An interlock is a safety mechanism designed to prevent a device from operating unless specific conditions are met. It refers to physical or electrical systems that protect both personnel and equipment from operational errors or unexpected failures.
1. Basic Concept of Interlock
Literally meaning "to lock together," an interlock ensures that a system will not function or certain actions will be prohibited unless a predefined set of safety criteria is satisfied.
Examples of interlock logic include:
Preventing high-voltage output while a protective cover is open.
Preventing a heater from activating while the cooling fan is stopped.
2. "Interlock" vs. "Safety Interlock"
While these terms are often used interchangeably, there are subtle differences in their professional nuances:
- Interlock
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Used in a broad sense, this can include mechanisms intended for procedural sequencing as well as safety.
Example: Switch B cannot be operated unless Switch A is turned on (used to prevent equipment damage or ensure correct operational steps).
- Safety Interlock
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This term specifically focuses on the prevention of human injury. Based on international safety standards (such as ISO 12100), its purpose is to physically interrupt hazardous operations while a person is in a danger zone.
Key Feature: These systems require high-reliability components, such as safety relays or switches with positive opening mechanisms, rather than standard control circuits.
3. The Role of "Interlock Terminals" in Power Supplies
The rear panels of industrial programmable power supplies often feature terminal blocks or connectors labeled "Remote Interlock."
In most power supply units, the interlock terminal functions as a "contact input":
- Closed (Short) State: The interlock is released, and power output is enabled.
- Open State: The interlock is activated. Output is immediately shut down, and the unit will not accept "ON" commands from the control panel.
At the time of shipment, these terminals are typically fitted with a shorting pin (jumper wire). While this allows for immediate operation, it is essential to remove this jumper and connect the terminals to an external safety device to ensure a safe working environment.
4. Practical Applications and Connection Examples
Interlock terminals are used to construct a "safety chain" for the entire system.
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Door Switches and Safety Fences
This is the most common application. Limit switches (door switches) are installed on test room doors or power supply rack enclosures.
- Configuration: Wired so that the switch is ON (closed) only when the door is fully closed.
- Effect: If an operator accidentally opens the door, the power output is forced to stop instantly.
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Integration with Emergency Stop (E-Stop) Buttons
The contacts of an Emergency Stop button are wired in series with the interlock terminal.
- Configuration: Pressing the E-Stop button opens the circuit, triggering an immediate power shutdown.
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Coordination with Other Equipment (Interlock Chain)
When synchronizing multiple devices, the alarm outputs (fault signals) of each device are connected in series to the power supply's interlock terminal.
Example: Chiller error detection + Ventilation fan confirmation + Door switch.- Effect: If any single element in the chain fails (opens), the "Safety Chain" is broken, and the power shuts down.
5. Precautions for Design and Operation
When utilizing interlock functions, please keep the following points in mind:
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Use of "Normally Closed" (NC) Contacts:
Interlock circuits should be designed with the logic that "closed equals safe." This ensures a fail-safe operation: if a wire is accidentally broken or disconnected, the circuit opens and safely shuts down the output. -
Prohibition of Automatic Recovery:
It is extremely dangerous for high voltage to resume automatically the moment a door is closed. The system should be configured (e.g., using a self-holding circuit) so that once an interlock is triggered, it requires a manual reset or restart by the operator after the cause is cleared. -
Electrical Noise Countermeasures:
Since interlock terminals often operate on low-level signals, long cable runs can pick up electrical noise, leading to malfunctions. Consider using shielded cables or isolating the signal using an external relay.