Protective Systems And Apparatus Therefor

Abstract

A protective system capable of being monitored for day and night use comprising a single electrical loop including a plurality of series-connected contacts arranged to be actuated by an intruder and a relay connected in the loop and arranged to be rendered operative to actuate associated contacts connected in an alarm circuit to indicate the presence of the intruder, selected ones of the contacts having diodes connected across the contacts such that the system can operate in accordance with two different modes. For day use, the current flow through the diodes is such that no alarm indication is provided if a series-connected contact spanned by a diode is actuated, while for night use, a switching unit is provided to reverse the direction of current flow through the loop to cause the relay to be rendered operative if any series-connected contact is actuated.

Description

This invention relates to protective systems and apparatus therefor such as intruder protection, fire and bandit warning systems including intruder detection and warning devices and fire and bandit warning alarms and detectors capable of providing a warning indication of any abnormal occurrence such as the entry of an intruder into a building, or the outbreak of heat, smoke or fire.

Heretofore, with some intruder protection systems, a closed electrical circuit loop has been provided having wired therein series connected, normally closed contacts associated with doors, windows and the like and arranged to open and close with opening and closing of the doors, windows etc., and parallel-connected, normally open contacts associated with floor pressure mats for instance and arranged to close under pressure.

When the intruder protection circuit is operative, a current flows through the series-connected contacts of the loop to effect energization of an electromagnetic relay until one or more of the series-connected contacts is opened in response to the opening of an associated door or window by an intruder, following which the relay is deenergized to effect actuation of associated relay contacts connected in an alarm circuit such that a warning indication is provided of the presence of the intruder. Furthermore, the application of pressure to a floor mat detector will close the associated parallel-connected contacts to bypass or "short" the relay winding thereby actuating the relay contacts in the alarm circuit and again providing an indication of the presence of an intruder. It will be appreciated that such a closed loop protective circuit provides a warning indication also if a short circuit or open circuit condition develops in the loop.

Open circuit loops have also been provided having normally open contacts connected across a battery or the like connected in series with the winding of a relay, such that closure of a contact in response to a break-in or similar occurrence effects a short circuit thereby deenergizing the relay winding to provide an indication of the presence of an intruder.

A disadvantage of the closed loop system is that when such a closed loop is installed in a building, it is necessary to switch-off the system during those periods when the building is occupied otherwise the alarm circuit would be operative to provide a warning indication each time a door or window was opened or a person closed the contacts of a floor mat detector. However, it is during the switch-off periods that the system may be inadvertently or deliberately damaged and such damage might escape detection until the system is switched on again when all personnel have vacated the building.

Such a situation leads to considerable inconvenience requiring as it does engineers standing by on call at peak closing hours in order to repair any such damage which might have occurred during the switched-off period.

A further disadvantage of the present system is that the whole system is switched off during those periods when the building is occupied whereas a certain degree of protection may be required during those periods at selected points for instance at basement windows, skylights, rear doors, etc. With present systems, in order to achieve such selective protection, a second wiring loop is provided but such an arrangement adds considerably to the cost of the overall installation.

Accordingly, it is an object of the present invention to provide a protective system and apparatus thereof including a single electrical loop but capable of providing selective protection when required and capable of being monitored 24 hours a day in order to overcome the disadvantages previously specified.

A further object of the present invention is to provide a protective system and apparatus therefor including a single electrical loop capable of operating in accordance with two different operative modes.

A still further object of the invention is to provide a two-wire protective loop capable of providing an alarm indication whenever the two wires are wholly or partially short circuited or one or other or both "open circuited".

According to the invention there is provided a protective system comprising a single electrical loop including a bistate control device having means associated with an alarm circuit and arranged to be actuated in response to a change in the operative state of said bistate control device, the loop being connected to a voltage source and also including a plurality of contacts arranged to be actuated whenever an abnormal situation against which protection is required, occurs, and the loop being arranged to operate in accordance with two alternative operating modes during one of which the current flowing through the loop is sufficient to allow said bistate control device to operate in accordance with a first operative state despite actuation of any of said contacts, and during the second mode the current through the loop being such as to cause the bistate control device to change to the other of said operative states whenever a contact is actuated.

