HOME AUTOMATION: and the power line carrier
Aug 1, 1999 12:00 PM,
Steve Filippini
The term home automation conjures images of futuristic, robotic dwellingsright out of a science fiction movie. In fact, home automation has beenaround for quite some time now. Your first brush with home automation mayhave been with the Clapper. Clap your hands to turn on a light; clap themagain to turn it off. The Clapper was nothing more than a device thatplugged into an AC receptacle with a plug for a lamp or appliance. Impactsound waves activated a relay that was either on or off depending on thestate in which the device was last used. The device was a big seller whenit was introduced. We used ours for the lights on the Christmas tree.
Today, lights and appliances are controlled by home security systems,desktop PCs and mini control modules that can be plugged into an AC outletand left to run a pre-programmed schedule. Regardless of the controllinghost panel, most of the devices used today rely on power line carriertechnology. Power line carrier (PLC) uses existing power lines that supplyAC to a home’s receptacles and light fixtures.
Let us first start with a basic course in home wiring and AC power. Fromthe street, the power company provides three cables to the home. One cableis called the neutral or common feed. The other two cables each carryapproximately 110V of electricity. These cables are terminated at your homeat a distribution box or breaker panel. Here, AC power is evenlydistributed to multiple AC lines that run throughout the home. Each line issent through a circuit breaker that prevents too much current from beingdrawn from any particular power line. Because there are two lines with 110Veach from the street, there is usually enough available for the homeowner.If the homeowner uses a clothes dryer rated for 220 V, the two lines with110 V are combined at the clothes dryer’s receptacle. In some instances,there may be a third line of 110V run to the home.
If you are not a licensed electrician, do not attempt any of the wiringdiscussed in this article on your own. If you have trouble keepinglow-powered 12 V systems from blowing fuses, leave the wiring to a skilledelectrician with a good medical plan.
AC power is viewed as a sine wave that cycles from a point called the zerocrossing, where it rises to an equal distance above, then below, then backto the starting point, and 60 of these cycles in a one minute time frame isknown as 60 Hz. The power company is required to regulate this value andkeeps it constant. The cleanest point of the sine wave is the zerocrossing. This is where you would look for and find a PLC signal. The PLCsignal is a 5 V peak-to-peak (5 Vpp), 121 kHz message that is induced intothe sine wave and rides along the AC wires with the AC power. The PLCsignal has coded instructions that can only be heard by devices that aredesigned to listen for and react to these instructions. The devices thathear the signal are commonly referred to as smart modules.
Smart modules come in two flavors – transmitter and receiver. A transmittercan be a desk top plug-in device or a wall mounted, wired-in module thathas a series of push buttons on the face. Pressing one of the buttons willsend a PLC signal into the home on all AC wiring. If a receiver is set toaccept that particular signal, it will react to the command. A receiver canbe a smart receptacle or light switch.
Sounds easy enough, but there is more to keeping the signal strong andstable than just popping in modules. The power company does its best toprovide a clean sine wave to the home, but like everything else, there areoutside forces that tend to get in the way. Radio signals, for example, canhop on the power cables and ride into the home with the AC power. InCalifornia, I lived near the KFI AM radio transmission tower and you couldhear the station bleed through some of the appliances that were designed toemit sound but not designed to play music.
Aside from the radio signals, you have to deal with the power company’sswitching stations and relay panels. These can create AC interference thatwill take the free ride into the residence. Once inside the home, there areappliances and light fixtures that make it difficult for the PLC signal toget the message across because they create interference as well. Dimmersare notorious for creating AC noise. Youmay have noticed an AC hum comingfrom your stereo, intercom or television whenever the lights are dimmed.
What you need are some filters, and there are at least two available. Thein-line filter is wired from one of the 110 V lines to the neutral line,and it creates a tunnel effect that allows a 5 Vpp signal to pass throughand blocks out everything else that may be on the sine wave. The filterdoes not affect AC power; it just cleans the noise from the lines.
Another common filter is the plug-in filter that isolates individualappliances that are creating more than their fair share of interference.Instant-on television, refrigerators, halogen light fixtures and even somecable television boxes create too much noise for the PLC signal toovercome. Simply plug the offending appliance into the filter and plug thefilter into the receptacle.
There is one popular product on the market that you cannot overcome withfilters – wireless intercom systems that use AC wiring to send the voicesignals from point A to point B. The wireless intercom plugs into areceptacle and uses the same zero crossing used for PLC to send voicesignals. You may have to decide between the intercom system or the homeautomation system. The intercom usually loses.
Now that we have addressed some AC noise, we need to address power phaselimitations. Remember that the power company usually provides three cablesto the home. Two of the cables carry 110 V each. When they wire into thebreaker panel, they rarely connect together unless there is a 220 Vappliance out there.
So you have two phases of power at the breaker panel, phase A and phase B.If your PLC Transmitter is plugged into phase A, and your PLC receiver isplugged into phase B, you are relying on the PLC signal to bleed acrossphases when the cables are near each other. What you need is a signalbridge. This device wires in at the breaker panel and logically connectsthe two phases, providing a path for the PLC signal to follow. It is highlyrecommended that every PLC installation has at least one signal bridge atthe main breaker panel. It does not hurt to have them at subpanels as well.You may also consider having an in-line filter at each phase and at thewire into the signal bridge. What about three-phase homes? Add anothersignal bridge and filter from the third phase to the neutral line. Thisounce of prevention goes a long way when the system is up and running andall of the AC players are involved.
What if the home is the size of a mansion with what seems like miles of ACwiring? The PLC signal can only go so far before it begins to drop insignal integrity. What you need is an amp or repeater (basically the samething named differently by many PLC manufacturers) to boost the signal andget it to the farthest reaches of the home. The amp or repeater has abuilt-in signal bridge and usually costs two to three times more than thesignal bridge. Using this device in large homes can be a true lifesaver.
