GB2407013A - Signalling via mains wiring - Google Patents

Abstract

Sending signals over the live and neutral wires of domestic wiring, encounters problems associated with appliance switching and varying impedance due to changes in the load. These problems are reduced by using the neutral and earth wires. A switch or sensor A actuates encoder B, which has a user set code e.g. of 12 bits, which modulates an FM modulator C. The modulator uses two different frequencies to represent binary values and the pulse width may also be different for the binary values. The RF pulses are impressed across the neutral and earth conductors of the mains wiring, via a capacitor G. Additionally the signal may also be applied to the live conductor via capacitor R.

Description

Sicnallin via the mains wiring The domestic mains wiring consists of 3

cables, Live Neutral and Earth and there exists examples of utilising these cables for communication purposes. The best known being the voice intercom frequently in the form of a baby alarm.

These intercoms could be plugged into any mains sockeyes and used Live and Neutral Lines onto which was superimposed the audio signal either as plain audio or in more elaborate arrangements the audio was incorporated as part of a modulated signal.

The disadvantages of this arrangement were twofold, both mains sockets must be wired to the same phase of the mains suppler, and the ever present 50 or 100 hz hum had to be eliminated from the audio circuits.

Nowadays there is a lot more noise superimposed on the mains wiring due to the huge increase in domestic appliances which rely on switch-mode power supplies. Examples are light dimmers, fluorescent lights, motor speed controls and computer power supplies as well as thermostats all of which can superimpose high amplitude spikes on the mains. The mains wiring is therefore much noisier now than it was when the mains intercoms were introduced. In addition, the X10 protocol addresses the use of Zero (grossing techniques when signalling via mains wiring using Lisle and Neutral.

The arrangement, which is the subject of this application, adopts the significant step of using the Neutral and Earth wires of the mains. This has the following advantages.

1) The amplitude noise mentioned before is mainly associated with the Live wire as it is the live circuit which is switched by Thyristors and switch-mode power supplies.

2) The RF Impedance of the Neutral and Earth wire tends to remain constant w hen compared with the Live-Neutral circuits. This allows a superior impedance match to be achieved. The:RF Impedance of the live and neutral circuits will vary during the course of a da: as equipment is switched on. and off and plugged in and out, This particularly applies to the modem use of E.MC Filters which often have fixed capacitors fitted across these lines.

3) In a building such as a school or an office block it is likely that a 3-phase supply is provided. The Neutral and Earth wires are common to a 3phase installation and signals flail] be present in all parts regardless of the phase to vhch the individual socket is connected. This means that it is not necessary, to provide a means of linking the phases together to allow RF signals to pass as would be required in a system using the Live wire.

A further enhancement of the operation can be achieved by coupling a part of the signal to the Live wire as well as the Neutral wire. This reduces the series inductance of the transmission path thus improving signal levels at the receiving end. It also helps overcome the attenuation encountered by the presence of Earth Leakage Trips and Miniature Circuit Breakers as these are frequency double wound and their impedances are equal and opposite.

The system to be described is not for voice transmission but is intended for, but not restricted tot the data necessary to identify a. particular sending station. In its basic form one sender and one receiver is used but a combination of many senders and rno,-e than one receiver is possible.

With reference to Fig 1 and Fig 2 where Fig 1 is the sender and Fig 2 the receiver.

A means of actuation - A - can be a switch, or external sensor initiates the Encoder -B- which generates a code according to its settings. Typically such encoders have address lines which are set by links or slide switches so that the code may be selected by the user thus enabling each sender in a system to have its over code.

The Modulator - C - consists of an oscillator with transmission being initiated by the actuator - A. The data Tom the encoder is used to frequency modulate the modulator such that the frequency shifts in accordance with the l's and O s become fl and f2. In practice these frequencies can be anything from audio to RF.

The modulated signal is sent to the output stage - D - which amplifies and matches the signal to the mains wiring.

The power supply - H - feeds all stages and is fed by transforn:er - F -. The output of the sender is coupled to the Earth wire - E - and the neutra! line - I] - via a capacitor -G-- which should be a good quality mains rated component. In addition the signal is also coupled to the Live conductor via capacitor R -, and this can be shown to provide advantages.

With reference to Fig 2 The signal across Neutral and Earth is passed to the receiver via Capacitor - O - The input stage - I - comprises an amplifier and discriminator which filters out the wanted FM signal and feeds this to the decoder - .T - . The decoder generates an output reflecting the encoder switch or link settings in a form that can be shown by the display unit - K - . This unit might be a numerical or indicator panel as appropriate. In practice the display unit can be latching and provide additional outputs to initiate actions as appropriate to the application.

The power supply -- M - powers the receiver, although it should be noted that the system will work in the event of a power failure as the Neutral and Earth wires will still be intact. It is assumed that a battery backup facility is provided in this case.

The enhancement which uses the Live wire mentioned earlier is connected to the receiver via Capacitor- P - . It has been mentioned that the noise on the mains causes problems for such arrangements but the system described here has immunity to such noise as follows.

Firstly the noise interference is of a pulsed amplitude nature and the use of FM allows the receiver to discriminate between AM and FM signals by conventional amplitude limiting techniques but an additional level of immunity can be provided by the choice olpulse widths as follows.

Noise interference from, say, a light dirnrrer is tied to the mains frequency of 50hz or 1 OOhz in the case of rectified mains and the switch spikes w 11 occur at this rate typically every 1 Om S. By making the bit rate period of the data signify cantly longer than thelOmS noise pulse period, the receiver can be made to ignore any signals either of an amplitude nature or with a pulse repetition rate of this value.

The pulse length representing O is 50mS and I is 1 OOmS both of these pulses is therefore many times longer than any mains related noise.

Pulse width and.FM applies all to improve noise immunity In the working version a 12 bit code is adopted and the pulse width set to 15mS Which gives a word length of 1 80mS meaning some 15 repetitions will talce place in the 3 second transmission.

Claims (3)

1. Claims i) A communication system using the Neutra] and Earth wires in an

   electrical installation to eliminate the distributed loads and the noise found on the Live to Neutral circuits.
2. 2) A conrwication system as in 1 and 2 which couples the live and neutral wires together at the same RF potential and phase to eliminate common mode and differential inductance and reduce series inductance.
3. 3) A communication system using the main Neutral and Earth independent of the live phase in a 3 phase installation 4) A communication system as in I to 4 which utilises pulse widths longer than the maximum duration of noise spikes found on the live to neutral. s