In one embodiment of the invention there is provided a protective system including a closed electrical loop connected across the winding of a relay having contacts connected in an alarm circuit, the loop including a plurality of series-connected contacts arranged to be actuated when an abnormal situation occurs, a respective unidirectional current-conducting device connected across selected ones of said contacts such that the system is capable of operating in accordance with one current-conducting mode and switching means for inhibiting current flow through said unidirectional current-conducting means such that the system operates in accordance with a second current-conducting mode.

In a further embodiment of the invention there is provided a protective system as described above wherein an additional relay is included in the loop together with normally open parallel-connected contacts connected in series with impedance means, the arrangement being such that whenever a parallel-connected contact is closed, the current flow through the additional relay is increased to an extent sufficient to change the state thereof but without effecting any change in the state of said first mentioned relay.

In another embodiment of the invention there is provided a protective loop system as described above having a two-wire loop terminated by impedance means and having a relay means arranged to be actuated whenever a whole or partial short circuit or open circuit condition occurs in the loop, second relay means operable to provide second channel operation and interlocking means associated with the two relay means to prevent operation of the second relay means should such a whole or partial short circuit or open circuit condition occur.

Several embodiments of the invention will now be described by way of example only with particular reference to the accompanying drawings wherein:

FIG. 1 is a circuit diagram of a conventional "closed loop" intruder alarm system;

FIG. 2 is a circuit diagram of a "closed loop" intruder alarm system according to the present invention;

FIG. 3 is a circuit diagram of a "closed loop" combined intruder and warning alarm system of the present invention;

FIG. 4 is a schematic circuit diagram of part of an "open loop" intruder alarm system;

FIG. 5 is a circuit diagram of part of an AC-DC circuit for an intruder alarm system and

FIG. 6 is a circuit diagram illustrating how the response time of a circuit is utilized to provide an intruder alarm indication.

Referring to FIG. 1 the closed electrical loop includes a plurality of normally closed series-connected contacts C1 to C5 which may be associated with windows, doors, etc., and arranged to be opened when the associated door or window is opened, and a pair of normally open parallel-connected contacts M1, M2, which may be associated with contact mats or the like each arranged to be closed when pressure is applied to the respective contact mat. The reference Ws indicates a wire screen provided for protection against intruders at windows, glass doors, etc. The contacts C1 to C5 are connected in series with a relay R1 and contacts M1 and M2 are connected across the relay which is normally energized when the series-connected contacts C1 to C5 are closed, as shown, and the parallel-connected contacts M1, M2, are open. The relay R1 has associated contacts R1/1 connected in an alarm circuit and the contacts R1/1 are arranged to be actuated upon deenergization of relay R1 to provide an alarm indication of the presence of an intruder, whenever one or more of the contacts C1 to C5 is or are opened, or one or more of contacts M1, M2, is or are closed, or the wire screen Ws is broken.

A battery B completes the loop and has a limiting resistor r.sub.1 connected in series therewith in order to protect the battery when a short circuit occurs.

With such an arrangement it is impossible to monitor the protective loop or provide selective protection for instance when a building protected by the loop is occupied and when the doors or windows protected by the contacts C1 to C5 are in use and personnel are liable to tread on the pressure mats protected by contacts M1, M2. Accordingly it is necessary to switch-off the system during occupation of the building, during which period the loop may be damaged and escape detection, and no selective protection is possible with such a prior art arrangement.

Referring now to the arrangement illustrated in FIG. 2 unidirectional current-conducting devices such as semiconductor diodes D1, D2, D5, are connected across the respective series-connected contacts C1, C2, C5, and a further diode D4 is connected in series with parallel-connected contact M2. For periods during which the premises protected by the loop are occupied, e.g., during daylight hours, the polarity of the current flowing through the loop is arranged such that diodes D1, D2 and D5 are forward connected such as to pass current whenever the respective contact C1, C2 or C5 is opened, whilst diode D4 is reverse connected such as to inhibit the passage of current when contact M2 is closed.

If any of contacts C1, C2 or C5 are opened such as by opening of the doors or windows associated with those contacts, the respective diodes D1, D2, D5 will conduct in the forward direction and will permit sufficient current to flow in the loop to retain relay R1 in the energized condition. Thus the contacts R1/1 are not actuated and no alarm indication is given.

