GB2126809A - Electrical supply lines - Google Patents

Abstract

To ensure continued operation of power consuming stations (S1-S4) that are connected in a two-wire ring (R1) driven from an a.c. power source (D1) via a pair of series limiting resistors (IR), the individual stations are connected into the live conductor of the ring via respective pairs of terminals (LT) and arranged to operate in a parallel mode when normal operation is effective. Each station has a transformer primary winding (PW) connected between the ring conductors by an associated mode- converting resistor (MR). The load (LS) is connected to a secondary winding (W2). A tertiary winding (W3) is connected between the line terminals (LT) in each station to assume operation in a series mode in the event of a short circuit occurring between the ring conductors. The loads may be loudspeakers, lamps or alarm bells. <IMAGE>

Description

SPECIFICATION Hybrid electrical supply lines The invention relates to hybrid electrical supply lines comprising a plurality of power-consuming stations in a two-wire ring circuit and a common a.c. input circuit connected between the wires.

In many electrical installations there is a need to provide a common drive to a plurality of loudspeakers, alarm bells, display elements, emergency light units, or other electrical signal or transducer means of a common type. For example, in hotels, factories, large aircraft and ships, at airports or other transport termini, and places of private or public assembly such as museums, theatres and office blocks, there may be a need for public address facilities enabling announcements, paging and/or alarm indications to be produced at a large number of individual signal stations. The feeding of audio-signals to loud-speakers is only one field of use, since similar security problems apply in the case of alarm installations for fire alarms, flood warnings, hazardous environments and other such warning arrangements.

Yet again, the ring circuit could be for power distribution, such as an emergency lighting installation intended to provide some low level of background illumination in the event of a power supply failure, whether resuiting from a break in mains distribution, a local fire, or any other cause.

In all these situations the operational reliability is of paramount importance, and may lead to the use of unnecessarily heavy wiring, excessive safety margins in component ratings and duplication of functional elements, which in turn considerably increases the cost of an installation.

Such precautions do not render a ring circuit of stations immune from total or partial failure in the event of a break in the wires of the ring or of a short circuit between the two wires at some intermediate point.

In accordance with one aspect of the present invention there is provided a hybrid electrical supply line comprising a plurality of powerconsuming stations in a two-wire ring circuit and a common a.c. input circuit connected between a live conductor and an earth return forming the two wires of the ring circuit, each powerconsuming station having its load element connected to a secondary winding of a transformer, a pair of line terminals for inserting the station in the live conductor of the ring circuit, a highly inductive primary winding connected in series with a limiting resistor between one of said pair of line terminals and the earth-return conductor of the ring circuit, and a relatively very low inductance tertiary winding connected between said pair of line terminals.

Preferably the load element is connected in series with its associated secondary winding between the pair of line terminals for the particular station.

In accordance with another aspect of the present invention there is provided a hybrid electrical supply line comprising a plurality of power-consuming stations in a two-wire ring circuit and a common a.c. input circuit connected between a live conductor and an earth-return forming the two wires of the ring circuit in such a manner that, in normal operation the stations are effectively connected in parallel, even if there is a break in the conductors, but remain operative as series connected stations in separate branches diverging from said common a.c. input circuit if there is a short-circuit between the conductors at any intermediate point in the ring circuit.

According to yet another aspect of the invention there is provided a power-consuming station constructed for use in a hybrid electrical supply line having a live conductor and an earthreturn conductor forming a ring circuit normally operating in a parallel mode, but constructed to remain operative in a series-mode in the event of any short circuit between the line conductor and earth-return conductor at any intermediate point in said ring, when connected therein.

The power-consuming stations may utilise loud-speakers or other electro-acoustic transducers capable of reproducing frequency modulated input signals, or alarm units such as bells or sirens, or visual display elements such as illuminated signs and/or emergency lighting units. Furthermore, the stations may be incorporated into a ring circuit of a fire alarm system.

Whereas a conventional ring circuit may be rendered totally or partially inoperative by a shortcircuit or break, the invention ensures that the stations remain fully effective, even if at reduced level. By careful design any loss in output signal level can be made insignificant.

The invention will now be described with reference to the drawing which schematically illustrates one exemplary embodiment, which shows four stations, S1 to S4 in a ring circuit R1 fed by a common a.c. input circuit driving source Dl. The ring circuit is completed by sections LR of a line conductor linking a terminal LT of one station to a terminal LT of the next station. These line sections are represented by theoretical resistor symbols LR to indicate that the section length in a large installation will have a significant resistance.

An earth-return conductor ER forms the other wire of the two-wire ring circuit, and is connected to an earth terminal ET at each station. To prevent a short-circuit between the two conductors at a point immediately adjacent the input circuit, having an immobilising effect, a separate series limiting resistor SLR is provided at the junction of the input circuit to each adjoining live conductor section LR.

In each station the two live conductor terminals LT in this embodiment are connected via the series combination of a secondary winding W2 of a matching transformer and the speech coil of a loudspeaker LS. In normal operation the loudspeaker is effectively driven by a high inductance primary winding PW of the matching transformer, which is connected between a terminal LT and the associated earth terminal ET via a series mode-converting resistor MR. A tertiary winding W3 of low inductance is provided on each matching transformer, and is connected directly between the associated terminals LT. The start of the primary winding PW is connected to the start of the associated secondary winding W2 and the end of the associated tertiary winding W3.In a typical embodiment, for every turn of the tertiary winding W3 there will be two turns of the secondary winding W2, and perhaps as many as 180 of the primary winding PW.

