Relay NC Contact symbol means
The Relay NC Contact symbol denotes a contact pair that is mechanically held in the closed (conducting) state by spring pressure when no coil current flows. This means current passes through the contact from In to Out in the normal (off) state. When the associated relay or contactor coil is energised, the armature overcomes the spring and the NC contact opens, interrupting the circuit.
In wiring and control schematics, the NC contact symbol is used to implement fail-safe or de-energise-to-trip logic: for example, a motor circuit where removing coil power returns the contact to closed, or an E-stop circuit that opens a circuit on coil energisation. Understanding that NC contacts are closed at rest is critical to correctly interpreting safety and interlock circuits.
How to identify the Relay NC Contact symbol
The Relay NC Contact symbol is drawn as two short horizontal or vertical lines (the fixed and moving contacts) with a diagonal slash or bar crossing the gap between them, indicating the contact is normally bridged (closed). The diagonal line represents the moving contact arm that bridges the two terminals in the rest state. This diagonal slash is the visual distinguisher from the NO contact symbol, which shows an open gap without a slash.
Function in a circuit
The NC contact functions as a normally-closed switch in a relay or contactor assembly. In the de-energised (normal) state the moving contact bridges In to Out, allowing current to flow. When the coil is energised, the armature moves the contact arm away, breaking continuity between In and Out. The contact returns to closed when coil power is removed. NC contacts are used for interlocking, safety shutdowns, and circuits that must be active in the absence of control power.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 (switching devices) defines the NC contact symbol as a diagonal line crossing the contact gap, indicating the bridged rest state. IEC 60947-5-1 specifies auxiliary contacts (NC included) for low-voltage control circuits. IEC 61810 covers elementary relays. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 Section 13 defines the NC contact (Form B) as a diagonal line across the switch gap, identical in appearance to IEC convention. The NC contact is also referenced as 'break contact' in IEEE documentation. |
| Key difference | IEC 60617 and ANSI Y32.2 / IEEE 315 use essentially the same NC contact symbol — a diagonal line crossing the open gap. The designator convention differs slightly: IEC labels the contact state explicitly (NC or '0' in position tables); IEEE 315 uses Form A (NO), Form B (NC), Form C (changeover) nomenclature. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Contact ratings vary by relay/contactor type. Typical auxiliary NC contact (IEC 60947-5-1): 10 A at 240 V AC, 10 A at 24 V DC. Contactor NC auxiliary: 6 A–10 A at 230 V AC. Operating time: typically 10–50 ms. Bounce time: 1–5 ms. Endurance: 1–10 million mechanical operations for relay-type contacts.
Where the Relay NC Contact symbol is used
- E-stop (emergency stop) circuits where the NC contact in the safety relay opens the motor contactor coil circuit when the E-stop button is pressed
- Interlock circuits preventing simultaneous activation of conflicting loads (e.g. forward and reverse contactors in a motor reversing circuit)
- Fail-safe control logic where removing power must open the load circuit (de-energise to trip)
- Monitoring circuits that generate an alarm signal when a relay coil energises (signal is present on NC path at rest, goes absent on coil energisation)
- Overload relay output contacts wired NC to trip a motor starter when thermal overload occurs
- PLC input circuits reading relay or contactor status via the NC contact position
Example
In a motor forward/reverse control circuit, an NC contact from the Forward contactor (K1) is wired in series with the coil of the Reverse contactor (K2); if K1 is energised, the NC contact of K1 opens, preventing K2 from energising simultaneously — this mechanical electrical interlock is a standard NC contact application.
Key facts
- The Relay NC Contact has two pins: In and Out; current flows from In to Out when the contact is in its de-energised (normal, closed) state.
- An NC contact is closed (conducting) when the relay coil is de-energised, and opens (non-conducting) when the coil is energised — the opposite of a Normally Open (NO) contact.
- IEC 60617-07 and ANSI Y32.2 / IEEE 315 both represent the NC contact with a diagonal slash crossing the switch gap, indicating the bridged rest-state position.
- NC contacts are designated Form B in ANSI/IEEE contact form terminology; NO contacts are Form A; changeover (both) are Form C.
- In safety and interlock circuits, NC contacts provide fail-safe operation: a broken wire or loss of coil power leaves the contact closed, maintaining the safe state unless the contact itself is the safety trip path.
- NC contacts experience electrical wear faster than NO contacts in circuits where the load current flows continuously through the closed state — contact erosion from sustained current carrying accelerates with load magnitude.
- Relay coil designator is K (e.g. K1); the associated NC contact on the schematic is labelled K1-NC or simply shows the K1 reference against the NC contact symbol to identify which coil operates it.
Frequently asked questions
What does the relay NC contact symbol mean in a schematic?
The Relay NC (Normally Closed) Contact symbol represents a contact that is conducting (closed) when the relay coil is off, and opens (non-conducting) when the coil is energised. It is drawn with a diagonal slash across the contact gap, indicating the bridged closed rest state.
What does the NC contact symbol look like?
The NC contact symbol shows two short lines (representing fixed and moving contact terminals) with a diagonal line crossing the gap between them — the diagonal indicates the contact arm is bridging (closing) the gap in the normal (de-energised) state. This diagonal slash is what distinguishes the NC symbol from the NO symbol, which has an open gap.
What is the difference between an NC and NO relay contact?
An NC (Normally Closed) contact is conducting when the coil is off and opens when the coil is energised. An NO (Normally Open) contact is open when the coil is off and closes when the coil is energised. NC contacts are used for fail-safe and interlock circuits; NO contacts are used to activate a load when the relay energises.
What standard defines the NC contact symbol?
IEC 60617-07 (switching devices) and ANSI Y32.2 / IEEE 315 Section 13 both define the NC contact symbol as a diagonal slash crossing the switch gap. The two standards use essentially the same graphical representation. IEC 61810 and IEC 60947-5-1 govern the electrical performance specifications for NC contacts.
What is a Form B contact?
Form B is the ANSI/IEEE designation for a Normally Closed (NC) contact — a single-pole contact that is closed in the de-energised (normal) state and opens when the coil is energised. Form A is Normally Open (NO); Form C is a Single-Pole Double-Throw (SPDT) changeover contact with both NO and NC positions.
Why use NC contacts in safety circuits?
NC contacts provide fail-safe operation: if the coil circuit wiring breaks or power is lost, the NC contact remains closed (or returns to closed), maintaining the safe default state. In E-stop circuits, the NC contact opens the motor circuit when the E-stop button is pressed (removing coil power), ensuring the motor stops on both button press and wiring failure.
How many terminals does an NC relay contact have?
An NC relay contact has two terminals — In and Out — through which current flows when the contact is closed (coil de-energised). A changeover (SPDT) relay contact has three terminals: COM, NC, and NO, where COM transfers between NC (at rest) and NO (when energised).
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