Using a PLC in Domestic System

themullet

I am going to start building my own house shortly and I will be wiring it myself. All of the lighting in the house will be LED low voltage. Part of me is thinking about putting in a PLC through which I can operate the lights, simply wire everything back to the PLC. Would this be against wiring regulations? I don't intend to ever sell this house.

2011

Great idea. I always wanted to do this in a domestic installation.


You are allowed to control anything electrical with a PLC once it is done properly.

To really make the most of it you should incorperate a touch screen HMI. Expect to pay about €1000 for this fantastic piece of kit. Also to make the most of it you should have the PLC controlling the heating and interfacing with the intruder alarm.

Discreet inputs are cheap enough, but analog can be very pricey. For this reason I would try to do as much as possible with digital I/O if I were you.

DublinDilbert

You would like to have local control in each room too... what would happen if the PLC was to go down?

I would get a neutral & cat 5 wired to each switch point so a "smart" switch can be installed which can be controlled centrally, with the power being switched locally. Also at any point in the future can a normal switch/dimmer be fitted.

2011

PLCs are used to control much more critical services than domestic lighting. For example PLCs are used for precise contol when making life saving medicens in the pharmaceutical industry.
They are also used on aircraft, missiles, power stations etc. If the PLCs on these things were to fail the consequences would be much more serious, yet they are "trusted" to PLCs.

Control of a few lights would present no problem even to the most basic of PLCs.

I take you point about the CAT 5s though. Always better to over wire and future proof.

DublinDilbert

2011 wrote: »

PLCs are used to control much more critical services than domestic lighting. For example PLCs are used for precise contol when making life saving medicens in the pharmaceutical industry.
They are also used on aircraft, missiles, power stations etc. If the PLCs on these things were to fail the consequences would be much more serious, yet they are "trusted" to PLCs.

Control of a few lights would present no problem even to the most basic of PLCs.

I take you point about the CAT 5s though. Always better to over wire and future proof.

I know all about PLCs, but have also seem them fail in my time programming them too. It doesn't happen often, but it can happen and usually at around 3am. Any safety critical system will have either a hardware back up (safety bag), or a second processor ready to kick in.

I would have no issue with a PLC controlling anything in my house, cause in general they are reliable, but what happens if the comms link or the hmi locks up? i would also make sure there was local control too.

2011

what happens if the comms link or the hmi locks up? i would also make sure there was local control too.

perhaps you could put it into stop and force the output? If that won't work then PLC will "fail safe" and you would be stuck with your lights on. In this case you could simply have a hard wired switches that can over ride each output from the PLC (or local control as you suggested). These switches could be located beside the PLC to make the wiring simple. They would not normally be required anyway.

Normally PLCs don't fail, but you are right they do sometimes. The chances are very slim and the consequences are not serious in this example.
PLCs are becoming cheaper and more reliable all of the time so I would think that they will become the norm in domestic installations in the future just as they are the norm for BMS systems.

Cheeble

Hi,

themullet wrote: »

Would this be against wiring regulations?

Not if you do everything in accordance with the regulations.

themullet wrote: »

I don't intend to ever sell this house.

That's not really relevant. It still has to be built in accordance with the regulations.

Cheeble-eers

themullet

Yeah the HMI is a good idea. I program PLC's for a living so getting my hands on the hardware at a good price isn't a problem.

I don't envisage that the PLC reliability will be a problem. PLC's in industrial applications do sometimes give trouble alright but this is a rare enough occurence. A PLC in a domestic situation would be under very little stress and the digital inputs/outputs would be only doing a fraction of the switching of the industrial ones.

I'm not sure how difficult it would be to control the temperature of each room through the PLC though?? I had intended to zoning the house using valves on the pipework and control the temperature of the radiators using stats. But if anyone could suggest an alternative to this I'd be glad to hear it.

I would be in favour of incorporating the alarm system though. Another possibility would be to put the PLC online so I could turn on/off lights whilst away on holiday or have the kettle just boiled for when I arrive home from work!

mac_iomhair

DublinDilbert wrote: »

You would like to have local control in each room too... what would happen if the PLC was to go down?

I would get a neutral & cat 5 wired to each switch point so a "smart" switch can be installed which can be controlled centrally, with the power being switched locally. Also at any point in the future can a normal switch/dimmer be fitted.

thats the way to go...

2011

Yeah the HMI is a good idea. I program PLC's for a living so getting my hands on the hardware at a good price isn't a problem.

Yeah, the HMI can show:

• The staus of the timer (on, off, auto, boost, domestic hot water, central heating, immersion etc.).
• The status of all zone valves, boiler and circulation pump.
• The status of rooms, below set point or above set point
• The status of domestic hot water
• Which lights are on in which rooms
• What presence detectors/PIRs are activated inside/outside
• With current relays any circuit drawing power could be identified

Each room could have it's own on timer. For example you could have it that the bedrooms will be off all day and will warm up just before bedtime and just beore getting up, but only if their temperatures are below their set points. The PLC could also controll the immersion.

If you have analog I/O and PT100s you could display the temperatures of each room on the HMI as well as the outside temperature and the temperature of the hot water cylinder. You could then use on/off control with deadband for each zone. Set points for each room could be entered via the HMI and could be displayed beside the current temperature for each room.

Do you mean that you can get whatever hardware you want cheap and/or free?

I don't envisage that the PLC reliability will be a problem. PLC's in industrial applications do sometimes give trouble alright but this is a rare enough occurence.

Exactly my thoughts. Also if they do go into fault they write a number into a register and from this number you can identify what type of fault it is. Normally the fault is due to a programming error such as asking the PLC to perform an illegal function like devide a number by zero.

I'm not sure how difficult it would be to control the temperature of each room through the PLC though??

Simple enough really.

But if anyone could suggest an alternative to this I'd be glad to hear it.

OK. Let's assume that you are using a thermostat with a volt free contact and a zone valve for each room or area.

The thermostat can either be:
(1) Closed circuit
(2) Open circuit

If (1) then the room that the stat is in is below the set point. If this is during an "on time" (according to the times programmed as on times in the PLC) then:

Open the corrisponding zone valve.

Wait for the valve to open. Feedback can be provided from the valves to the PLC via the auxillary contacts.

Turn on the circulation pump (if not on already).

Fire the boiler (if not on already).

Do all of the above until each stat is "satisfied" i.e. it is open circuit. Once a stat is satisfied close it's corrisponding zone valve. However ensure that at least one valve (such as hall) is open until the circulation pump has stopped to ensure that the pump is not pumping against a "dead head".

Once all stats are satisfied then turn off the boiler. Keep the circulation pump running until the pipe stat (normally located near the boiler) is satisified. This ensures that the water is circulated until it has cooled sufficiently.

Once the pump stops running close all zone valves.

I would be in favour of incorporating the alarm system though

Most alarm systems can close a volt free contact when they go into alarm. This could be used as a DI for the PLC. Once this input goes high the PLC could turn on all lights in the house and flash all outside lights.

Also the PLC could switch on and off lights at appropiate times when in "holiday mode" to give the impression that people are home.

Another possibility would be to put the PLC online so I could turn on/off lights whilst away on holiday or have the kettle just boiled for when I arrive home from work

Easy enough to do!

DublinDilbert

mac_iomhair wrote: »

thats the way to go...

does anyone know if you can get a "smart" switch with an RS485 interface, which the PLC could talk to?

DublinDilbert

why not use X10 switches in each room, then get an X10 to RS232/485 converter to talk to them from the PLC?