PLC Modules
With the groundwork out of the way, let us look at the modules themselves.Power line carrier is also known throughout the industry as X-10, which isthe name of a company that manufactures and distributes these modules. X-10is also the signal protocol used for PLC transmissions. X-10 may havedeveloped the technology, many companies have incorporated it into theirproduct lines. Some of these modules are of the plug-in variety. Plug theappliance or lamp into the module, plug the module into a hot receptacle,and you are set.
Some modules require a licensed electrician for their installation. Thesemodules actually replace the existing light switches and receptacles. Whenyou get past the packaging and marketing terms, the modules are basicallythe same. I bring this up because with so many PLC providers, you may beasked by the customer if existing smart modules within the home arecompatible with the newer modules you are installing. They are not onlycompatible, but they are also practically the same thing.
Each module has two code wheels on them. The first code wheel has 16positions labeled A through P. This is the house code setting. The secondcode wheel also has 16 positions, but they are labeled 1 through 16. Thisis the unit code setting. The PLC transmitter (depending on the model) mayhave similar code wheels that will match with the code wheels on thereceiver. You may have the front porch light fixture set at a code of F-5and the kitchen lights set to a code of F-7.
Some tips when setting the house codes. Do not use house code A. Themanufacturers ship the modules with A pre-coded. Do not use house code Bbecause everyone who knows not to use A always clicks once to B. Do not useP; it is used with some PLC test units and signal measuring tools.
Some modules available are light switches that can dim a light fixture upto 500 W; others just turn the light fixture off and on. The dimmer modulesare either two-wire or three-wire models, the difference being that thetwo-wire dimmer wires in-line with the light fixture, and the three-wireuses a neutral wire to help stabilize the dimming process. My experiencehas taught me that any time you can use a three-wire dimmer, use it. Thenon-dimming modules are actually large relays that can handle standardlight fixtures and switched receptacle loads at 15 amps to 20 amps. Youknow when they activate by the hollow sound emitted from inside the wallbehind the module. There is also a three-way smart module that can beupgraded to a four-way or higher application. Read the wiring directionsincluded with the module; it is not the same as a standard three-way wiringscheme.
There are dual receptacles that will switch either one or both receptacles,depending on the module. Now, what if you have a table lamp plugged intothe smart receptacle, and you need the module to provide power when it isin an off state? The receptacle module is designed to provide power ondemand. Just turn the lamp on, then off, then on again, and the module willturn on, providing AC power. Leave the lamp on, and use a controller toturn the module off. If you turn the lamp off, the module is useless. Awall-mount controller is a transmitter that can control one or more smartmodules from one location. It also has a set of code wheels. A desktopcontroller can easily access many smart modules individually or through aseries of time-scheduled events. Through the use of a PC, you can set uplighting schedules that produce a lived-in look while the resident is awayfrom home, or as a matter of convenience, turns lights on at dusk and offat midnight or dawn.
Here is a quick tip that all installers should keep in mind because theywill encounter it at one time or another. Do not attempt to control astandard receptacle with a dimming light switch module. The first time youplug a vacuum cleaner into the receptacle and turn it on, you will smellsmoke and quite possibly see flames flash out of the 500 W light switchmodule – not that I have ever done that.
Some controllers can be tied into a security system that handles the timedevents and uses the controller only as an interface connection to thehome’s AC wiring. Some security systems produce and transmit the PLCsignals without the use of a controller. The systems use the step-downpower transformer that powers the security system as the path to send PLCsignals onto the AC power lines.
OK, you have installed the hardware, programmed the software, and youcannot get some of the modules to activate as programmed. You will needsome test equipment to isolate the problem. Each manufacturer of PLCequipment provides some form of test equipment to measure the strength andintegrity of the signal. X-10 and Leviton both use a pair of testers thatmeasure signal strength. The first test device plugs into the samereceptacle used for the desktop controllers or security system interfacemodules. This test device transmits a 2 Vpp PLC signal of P-1 on, P-1 offover and over again. The other test device plugs into any receptacle thatis not behaving properly. This device listens for the code of P-1 and measures the strength of the signal. You need a signal that measures at least 100 mV for the smart modules to respond. It can also tell you if outside noiseis interfering with the decoding of the signal. If you want to have somefun, set all of your smart modules to P-1 and watch the home slowly strobe.
Sometimes you will encounter the haunted house syndrome – lights andappliances turning themselves on and off for no apparent reason. The firstthing to check is that the neutrals in all of the splices are tight andsecure. If the wiring is stable, you may be hearing PLC signals from aneighbor’s home automation system. The easiest fix is to change the housecode. If that does not alleviate the problem, you may need to have anelectrician install a signal block at the breaker panel. It is large andexpensive, but it keeps foreign signals out of the wiring.
Other test fixtures actually decode the PLC signal and record it onto a PCfor later reference. This type of tester is usually designed and used bythe company the installer represents. Some are available through themanufacturer. The companies that provide these modules offer a softwarepackage for programming controllers, and they give you the transmissioncode that allows you to create your own PLC transmission source or tester.
PLC modules are also available to flash lights off and on when an alarm hasbeen detected. There are modules that accept a dry contact closure that, inturn, transmits a PLC signal into the home. Other modules will accept a PLCsignal and provide dry contact closures. Some PLC signals can be generatedfrom pendants and keyfobs while others can be generated by a phone callfrom the customer. Some PLC modules even have built-in horns and sirensthat can be installed in the event the installer cannot get system wires toa particular part of the home.
One little point to bring up. If the AC power to the home is turned off ornot available by generator during a blackout, none of the above moduleswill function. The technology does, admittedly, have its limitations.