Similarly, if contact M2 is closed by pressure applied to the associated mat, since the diode D4 is reverse connected, no current will flow therethrough and no shunting of relay R1 will occur. Thus the relay R1 will remain energized and no alarm indication will be given.

If the contacts C3 or C4, are opened or screen Ws is broken, since these contacts are not shunted by a diode and are effective to provide an indication of the passage of an intruder through the window or door associated therewith, the relay R1 will be deenergized to actuate contacts R1/1 and provide an alarm indication of the break-in. Similarly contact M1 has no diode in series therewith and if the mat associated with contact M1 has pressure applied thereto, the contact M1 will close to short circuit relay R1 thereby deenergizing the relay and actuating contacts R1/1 to provide an alarm indication.

It will be appreciated that the above arrangement allows the circuit to be monitored during the period when the building is occupied and provides protection at selected points of the building.

In order to provide complete protection during those periods when the building is unoccupied a changeover switch (not shown) is provided whereby the direction of current flow through the loop is reversed. Diodes D1, D2 and D5 will now be reverse-connected and diode D4 will be forward connected. Thus, if contacts C1, C2 or C5 are opened by opening of the door or window protected thereby, the leakage current through the reverse connected diode will be insufficient to maintain energization of the winding of relay R1. Accordingly, contacts R1/1 will be actuated to provide an alarm indication of the presence of an intruder. Furthermore since diode D4 is now forward connected, if contact M2 is closed diode D4 applies a short circuit across the relay winding and thus relay R1 is deenergized and the alarm indication given. With the above arrangement complete protection is available at all detection points and thus selective protection is possible when the current is flowing in one direction through the loop and complete protection is provided when the direction of current flow is reversed.

Referring now to the circuit illustrated in FIG. 3 of the drawings, the same contacts C1, C2, C5 and M2 have been provided with respective diodes D1, D2, D5, D4, but in addition thereto, normally open detection or alarm contacts F1, F2, F3, F4, have been connected across the loop in series with respective resistors r.sub.2, r.sub. 3, r.sub. 4, r.sub. 5, and an additional relay R2 has been included in the loop with the winding thereof in series with battery B in place of resistor r.sub. 1. The contacts R2/1 associated with relay R2 may be connected in a fire-warning alarm circuit or in a bandit-warning alarm circuit.

The circuit operates in the "selective" and "complete" protection modes as described above for intruder protection and for additional fire or bandit protection. The resistance and ampere-turns of coil R2 are chosen such that with both relays R1 and R2 in series, the current passing through the loop is sufficient to energize relay R1 but insufficient to energize relay R2. If necessary, the applied voltage V may be increased to allow for the additional voltage drop across the winding of relay R2.

If any of the contacts F1 to F4 close in response to the detection of heat, fire or smoke, or in response to depression of a bandit or "hold-up" alarm switch or the like, relay R1 will be shunted by the respective resistor r.sub. 2 to r.sub. 5. However, the individual ohmic value of resistors r.sub. 2 to r.sub. 5 is selected such that the shunting effect on relay R1 is insufficient to deenergize this intruder alarm relay R1. Nevertheless, the current which now flows through the resistor is sufficient to effect energization of relay R2 thereby actuating contacts R2/1 and, in the case of a fire alarm system, provide an alarm indication of the existence of the fire, heat or smoke.

A latching contact R1/2 is provided associated with the reset switch S1. As before, closure of contacts M1 or M2 will effect deenergization of relay R1, but at the same time, the current through the loop is increased due to a virtual short circuit caused by closure of contacts M1 or M2. Consequently, the latching contact R1/2 is included in the circuit to prevent energization of relay winding R2 in response to closure of contacts M1 or M2, and to prevent a false fire alarm or hold-up alarm from being given. When relay R1 is deenergized, the latching contact R1/2 is actuated to open the loop circuit thereby retaining relay R2 in the deenergized state and preventing an alarm from being given. The loop will remain in the open circuit condition until reset switch S1 is operated.

When one of the contacts F1 to F4 close in response to the detection of heat, fire or smoke or in response to depression of a "bandit" or "hold-up" alarm switch, relay R1 is not affected and remains energized and the loop circuit remains closed via latching contact R1/2. An increased current then flows due to the short circuit initiated by closure of a contact F1 to F4, and relay R2 is energized to effect actuation of contacts R2/1 thereby providing a fire or bandit warning alarm indication.

