I was tasked to provide a door interlock system. The system comprises 3 doors and an airlock; plus some rules.

Doors 1 is a 230V shutter to outside; door 2 is a 415V shutter to a clean area. Door 3 is a fire door into a sterile room.

• When door 1 is open neither doors 2 or 3 may open.
• When doors 2 or 3 are open door 1 must not be allowed to be raised.
• A green break glass override must be provided.
• Safety beams are required on door 1.
• Door 1 must operate on a 5 minute timer so if the door is left open; clear of obstruction AND the area is unoccupied then it must close fully.

I achieved this using a programmable logic controller (PLC) and using 11 ladder networks and some cunning logic the above was all achieved in 2 days including all installation. It was important that the system was fitted as cleanly as possible with a minimum of containment and so I managed to rod most of the wiring down the backs of the walls as they were plasterboard over stud.

It took 8 inputs and 4 outputs to control the system.

1. A 12V normally low input from a push button station (up button)
2. A 12V normally low input from a push button station (down button)
3. A 12V normally high input from a sensor telling the system shutter 2 is in the lowered position.
4. A 12V normally high input from a sensor taking the system the clean room door is closed.
5. A 12V normally high input from the safety beam system on shutter 1.
6. A 12V normally high input to indicate that shutter 1 is in the lowered position.
7. A 12V normally high input from the green safety release buttons; wired in series.
8. A PIR to detect occupancy of the area (this signal rising to 12V starts a 5 minute timer to close the shutter; this timer is reset if either door button is pressed, if occupancy is again satisfied or if the beams are interrupted).

Two flags are created inside the system; safe to open and safe to close – these pertain to shutter 1. If at any point the green emergency release buttons are pressed then all sensors are set to high and the system is cleared to operate all doors manually only. The automatic close no longer functions when this situation occurs.

1. Output 1 is the relay for the door to raise.
2. Output 2 is the relay for the door to lower.
3. Output 3 controls the mag-lock keeping the clean room sealed.
4. Output 4 controls a relay breaking the signal from the up button of Shutter 3 to the controller.

I just wish I could have got raise and lower buttons quickly enough, but instead I got red and green and I’ve since labelled them using my P-Touch and clear label tape. Green takes you to a safer situation (door open) whilst red takes you to a more dangerous condition (door closed) and so it does satisfy best practice.

I might add that this is also why “on” for a circuit breaker is red whilst off is green.

Finally the big mushroom is the motor panic button. This overrides all the electronic systems and is physically connected to the motor on shutter 1. It is a full kill switch and kills power to the motor should interlocks or systems fail.

I know for you guys my work isn’t always that interesting; but this job has fascinated me as I love doing PLC work; after all what are fire alarms except for lots of logic gates.