Thrown into it.

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.

I know this is nerdy stuff, but today I encountered the highest PSC (Prospective Short-Circuit Current) that I’ve ever encountered on an installation.

23kA was the figure I found, with an external loop impedance of just 0.01A

So, why you ask am I sharing this?

Let me briefly explain.

All breakers have a maximum “breaking capacity”. It’s a number, usually in a box, on the front of the device. Most domestic ones are 6kA, industrial tend to be 10, 12, 15 or even 25.

If a short circuit occurs which takes the fault current above the breaking capacity of the breaker then the breaker may not actually trip, or it might fail, or the it may trip but the spark will then “hop the gap”.
So, what do we do in this circumstance?

Well we could protect the installation with BS1361 fuses, however, that is not guaranteed to get your 0.4 second trip time.

My chosen method is to put an MCCB (Moulded case circuit breaker) with a 25kA breaking capacity into the meter tails. Not only do you then get your 0.4s trip time, but you get a guaranteed trip at high PSC’s.

Rewiring complete… for now

We now have lots of new sockets, and lights working throughout the house again after I replaced sections of this stuff (pictured) with modern PVC cable.

The stuff I replaced was in a really sorry state, with the insulation cracking off as soon as I touched it. Rather scarily – this stuff has no earth (it carries just Live and Neutral) which means that the lights in the front of the house have never been earthed… something I did suspect to be honest.

The work is complete for now, but I’ve left the floorboards loosely fitted in readiness for more new sockets at the front of the house this coming year when we decorate the living room, taking in the front bedroom and small 3rd bedroom thus allowing me to disconnect and turn off the upstairs ring main and replace it with a much safer radial circuit.

A Busmans Holiday

Yesterday and today I’ve been doing general electrical work on our own house (yes… our own house). I’ve fitted 4 new power points (kitchen, living room x 4,in 2 places, landing) and have had to disconnect two lights in the house because the wiring is the old fashioned rubber cable and is in a pretty rough state.

I’ve also fitted a new radiator in the bedroom (replaced the old one because it was too small).

Christmas was very nice… we went to Hannah’s parents for Christmas Dinner and Boxing Day. I got Zelda – Spirit Tracks and Professor Layton – Pandora’s Box, as well as many other presents.

Now what to do… well eat pie and peas first of all cos they’ll be done soon!