It will be appreciated that an advantage of the circuit shown in FIG. 3 is that since the "detection" contacts F1 to F4 have no diodes or similar unidirectional current-conducting devices connected in series or in parallel therewith, the contacts are not polarity sensitive, i.e., they are operative irrespective of the direction of current flow through the loop and thus detection facilities are provided for 24 hours of every day if necessary.

The invention is in no way limited to the closed circuit loop shown in FIGS. 2 and 3 and an open loop is shown in FIG. 4 having parallel-connected contacts M3, M4, connected across the battery B and relay R1. FOr dual-mode operation, a semiconductor diode D5 is connected in series with contact M4 and the circuit operates in a similar manner to that part of the circuit already described with reference to FIG. 2 and includes contact M2, diode D4 and relay R1.

Furthermore it will be appreciated that the invention has been described by way of example only and is susceptible of considerable modification. For instance, electrolytic capacitors may be used as the unidirectional current-conducting devices, instead of the semiconductor diodes. An electrolytic capacitor will, when connected in circuit with a voltage supply become charged such as to inhibit the flow of current in the circuit. If the polarity of the applied voltage is reversed, the capacitor behaves as an impedance and permits current to flow continuously since the capacitor is no longer being charged. Thus, any device which changes the magnitude of the current flowing in a circuit when a change of polarity occurs, may be used, and the term "unidirectional current-conducting device" is to be construed to include any such device.

From the foregoing it will be appreciated that the present invention provides a protective system capable of operating in accordance with two different modes which may be termed conveniently "day" and "night" modes respectively. However, it is not necessary for the change in mode to be dependent upon a change in the direction of the current flowing through a loop or upon the use of unidirectional current-conducting devices. The change in mode may be effected by means of a change in the applied voltage or by changing the response time of the circuit, which for the purposes of the following explanation, includes the relay winding or windings.

A change in the applied voltage may be effected by

a. a reversal of polarity, e.g., of the battery B in order to produce a change in the direction of current flow through the loop as already described;

b. a change in the waveform of the voltage signal, e.g., from an AC voltage to a DC voltage;

c. a change in the value of the voltage applied to the relay or relays and

d. a change in the response time of the circuit which may be conveniently effected by changing the response time of the relay or relays.

Referring to (b) above, a circuit for changing from an AC voltage during the day to a DC voltage at night is shown in FIG. 5. The series-connected contacts C1, C2, C3, are arranged to be connected in circuit with terminals a, b, of an AC voltage source via switches S2, S3 during the daytime such as to operate in accordance with one circuit mode, and to be connected via switches S2, S3, to terminals c, d, of a DC voltage source for night operation. Contacts C1 and C3 have capacitors C10, C11, shunted thereacross and a parallel-connected contact M1 is connected across the relay R1. An associated relay contact R1/1 is normally closed and arranged to be opened when the relay R1 is deenergized.

During the day, an AC voltage is applied to the loop and if one of the contacts C1 or C3 shunted by capacitors C10, C11 respectively, is opened, current continues to flow to maintain the relay energized. The value of the current is determined by the resistance of the capacitor. For night security, the contacts S2, S3 are switched to terminals c, d, respectively. If a contact C1 or C3 opens, the respective capacitor charges to inhibit the flow of current through the loop and the relay R1 is deenergized to actuate the associated relay contacts in the intruder-warning alarm circuit. Similarly, the relay will become deenergized when a short circuit occurs.

Referring to paragraph (c) above, any contacts which might be opened during daytime use are shunted by resistors such as to render the opening of a respective contact ineffective. When a contact is opened, the resistor is switched into circuit and the voltage across the protective relay decreases, but remains of sufficient magnitude to retain the relay energized.

For night security, the value of the applied voltage is decreased and the additional resistance produced by the opening of a contact, decreases the relay voltage to a value sufficient to cause deenergization thereof.

Referring to paragraph (d) above, and as shown in FIG. 6, all contacts required to remain ineffective during one circuit mode, are of the single pole changeover type with the contacts looped as shown at x and y. During the day, if a contact C1 or C3 is opened, a momentary break only occurs. During the contact changeover period, the relay R1 is held energized by the capacitor C12 across the winding thereof. For night security purposes, the capacitor is taken out of circuit by actuation of switch S3 and a momentary break in the circuit will then deenergize the relay R1 which trips via its associated latching contact R1/1.

