Contact Blocks Explained: NO vs NC, How to Choose
The contact block is the electrical heart of a push button — it's the part that actually switches the circuit. Understanding NO vs NC is essential for correct wiring and safe operation.
What is a contact block?
Most industrial push buttons are modular — the button head (the part you press), the body/mounting, and the contact block (the electrical switch) are separate pieces. The contact block snaps or screws onto the rear of the button body inside the panel.
This modularity means you can often replace just the contact block when a button starts working intermittently — the head and body are usually fine. Contact blocks are consumable parts, rated for a specific number of operations (typically 1-10 million cycles).
NO vs NC: the fundamental difference
NO — Normally Open
Circuit is open (off) until you press the button. Pressing closes the circuit and allows current to flow.
Use for: Start, Enable, Acknowledge, Go
Example: Schneider ZB4-BZ101, Eaton M22-K10
NC — Normally Closed
Circuit is closed (on) until you press the button. Pressing opens the circuit and stops current.
Use for: Stop, Emergency Stop, Safety circuits
Example: Schneider ZB4-BZ102, Eaton M22-K01
Why stop buttons use NC contacts (safety)
Stop buttons and emergency stops must use NC contacts. Here's why: if a wire breaks or comes loose from an NC circuit, the circuit opens — which is the same as pressing the stop button. The machine stops safely.
If you wired a stop button with an NO contact and a wire broke, nothing would happen — the machine would keep running and you'd have no way to stop it with that button. This is called fail-safe design and it's required by IEC 60204-1 (Safety of Machinery — Electrical Equipment).
Stacking contact blocks
Most modular button systems allow you to stack multiple contact blocks on a single button. Common configurations:
- 1NO + 1NC — Start button that also signals "start pressed" to the PLC. The NO energizes the motor contactor, the NC sends a discrete input to the PLC.
- 2NO — Redundant start contacts for critical applications, or one NO for the motor and one NO for an indicator light.
- 2NC — Redundant stop contacts for safety circuits (each NC feeds a separate safety relay channel).
When to replace a contact block
Signs that a contact block is failing:
- Intermittent operation — button works sometimes but not others. The silver contacts are pitted or burned.
- Requires hard press — contact spring has weakened, needs more force to make/break.
- Visible arcing — black marks or smell of burning from the contact area.
- Exceeded rated cycles — if a button is pressed 200 times per day and the block is rated for 1M cycles, replace it every 13-14 years as preventive maintenance.