A zero current crossing capacitive switching scheme for controlling the switching of a capacitor into and out of an HID lead ballast circuit at a time when a current through the capacitor is at or near zero. The capacitor switching enabling bi-level operation of an HID lamp such that the HID lamp operates at full power mode or in a reduced power mode. The zero current crossing is achieved by delaying the capacitor switching, a duration equal to about 5.degree. to 10.degree. lag from the zero volt crossing. The device can be used in a method of controlling a plurality of HID lamps using an isolated class 2 wiring scheme. Potential damage to the HID lamp caused by aborted lamp ignitions is also disclosed. The method includes suppressing off input signals during the warm up period.

A communication method particularly suited for lighting control employs encoding of perturbations in a voltage signal having a fundamental period with a nominal waveform. A control period includes a pre-selected number of fundamental periods of the voltage signal. Different control commands are indicated by imposing a selected perturbation, such as a phase cut, on the nominal waveform with a respective occurrence signature within the control period. The control commands are decoded from the voltage signal by detecting the occurrence signature of the perturbations within each control period. Decoding is accomplished by differentiating the nominal waveform to detect encoded perturbations. The technique is particularly suited for two-wire dimming applications for gas discharge lamps, in which the voltage signal carrying the encoded commands is the sinusoidal mains voltage powering the ballast, since the encoded perturbations may be small enough so that lamp flicker is avoided even with encoded signals on the line voltage. In a favorable embodiment for dimming, no perturbations are introduced on the line voltage unless a change in the operating characteristic, for example the light level, of the electric lamp is desired. This has the advantage that when lamp operation is to be kept constant, no distortions are imposed on the mains voltage, so that flicker of the lamp(s) is completely avoided and there are no adverse effects to the power factor, THD or component stress.

A system for communicating over electrical wiring in a house or other building is presented. Components are grouped and each group is assigned a group identifier code. Components communicates only with components of the same group, using the group identifier code. Each message includes the group identifier code, message data, and a message authentication code (MAC) that is calculated for each message. A receiving component disregards any message whose group identifier code is not the same as that of the receiving component. MACs are calculated using a shared key value and a one-way hash function. The shared key value, in turn, is taken from an ordered sequence of key values that is defined for each component group based on a counter value. To change to a new key value, one component of the group simply starts using the new key value. When a receiving component receives a message that does not.

Switching of parallel capacitors in an HID bi-level lighting control system is accomplished through use of transient voltage suppression across an electronic relay for discharge of residual charge from a switched capacitor when combined peak voltage exceeds clamping voltage, thereby allowing maximum switch voltage rating to be lower than is possible through the use of conventional switching methods and circuitry. The invention contemplates method and apparatus permitting capacitive switching at voltage levels higher than are possible in conventional capacitive switching arrangements including capacitive switching arrangements used in lighting control systems.

A system communicates over electrical wiring in a house or other building. Messages between components include a group identifier code, message data, and a message authentication code (MAC) that is calculated for each message. MACs are calculated using a shared key value and a one-way hash function.