Whole House Blocker


This device prevents X-10 commands from your neighbor from entering your house.


In April 1995, I started receiving X-10 commands from my neighbor down the street. Since our homes are wired for 200Amp service, I had previously assumed that we didn't share transformer secondaries. This assumption was either incorrect, or commands were somehow bridging the secondaries. Note that he does not use RF components.

The Device

An add from Worthington industries in an issue of Electronic House proclaimed the arrival of a new device from ACT that promises to isolate your house from the rest of the street for $75.00. In addition, the unit also functions as a phase coupler. As I will explain below, this latter function is very important for proper operation of the blocker. After ordering one unit and setting up a bench test circuit at home, I was able to use my scope to measure that the device indeed attenuates X-10 signals by a factor of four to five. Technical literature from their company president Rick Scholl stated that if the amplitude of the offending X-10 command exceeds 100mVolts, two such filters will be needed (one at each house's breaker panel) to block all signals.

Operating Principle

The device's principle of operation is quite clever. In a conventional design, you would have to insert an inductor of sufficient value and current rating in series with each incoming power phase. The size of such a device can be enormous. Instead of that approach, the ACT blocker is a toroidal transformer. The installer passes the neutral conductor to the house's breaker panel thru a hole in the noise blocker and connects the incoming neutral and power phase connections to screw terminals on the device. Note that the device is not in series with the house, thus mega size conductors are not needed. The actual operation of the circuit is not 100% clear to me, but it functions roughly as follows: when an offensive signal is detected by the module, it somehow figures out its source (either from in or outside the house), and then magnetically induces a voltage of the proper polarity on the segment of the neutral cable that is inside the toroid's hole. The effect is that the signal will be cancelled out. For example, if there appears 10mV of signal on phase A, it will be automatically be coupled also to phase B. ACT's blocker then induces roughly 10mV of signal onto the neutral, and the effect is that the house 'sees' no signal, since there exist an equeal amount of 'noise' on all three connections.

Article by 'Uncle Phil' on the CP303.

The whole-house blocker installed into the breaker panel (right).  Note the thick uninsulated
neutral going thru the middle of it.  The items clamped around the two power feeds are the current
transformers for my power-line monitor.


  • The device is small. It is approximately the size of a coffee mug. In my case, this allowed an inexpensive installation--right into the breaker panel. If it were much larger, it would require a much costlier and lenghtier installation. The electrician I hired needed one hour, a total cost of $80.
  • In principle, there is no reason that the frequency response of the device be limited to the X-10 carrier. Perhaps the unit can / does function as an active high frequency fire wall for the house. In this case, it can add a side benefit of reducing line noise in general. I wonder then if this device can also be useful with the next generation of powerline carrier control such as CEBUS and Echelon.
  • Leviton is also building a whole house block, and has been advertised for several months in Home Control Concept's flyer. As of Feb '95, this unit will not be available till late Summer '95, and is rumored to cost a few hundred dollars. I do not know how Leviton's device works, but from the drawings that I have, I do not see any thru holes. In addition, the Leviton device handles only two phases (up to 100Amps each), while ACT's can handle up to three (up to 200Amps each). I wonder if Leviton's device is just two big inductors.
  • If the amount of attenuation of intruding signals is not sufficient, it would appear to be possible to install capacitors across the power lines on the meter side of the blocker to suppress these signals. I say this hesistantly as there are extraneous issues associated with doing this, so tread carefully.
  • This device is very new. The serial number of mine is #109.
  • Worthington's phone number is 800-282-8864. The part number is CP303, and it is UL listed.


I am very enthusiastic about this device. Everyone knows intruding commands is one big liability with X-10. There is a loss of privacy associated with commands passing freely between dwellings. My family and I are glad to have regained a great deal of it (the privacy that is). I am curious if this device and my post generate any interest. I would be happy to discuss its use further with those that need something like this. I would like to thank Rick for his assistance. I was very skeptical and asked many questions before purchase, which he answered patiently.


Note that I have no relationship to any of the companies that I mentioned above whatsoever. I am receiving no compensation for writing this.

Long term log

  • 4/15/95 - The device has been installed in my home for a few days, and since then there has been a complete cessation of outside commands. I suppose that I would be willing to accept one or two commands from the other homes per week, and so far, we are below that threshold. X-10 performance inside the home has been unchanged.
  • 11/01/02 - This device has continued to work effectively.
  • 7/03 - Installed an active repeater into the system to address a few outlets with problems.
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