Instead of a battery B, the DC voltage may be derived from a DC power pack supplied from the AC mains. As previously described, for periods during which a building is occupied, the applied voltage may be increased above the nominal voltage to compensate for the voltage drop across the diodes. The voltage supply during the "occupied" periods may be separate from that employed during "unoccupied" periods. The former may be derived from a DC powerpack and the latter from a battery. Alternatively, for "unoccupied" periods, the voltage may be derived from a powerpack with standby battery facilities; this facility being less necessary during "occupied" periods when alarms due to a supply failure are unimportant. Where a battery standby is available, this may mask a supply fault, such as a blown fuse, in which case the battery would be used unnecessarily, thus shortening the working life of the battery. If no standby battery is employed during "occupied" hours, such faults would be immediately detected.

The diodes connected across the series-connected contacts or connected in series with the parallel-connected contacts of the loop may be mounted separately from the contacts or incorporated within a switch unit, particularly where the contacts may be of the magnetic reed type. Such a switch unit is described in copending British Pat. application No. 50,385/68 and the switch unit may include a switch and at least one unidirectional current-conducting device and/or impedance means connected in series with or across the contacts of the switch.

The relays employed in the present invention may be of any suitable type such as electromagnetic or transistor relays.

For an intruder alarm circuit only such as illustrated in FIG. 2, only one relay is used in the protective loop but since this relay is self-resetting, it must be provided with a latching contact, or may operate a second relay which latches and completes the alarm circuit. The loop relay R1 should have low-current consumption and be sufficiently robust not to be operated by vibration.

For a combined intruder alarm fire or bandit alarm circuit (FIG. 3) the two relays R1, R2 are chosen to have a predetermined resistance/ampere turn relationship, and the ohmic value of resistors r.sub.2 to r.sub.5 will depend upon the characteristics of the relays R1, R2.

The system of the present invention provides complete 24 hour monitoring which implies that relay R1 (FIGS. 2 and 3) is continuously energized, being deenergized only when a detector is operated.

This condition is unsatisfactory since the relay may become "stuck" in the energized condition, i.e., an "unsafe" fault condition. When switching from the "occupied" to the "unoccupied" condition, or vice versa, the relay should preferably be deenergized and an indication of the condition provided. An "unsafe" relay condition would thus not escape detection. During the momentary period during which the relay was deenergized on test, the alarm devices would be switched out of circuit.

The present invention relates to fire protection circuits employing either fire detection devices or fire alarm warning devices, or both. Fire detection devices include devices which detect the presence of fire, heat or smoke where fire alarm warning devices are devices which are manually operable push buttons operated to give a warning indication of the existence of an outbreak of fire. The contacts F1 to F4 (FIG. 3) may include either or both types of device.

Although the series-connected contacts have been described with particular application to doors, they may be the contacts of any form of intruder detection device such as for instant invisible infrared ray units ringing the exterior of a building or located within the building.

A manually operable control unit for use with the protective system of the present invention is described in copending British Pat. application No. 51,843/68 in which switching mechanism is provided to enable the system to be switched from one operative mode to the other and including locking means for preventing the switchover to the second operative mode whenever a fault or alarm condition prevails.

In a modification of the present invention, the loop comprises only two wires terminated by a resistor and no return to the control unit is required in such an embodiment. Current is arranged to flow through the loop to maintain a transistor relay in the "OFF" condition when the loop is operative, i.e., giving complete protection. If the two wires of the loop are wholly or partly short-circuited, or one or both are "open-circuited", the transistor is rendered conductive to energize or deenergize the winding of the relay and give an immediate alarm. The term "transistor relay" is intended to indicate at least one transistor connected in circuit with an electromagnetic relay, but the latter may if required be replaced by a power transistor or other solid-state device such as a silicon-controlled rectifier.

