Let us first start with a basic course in home wiring and AC power. From the street, the power company provides three cables to the home. One cable is called the neutral or common feed. The other two cables each carry approximately 110V of electricity. These cables are terminated at your home at a distribution box or breaker panel. Here, AC power is evenly distributed to multiple AC lines that run throughout the home. Each line is sent through a circuit breaker that prevents too much current from being drawn from any particular power line. Because there are two lines with 110V each 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 with 110 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 wiring discussed in this article on your own. If you have trouble keeping low-powered 12 V systems from blowing fuses, leave the wiring to a skilled electrician with a good medical plan.

AC power is viewed as a sine wave that cycles from a point called the zero crossing, where it rises to an equal distance above, then below, then back to the starting point, and 60 of these cycles in a one minute time frame is known as 60 Hz. The power company is required to regulate this value and keeps it constant. The cleanest point of the sine wave is the zero crossing. This is where you would look for and find a PLC signal. The PLC signal is a 5 V peak-to-peak (5 Vpp), 121 kHz message that is induced into the sine wave and rides along the AC wires with the AC power. The PLC signal has coded instructions that can only be heard by devices that are designed to listen for and react to these instructions. The devices that hear the signal are commonly referred to as smart modules.

Smart modules come in two flavors - transmitter and receiver. A transmitter can be a desk top plug-in device or a wall mounted, wired-in module that has a series of push buttons on the face. Pressing one of the buttons will send a PLC signal into the home on all AC wiring. If a receiver is set to accept that particular signal, it will react to the command. A receiver can be a smart receptacle or light switch.

Sounds easy enough, but there is more to keeping the signal strong and stable than just popping in modules. The power company does its best to provide a clean sine wave to the home, but like everything else, there are outside forces that tend to get in the way. Radio signals, for example, can hop on the power cables and ride into the home with the AC power. In California, I lived near the KFI AM radio transmission tower and you could hear the station bleed through some of the appliances that were designed to emit sound but not designed to play music.

Aside from the radio signals, you have to deal with the power company's switching stations and relay panels. These can create AC interference that will take the free ride into the residence. Once inside the home, there are appliances and light fixtures that make it difficult for the PLC signal to get the message across because they create interference as well. Dimmers are notorious for creating AC noise. Youmay have noticed an AC hum coming from your stereo, intercom or television whenever the lights are dimmed.

What you need are some filters, and there are at least two available. The in-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 through and blocks out everything else that may be on the sine wave. The filter does not affect AC power; it just cleans the noise from the lines.

Another common filter is the plug-in filter that isolates individual appliances that are creating more than their fair share of interference. Instant-on television, refrigerators, halogen light fixtures and even some cable television boxes create too much noise for the PLC signal to overcome. Simply plug the offending appliance into the filter and plug the filter into the receptacle.

There is one popular product on the market that you cannot overcome with filters - wireless intercom systems that use AC wiring to send the voice signals from point A to point B. The wireless intercom plugs into a receptacle and uses the same zero crossing used for PLC to send voice signals. You may have to decide between the intercom system or the home automation system. The intercom usually loses.

Now that we have addressed some AC noise, we need to address power phase limitations. Remember that the power company usually provides three cables to the home. Two of the cables carry 110 V each. When they wire into the breaker panel, they rarely connect together unless there is a 220 V appliance 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 is plugged into phase B, you are relying on the PLC signal to bleed across phases when the cables are near each other. What you need is a signal bridge. This device wires in at the breaker panel and logically connects the two phases, providing a path for the PLC signal to follow. It is highly recommended that every PLC installation has at least one signal bridge at the 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 the wire into the signal bridge. What about three-phase homes? Add another signal bridge and filter from the third phase to the neutral line. This ounce of prevention goes a long way when the system is up and running and all of the AC players are involved.

What if the home is the size of a mansion with what seems like miles of AC wiring? The PLC signal can only go so far before it begins to drop in signal integrity. What you need is an amp or repeater (basically the same thing named differently by many PLC manufacturers) to boost the signal and get it to the farthest reaches of the home. The amp or repeater has a built-in signal bridge and usually costs two to three times more than the signal 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 is the name of a company that manufactures and distributes these modules. X-10 is also the signal protocol used for PLC transmissions. X-10 may have developed the technology, many companies have incorporated it into their product lines. Some of these modules are of the plug-in variety. Plug the appliance 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. These modules actually replace the existing light switches and receptacles. When you get past the packaging and marketing terms, the modules are basically the same. I bring this up because with so many PLC providers, you may be asked by the customer if existing smart modules within the home are compatible with the newer modules you are installing. They are not only compatible, but they are also practically the same thing.

