# NC — Normally closed

> Source: https://rodlesspneumatic.com/resource-center/pneumatic-glossary/nc/
> Language: en-US
> Category: Valves & circuits

## Definition

Normally closed: Flow path is closed in the defined normal or de-energized condition. Confirm which ports the catalog calls normally closed. Read the applicable product data and technical standard before treating a catalog value as interchangeable.

## Key facts

- **Term:** NC
- **Full form:** Normally closed
- **Selection effect:** Confirm which ports the catalog calls normally closed.

## NC: definition and engineering context

Normally closed. Flow path is closed in the defined normal or de-energized condition.

The term belongs to the valves & circuits context. Its exact limits can depend on the cited standard, catalog convention, unit system, reference state, and component configuration.

## Why NC affects pneumatic selection

Confirm which ports the catalog calls normally closed.

- NO: Use when the required fail-state needs flow.
- Pilot operated: Check minimum pilot or differential pressure.
- Direct acting: Useful where zero differential operation matters, usually with lower maximum flow.
- Spool: Leakage, overlap, and contamination tolerance affect application behavior.

## How to evaluate NC in a catalog or replacement project

Read the value together with its unit, test basis, operating pressure, temperature, circuit state, and manufacturer definition.

For replacement work, compare the complete datasheet and drawing. A matching abbreviation or nominal size does not by itself prove functional or dimensional interchangeability.

- Normally open: Flow path is open in the defined normal or de-energized condition.
- Pilot-actuated valve: Main element is shifted using pilot pressure.
- Direct-actuated valve: Solenoid or mechanical input acts directly on the sealing element.
- Sliding valve element: Internal element routes air between valve ports by moving along a bore.

## What to verify before selection

- Confirm which ports the catalog calls normally closed.
- Use when the required fail-state needs flow.
- Check minimum pilot or differential pressure.
- Useful where zero differential operation matters, usually with lower maximum flow.

## Common mistakes

- Treating NC as a universal definition without checking the manufacturer or standard context
- Mixing gauge and absolute values, incompatible units, or different reference conditions in one comparison
- Selecting a component from the term or nominal value alone without checking the complete operating point and drawing

## Frequently asked questions

### What does NC mean in pneumatics?

Normally closed. Flow path is closed in the defined normal or de-energized condition.

### Why does NC matter when selecting a component?

Confirm which ports the catalog calls normally closed.

### Can NC values be compared directly between catalogs?

Only after the unit, reference condition, test method, operating point, and manufacturer definition have been reconciled.

## Technical references

- [ISO 5598:2020](https://www.iso.org/standard/67985.html): Provides the international fluid-power vocabulary used to interpret pneumatic terminology.
- [ISO 1219-1:2012](https://www.iso.org/standard/60184.html): Defines graphical symbols and circuit-documentation principles for fluid-power systems.