Today, Wuxi Rihuan Sensing Technology Co., Ltd. will introduce to you the situation where the safety gratingimmediately locks up and fails to start after power-on - what could be the reason?

The failure where the safety grating locks up and fails to start immediately after power-on is one of the most common and troublesome faults in the field. Many people initially think that "the grating is broken", but in most cases, the problem does not lie in the grating itself, but in the power supply, wiring, or selection.

The following outlines the four most common causes and corresponding troubleshooting methods.

1. Power supply issue: Voltage not meeting standards or reverse polarity connection

The safety grating usually uses DC24V power supply. The first troubleshooting direction after power-on lock-up is power supply:

Voltage too low: The power supply line is too long, resulting in voltage drop, or the power supply itself has insufficient output, causing the actual working voltage of the grating to be lower than the rated range (usually 23-25V).

Reverse polarity connection of positive and negative poles: The red and black wires of the power supply line are connected in reverse, although the indicator light may be slightly bright, the internal circuit of the grating cannot work normally and directly locks up.

Troubleshooting method: Use a multimeter to measure the voltage at the power supply terminals of the grating to ensure it is stable within the range of 23-25V, and check if the polarity connection is correct.

2. Synchronization line failure: The transceiver unit loses synchronization

The safety grating consists of two main components - the transmitter and the receiver. They communicate synchronously through the CP synchronization line, synchronizing the timing of the light beam transmission and reception. Once the synchronization line fails, the transmitter and receiver "sing different tunes", and the grating immediately enters the lock-up state.

Common problems include:

Synchronization line terminal loose or poor contact

Internal break of the synchronization line

Incorrect wiring of the synchronization line (such as reversing or connecting the wrong pins)

Troubleshooting method: Check if the terminals at both ends of the synchronization line are securely plugged in, re-press the connection or replace the synchronization line. When the synchronization line is broken, the grating will directly lock up and stop - this is a normal safety protection mechanism, and it is strictly prohibited to shield or short-circuit it.

3. NPN/PNP selection mismatch: Signal "language"不通

This is the most common pitfall in the field. The output mode of the safety grating is divided into NPN (low-level output) and PNP (high-level output). There is no performance difference between them； the difference lies only in the signal logic and wiring method.

The core principle is： PLC open-collector input matches NPN， PLC source-type input matches PNP. If the selection is wrong， the grating may immediately lock up after power-on.

NPN output： When triggered， the output terminal is pulled low to 0V， suitable for common anode PLC input modules

PNP output： When triggered， the output terminal is pulled high to 24V， suitable for common cathode PLC input modules

Eight out of ten problems with the absence of signal， non-triggering， and abnormal linkage of safety gratings in the field are caused by selection mismatch or wiring errors， and are unrelated to the quality of the equipment.

Troubleshooting method： Refer to the PLC input module manual to confirm whether it is "common anode" or "common cathode"； check if the output type of the grating matches. If you are unsure about the control end input type， you can choose a dual-compatible grating model that supports NPN/PNP switching.

4. Normally open/normally closed wiring error： Safety logic reversed

The output contacts of the safety grating are divided into normally open (NO) and normally closed (NC) modes. High-risk equipment's safety circuits must use normally closed contacts.

Normally closed (NC)： The contact closes and conducts when there is no obstruction， and opens and disconnects when there is obstruction or a line fault. It triggers the stop mechanism. Follow the "fail-safe" principle - when the line breaks or the grating fails， the circuit automatically disconnects， and the equipment is forced to stop.

Normally open (NO)： The contact opens when there is no obstruction， and conducts only when there is obstruction. If the line breaks， the normally open circuit is permanently disconnected， and the equipment does not receive the stop signal.

If the normally open signal is mistakenly connected to the controller that requires a normally closed signal， the safety circuit of the grating cannot be established after power-on， and directly enters the lock-up state.

Troubleshooting method: Confirm whether the output mode of the grating (NO/NC) matches the equipment control system. For gratings with relay contacts output, the terminals will be marked as NC and NO. Before wiring, make sure to check the markings.

V. EDM (External Equipment Monitoring) Fault: Relay does not "feedback"

Some safety gratings support EDM (external equipment monitoring) function, which is used to continuously monitor the contact status of external relays or contactors. If the relay contacts do not operate properly (such as sticking, delay), EDM will detect the abnormality and lock the grating.

For example, after the grating control output changes from ON to OFF, if the normally closed contact does not close within 300ms, EDM determines an error and the grating immediately locks.

Troubleshooting method: Check if the EDM circuit is closed, measure the open and closed states of K1/K2 normally closed contacts; confirm if the auxiliary contacts of the contactor are wired correctly.

Summary of troubleshooting sequence

Step Check content Judgment standard

1 Power supply voltage Measure with multimeter, 23-25V range, positive and negative poles correct

2 Synchronization line Plug firmly, no breakage, correct wiring

3 NPN/PNP selection Match the type of PLC input module

4 Normally open/normally closed setting Safety circuit must use normally closed (NC)

5 EDM circuit Relay contacts normally closed, correct wiring

The power-on locking of the safety grating is mostly due to power supply, synchronization line or wiring issues, rather than damage to the grating itself. Follow the above steps to troubleshoot one by one, and most faults can be located and resolved within half an hour. After troubleshooting, usually, a power-off restart or pressing the reset button is needed to release the locking state.

The safety grating products of Rihuan Sensor support NPN/PNP dual output options, are compatible with various PLC input types, and are equipped with intuitive fault indicator lights to help users quickly locate abnormal states such as power supply, synchronization, and EDM. The products comply with IEC 61496 Type 4 safety standards and provide complete wiring guidance and fault troubleshooting documents. Welcome to follow Dayuan Sensor to obtain professional safety grating selection and technical support.