Each module has two code wheels on them. The first code wheel has 16 positions labeled A through P. This is the house code setting. The second code wheel also has 16 positions, but they are labeled 1 through 16. This is the unit code setting. The PLC transmitter (depending on the model) may have similar code wheels that will match with the code wheels on the receiver. You may have the front porch light fixture set at a code of F-5 and the kitchen lights set to a code of F-7.

Some tips when setting the house codes. Do not use house code A. The manufacturers ship the modules with A pre-coded. Do not use house code B because everyone who knows not to use A always clicks once to B. Do not use P; it is used with some PLC test units and signal measuring tools.

Some modules available are light switches that can dim a light fixture up to 500 W; others just turn the light fixture off and on. The dimmer modules are either two-wire or three-wire models, the difference being that the two-wire dimmer wires in-line with the light fixture, and the three-wire uses a neutral wire to help stabilize the dimming process. My experience has taught me that any time you can use a three-wire dimmer, use it. The non-dimming modules are actually large relays that can handle standard light fixtures and switched receptacle loads at 15 amps to 20 amps. You know when they activate by the hollow sound emitted from inside the wall behind the module. There is also a three-way smart module that can be upgraded to a four-way or higher application. Read the wiring directions included with the module; it is not the same as a standard three-way wiring scheme.

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 into the smart receptacle, and you need the module to provide power when it is in an off state? The receptacle module is designed to provide power on demand. Just turn the lamp on, then off, then on again, and the module will turn on, providing AC power. Leave the lamp on, and use a controller to turn the module off. If you turn the lamp off, the module is useless. A wall-mount controller is a transmitter that can control one or more smart modules from one location. It also has a set of code wheels. A desktop controller can easily access many smart modules individually or through a series of time-scheduled events. Through the use of a PC, you can set up lighting schedules that produce a lived-in look while the resident is away from home, or as a matter of convenience, turns lights on at dusk and off at midnight or dawn.

Here is a quick tip that all installers should keep in mind because they will encounter it at one time or another. Do not attempt to control a standard receptacle with a dimming light switch module. The first time you plug a vacuum cleaner into the receptacle and turn it on, you will smell smoke and quite possibly see flames flash out of the 500 W light switch module - not that I have ever done that.

Some controllers can be tied into a security system that handles the timed events and uses the controller only as an interface connection to the home's AC wiring. Some security systems produce and transmit the PLC signals without the use of a controller. The systems use the step-down power transformer that powers the security system as the path to send PLC signals onto the AC power lines.

OK, you have installed the hardware, programmed the software, and you cannot get some of the modules to activate as programmed. You will need some test equipment to isolate the problem. Each manufacturer of PLC equipment provides some form of test equipment to measure the strength and integrity of the signal. X-10 and Leviton both use a pair of testers that measure signal strength. The first test device plugs into the same receptacle used for the desktop controllers or security system interface modules. This test device transmits a 2 Vpp PLC signal of P-1 on, P-1 off over and over again. The other test device plugs into any receptacle that is 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 m V for the smart modules to respond. It can also tell you if outside noise is interfering with the decoding of the signal. If you want to have some fun, set all of your smart modules to P-1 and watch the home slowly strobe.

Sometimes you will encounter the haunted house syndrome - lights and appliances turning themselves on and off for no apparent reason. The first thing to check is that the neutrals in all of the splices are tight and secure. If the wiring is stable, you may be hearing PLC signals from a neighbor's home automation system. The easiest fix is to change the house code. If that does not alleviate the problem, you may need to have an electrician install a signal block at the breaker panel. It is large and expensive, but it keeps foreign signals out of the wiring.

Other test fixtures actually decode the PLC signal and record it onto a PC for later reference. This type of tester is usually designed and used by the company the installer represents. Some are available through the manufacturer. The companies that provide these modules offer a software package for programming controllers, and they give you the transmission code 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 has been detected. There are modules that accept a dry contact closure that, in turn, transmits a PLC signal into the home. Other modules will accept a PLC signal and provide dry contact closures. Some PLC signals can be generated from pendants and keyfobs while others can be generated by a phone call from the customer. Some PLC modules even have built-in horns and sirens that can be installed in the event the installer cannot get system wires to a particular part of the home.

One little point to bring up. If the AC power to the home is turned off or not available by generator during a blackout, none of the above modules will function. The technology does, admittedly, have its limitations.