An emergency stop must act through hardwired safety components such as a safety relay that drops the contactor supply; a standard PLC like the AC500-eco only monitors the safety relay and adds operational interlocks. In ladder, the motor rung includes the safety relay OK input and other permissives, the start request is cleared on any stop, and the machine never restarts by itself after the E-stop is reset.
- Safety function in hardware (safety relay / safety PLC), not in standard PLC code
- The PLC monitors and adds restart prevention and diagnostics
- Every permissive is a separate, named contact in series
- After any stop, a new deliberate Start is required
This lab is about doing it properly. Standards such as IEC 60204-1 and ISO 13850 require the emergency stop to work independently of standard PLC software. If your risk assessment needs a programmable safety solution, use a safety PLC such as AC500-S, not code in a standard CPU.
Step 1: Understand the Safety Architecture
Click a device to see whether it is part of the safety function.
| Job | Done by | Why |
|---|---|---|
| Remove motor power on E-stop | Safety relay + contactors | Must work even if the PLC has failed |
| Detect welded contactor | Safety relay EDM input | Part of the safety function |
| Prevent automatic restart | Relay manual reset + PLC start logic | Both layers |
| Tell the operator what happened | PLC alarms and HMI | Diagnostics |
| Flash reset lamp when reset is possible | PLC | Usability |
Step 2: Write the Permissive and Restart Logic
E-stop and interlock simulator
Start the motor, open the guard or press E-stop. Release it: nothing restarts. Reset the safety relay (lamp flashes when possible), then press Start again.
Green contact = passing power. Orange moving wire = rung is true and the output is energised.
Because the seal-in contact DO_Motor_Run is in the same rung as Relay_OK, losing the relay drops the seal permanently. When the relay comes back, the rung stays false until Start is pressed again. That is restart prevention in the PLC layer.
Step 3: List Every Interlock
| Permissive | Tag | Blocks | Type |
|---|---|---|---|
| Safety relay healthy | DI_EStop_OK | All motion | Safety monitoring |
| Guard closed | DI_Guard_Closed | All motion | Safety monitoring |
| MPCB healthy | DI_MPCB_OK | Conveyor motor | Operational |
| No motor alarm | NOT Any_Motor_Alarm | Conveyor motor | Operational |
| Mode valid | NOT Alm_Selector | Start in any mode | Operational |
| Forward not active | NOT DO_Motor_Run | Reverse | Mutual interlock |
| Drive ready | Sts_VFD_Ready (Lab 20) | Drive start | Operational |
Step 4: Validate the Stop Functions
| ID | Check | How to check | Pass criterion | Result |
|---|---|---|---|---|
| Stop and reset tests | ||||
| E-01 | E-stop stops | Press each E-stop while running | Contactor drops immediately, independent of PLC | |
| E-02 | PLC off test | Stop the CPU, check relay still acts | E-stop still removes power | |
| E-03 | No restart | Release E-stop | Nothing moves | |
| E-04 | Reset | Press reset | Relay ON, lamp stops flashing, still no motion | |
| E-05 | Restart | Press Start | Motor starts only now | |
| E-06 | Guard | Open guard while running | Stop; same reset sequence | |
Practice Exercise
Add a second guard door. Decide whether it goes in the safety relay chain or only to the PLC, based on the hazard. Write your reasoning in one paragraph for your trainer.
Troubleshooting
| Symptom | Likely cause | Fix |
|---|---|---|
| Motor restarts after E-stop reset | Start request held in a SET bit | Use a seal-in through the permissives or reset the request on stop |
| Relay will not reset | Contactor feedback (EDM) open or channel fault | Check feedback loop and both channels |
| E-stop alarm but relay OK | Aux contact wiring to PLC | Check DI_EStop_OK wiring |
| Reset lamp never flashes | Reset_Possible logic wrong | Lamp only when E-stops released and guard closed |
Frequently asked questions
Can I program an emergency stop in a standard PLC?
No. The E-stop function must be performed by hardwired safety components or a safety-rated controller. A standard PLC can monitor it and add restart prevention and diagnostics.
What is restart prevention?
After a stop, the machine must not start again by itself when the E-stop is released or power returns. A deliberate Start command is needed.
What is an ABB safety PLC?
AC500-S is ABB's safety PLC family, with safety CPU and safety I/O modules certified for functional safety applications.
Why monitor the safety relay with the PLC?
To stop the logic cleanly, block restarts, show alarms on the HMI and guide the operator through the reset.