Today, Wuxi Rihuan Sensor Technology Co., Ltd. will introduce to you how to configure the "safety relay" for the safety door lock - the dual-channel wiring and EDM function.

The output signal of the safety door lock cannot be directly connected to the input points of a regular PLC. Regular PLCs do not have a safety redundancy mechanism. If the contacts are stuck or the circuit is short-circuited, the PLC may still receive the "door is closed" error signal, causing the equipment to operate normally when the protective door is opened. The safety door lock must be connected to a safety relay or a safety controller, and the safety relay will execute the "detect - judge - cut-off" safety logic.

1. Why must we use a safety relay?

Safety relays have an essential difference from ordinary intermediate relays:

Forced directional contacts: Normally open and normally closed contacts are mechanically linked. Once the contacts are welded and stuck, the normally closed contact cannot close normally. The safety relay immediately detects the fault and locks it, while ordinary relays cannot achieve this.

Dual-channel redundancy + mutual check: The two input signals are mutually verified. When a single component fails, the safety function remains effective.

EDM function: It can monitor whether the downstream contactors have truly disconnected the power circuit.

Using ordinary relays to build a safety circuit cannot pass the safety inspection, and in case of an accident, it is impossible to prove that the circuit is effective.

2. Dual-channel wiring: The two normally closed contacts are respectively connected to

The safety door lock is usually equipped with two independent normally closed (NC) contacts, which are connected to the two independent input channels of the safety relay. Take Pilkiz PNOZ C1 as an example:

Channel 1: The normally closed contact of the door lock is connected to the S11 and S12 terminals of the safety relay.

Channel 2: The other normally closed contact of the door lock is connected to S21 and S22 terminals.

The two sets of channels must come from the same safety device (i.e., the same safety door lock), and the contacts of two different door locks cannot be mixed and connected in the same set of channels.

Working principle: When the door is closed and the door lock is locked, both sets of normally closed contacts are closed, and the safety relay's two input channels receive signals. The internal logic determines that "the safety condition is met", and the safety output contacts close, allowing the equipment to start. Once the door is opened, both sets of normally closed contacts simultaneously open, and the safety relay immediately cuts off the safety output, causing the equipment to stop.

Why must it be dual-channel? If only one channel is connected, once this signal line breaks or the contacts stick, the safety relay cannot determine whether it is "the door is really closed" or "there is a problem with the circuit", resulting in a single-point failure risk. The dual-channel design ensures that if any channel fails, the safety function remains effective.

Special attention: The output types of different door locks are different. Mechanical contact (dry contact) output can be wired according to the above dual NC connection method; if the door lock is an OSSD electronic output, the double NC wiring diagram cannot be applied, and it must be wired according to the corresponding OSSD input mode of the safety relay.

3. EDM function: Monitoring downstream contactors

EDM (External Device Monitor) is used to detect whether the contactors downstream of the safety relay are operating normally.

Why is EDM needed? After the safety relay cuts off the output, the contactors in the power circuit must truly disconnect for the equipment to stop. If the main contacts of the contactors are welded and stuck due to frequent on-off, even if the safety relay has sent an OFF signal, the contactors still remain conductive, and the motor continues to operate with electricity, completely losing safety protection.

EDM working principle: The normally closed auxiliary contacts of the two contactors (K1, K2) are connected in series and then connected to the EDM feedback input terminal of the safety relay. The safety relay checks the EDM circuit for closure within a specified time (usually about 300ms) after each safety output switches from ON to OFF. If the contactor is properly disconnected, its normally closed auxiliary contact closes, and the EDM circuit is connected, confirming that the safety relay operates normally. If the contactor contacts are stuck together, the normally closed auxiliary contact cannot close, and the EDM circuit remains disconnected. When the safety relay detects an anomaly, it enters a locked state, preventing the equipment from restarting.

Key wiring points: The EDM must use a forced-direction (force-direction) contactor, and its normally closed auxiliary contact mechanically links with the main contact - when the main contact is stuck, the normally closed auxiliary contact cannot be reset, and is detected by the EDM circuit. The auxiliary contact of an ordinary contactor does not have mechanical linkage with the main contact and cannot be used for EDM monitoring.

IV. Complete Wiring Structure

The complete circuit for a safety door lock connected to the safety relay includes three parts:

Circuit Connecting Content Function

Dual-channel input NC contacts of the door lock are connected to S11-S12 and S21-S22 respectively To detect the door lock status, any channel being disconnected triggers a shutdown

EDM feedback NC auxiliary contacts of the two contactors are connected in series to the EDM terminal To monitor whether the contactor has truly disconnected

Safety output The safety contacts inside the safety relay control the contactor coil To cut off the power circuit and stop the equipment

Common wiring errors:

Only connecting a single channel, the safety relay cannot detect a short circuit between channels;

Connecting the door lock output of OSSD in a double-NC diagram, the input mode does not match, and the safety relay cannot work properly;

The EDM circuit is not connected or connected incorrectly, and the contactor is stuck together, making it impossible to detect, and there is a blind area in the safety circuit.

The correct cooperation between the safety door lock and the safety relay is essentially a complete closed loop of "Detection - Judgment - Execution - Feedback": The dual channels of the door lock are responsible for detection, the safety relay is responsible for judgment and output, the contactor is responsible for execution, and the EDM circuit is responsible for confirming that the execution is complete. Any one of these four steps being faulty makes the safety protection incomplete.

Rihuan Sensing provides a full range of safety door lock products, compatible with the dual-channel input mode of mainstream brand safety relays. The product contacts are forcibly disconnected, in line with EN ISO 13849 safety standards. Welcome to follow Rihuan Sensing to obtain professional selection guidance for safety door locks and safety relays.