The input circuit may be a 50v P/P signal voltage source connected to a 50v line via the series limiting resistors SLR each having a value equal to that of the value of the resistor MR in each station. For the purpose of explaining the mode conversion advantages that are provided when the invention is utilised in a large installation it will be assumed that the value of resistance LR introduced by the live conductor of each line section has a common value.

In normal operation the current flowing via a tertiary winding W3, resistor MR and primary winding PW will combine to produce an effective output voltage in the secondary winding W2, the contribution from the tertiary being negligible in normal, parallel-mode operation, and the voltage at each loudspeaker being typically 0.9v P/P.

Because of the adverse effect of the line resistance in large installations, the simpie ring circuit safeguard does not ensure effective operation in the event of a short-circuit, since the voltage available across any primary winding may be seriously reduced. In these circumstances the presence of the resistor MR prevents any reflected short-circuit appearing at the primary winding PW of an affected station, and the station then operates as a series-mode connection, the current flowing through the tertiary winding W3 becoming the dominant factor in driving the secondary winding W2, and the sum of the potentials across these windings is applied to the loudspeaker coil. In the drawing there are five short-circuit positions SC1 to SC5, which each represent a possible short-circuit between the ring conductor due to some accident or fire, for example.In the absence of any safeguard this type of short-circuit would render the adjacent stations inoperative, and could reduce the output at all stations to a negligible level, depending upon the actual resistance introduced by intervening line sections.

In the circuit shown, assuming an equal resistance in each line section giving a total line resistance that is equivalent to some 700 metres of a typical cable, the signal voltages obtained at the loudspeaker LS in each of the stations S1 to S4 in the event of a short-circuit at any of the points SC1 to SC4 has been found to be as shown in the following table:- S1 S2 S3 S4 SC1 0.8 0.9 0.55 0.55 SC2 0.85 0.9 0.7 0.65 SC3 0.7 0.8 0.8 0.8 SC4 0.6 0.7 0.75 1.0 SC5 0.55 0.7 0.75 0.75 Thus, even in the most adverse case there is still an effective residual signal output of more than 50%; the normal voltage being lv plus or minus 10%.

It should be noted here that it is possible to connect all the stations into the ring in a similar configuration, but the circuit shown utilises reflected connections, stations S1 and S2 being connected in one configuration, and stations S3 and S4 being connected in the mirror-image configuration, as this has been found to improve the effectiveness by giving a more uniform energy distribution. If the laod is not connected in series with its secondary, as in the illustrated embodiment, but is only inductively coupled, then the turns ratio of W2:W3 must be adjusted to maintain effective series mode working.

It will be readily understood that the load to be supplied may take many forms, such as an electro-acoustic transducer of PZT, a bell or siren, or a lamp or optical display. When used in combination with a fire alarm system the provision of loudspeakers to issue warning statements and to control or advise can help to reduce panic and improve safety by giving information regarding exit routes, for example.

Thus a power-consuming station can provide a loudspeaker within each fire-detector, or in selected detectors.

In an emergency lighting installation the proposed system gives greater reliability and reduces the likelihood of total darkness conditions in hazardous circumstances when the ring wiring may well be liable to damage.

By monitoring the output power it is possible to provide a warning indication in the event of a line fault causing series-mode operation to become effective. Normal loop current monitoring in a conventional manner can obviously be utilised to detect any break in a conductor, if required.

Claims (6)

Claims

1. A hybrid electrical supply line comprising a plurality of power-consuming stations in a twowire ring circuit and a common a.c. input circuit connected between a live conductor and an earth return forming the two wires of the ring circuit, each power-consuming station having its load element connected to a secondary winding of a transformer, a pair of line terminals for inserting the station in the live conductor of the ring circuit, a highly inductive primary winding connected in series with a limiting resistor between one of said pair of line terminals and the earth-return conductor of the ring circuit, and a relatively very low inductance tertiary winding connected between said pair of line terminals.

2. A hybrid electrical supply line as claimed in Claim 1, in which the load element is connected in series with its associated secondary winding between the pair of line terminals for the particular station.

3. A hybrid electrical supply line comprising a plurality of power-consuming stations in a twowire ring circuit and a common a.c. input circuit connected between a live conductor and an earthreturn forming the two wires of the ring circuit in such a manner that, in normal operation the stations are effectively connected in parallel, even if there is a break in the conductors, but remain operative as series connected stations in separate branches diverging from said common a.c. input circuit if there is a short-circuit between the conductors at any intermediate point in the ring circuit.

4. A hybrid electrical supply line substantially as described with reference to the drawing.

5. A power-consuming station constructed for use in a hybrid electrical supply line having a live conductor and an earth-return conductor forming a ring circuit normally operating in a parallel mode, but constructed to remain operative in a series-mode in the event of any short circuit between the line conductor and earth-return conductor at any intermediate point in said ring, when connected therein.

6. A hybrid electrical supply line powerconsuming station substantially as described with reference to the drawing.