Previously, two-wire loop systems have been of the "open circuit" type but this embodiment of the invention has numerous advantages over present two and four wire systems and in particular, two-channel operation is obtainable, as follows:

Preferably normally closed contacts are associated with doors, windows and like points of entry and the contacts are arranged to be held open when the door or window is closed. When a door is opened, the contact is closed to effectively short circuit the protective loop. The diodes are connected in series with the contacts and if a resistor is connected in series with the diode, the short circuit current will be limited in value. The value of the resistor is chosen such as to prevent transistor relay from operating but a second "transistor relay" is arranged to operate to give second channel operation. By interlocking the two relays, the second one is prevented from operating under short circuit conditions.

Claims

I claim:

1. A protective system comprising a single electrical loop including a bistate control device associated with an alarm circuit, said bistate control device operatively changes from one state to another whenever an abnormal situation against which protection is required occurs, the loop being connected to a voltage source and also including a plurality of detection contacts actuated whenever such abnormal situation occurs; a respective unidirectional current-conducting device connected across at least selected ones of said contacts such that the system is capable of operating in accordance with one current-conducting mode despite actuation of any of said at least selected ones of said contacts and switching means to inhibit current flow through the unidirectional current-conducting devices to permit the bistate control device to change to the other of said operative states whenever any one of said contacts is actuated such that the system operates in accordance with a second current-conducting mode and thereby provides continuous monitoring of the loop.

2. A protective system comprising a single closed electrical loop including the winding of a relay having contacts associated with an alarm circuit, the loop also including a plurality of series-connected contacts arranged to be actuated when an abnormal situation occurs, a respective unidirectional current-conducting device connected across at least selected ones of said contacts such that the system is capable of operating in accordance with one current-conducting mode and switching means to inhibit current flow through said unidirectional current-conducting devices such that the system operates in accordance with a second current-conducting mode.

3. A protective system as claimed in claim 2 wherein said series-connected contacts are each normally closed and arranged to be opened whenever the presence of an intruder is detected, such as to change the state of said relay, said unidirectional current conducting devices each comprising a semiconductor diode connected across a respective series-connected contact, such that for said one current-conducting mode, whenever a series-connnected contact is opened, current continues to flow around the loop to maintain the normal state of said relay, and said switching means being arranged to reverse the direction of current flow through the loop such that thereafter during said second current-conducting mode, whenever a series-connected contact is opened, the current flow through the loop is interrupted to change the state of said relay and provide an alarm indication of the presence of an intruder.

4. A protective system as claimed in claim 2 wherein in addition to said series-connected contacts, at least one parallel-connected contact is provided with a unidirectional current-conducting device connected in series therewith.

5. A protective system as claimed in claim 4 wherein the parallel-connected contacts are normally open and arranged to be closed whenever the presence of an intruder is detected said at least one unidirectional current-conducting device comprising a semiconductor diode connected in series with the associated parallel-connected contact such that whenever the contact closes during said one current-conducting mode, the reverse connection of said diode substantially inhibits the passage of current therethrough and no short circuiting of the relay winding occurs, said switching means being arranged to reverse the direction of current flow through the loop such that thereafter, during said second current-conducting mode, whenever a parallel-connected contact is closed, the diode passes current to decrease the flow of current through the winding of the relay and provide an alarm indication of the presence of an intruder.

6. A protective system as claimed in claim 2 wherein an additional relay is included in the loop together with a plurality of additional parallel-connected contacts connected in series with impedance means such that when an additional parallel-connected contact is actuated, the current flow through said additional relay is increased to an extent sufficient to change the state thereof but without effecting a change in the state of said first-mentioned relay.

7. A protective system as claimed in claim 6 wherein said additional parallel-connected contacts are normally-open and arranged to be closed whenever heat, fire or smoke is detected and said additional relay having associated contacts connected in circuit with a fire warning circuit such as to provide an alarm indication of the presence of said fire, heat, or smoke.

8. A single closed electrical loop for the protective system of claim 2 including a single relay winding connected in series with a DC voltage source and a plurality of series-connected contacts each arranged to provide intruder protection at a respective location and to be actuated by an intruder and selected ones of said series-connected contacts each having a unidirectional current-conducting device connected in parallel therewith, said unidirectional current-conducting devices being connected such that when the current flows through the loop in one direction opening of one of said selected ones of said series-connected contacts fails to interrupt the current flow through the winding of the relay and switching means for changing the direction of current flow through the loop whereby the unidirectional current-conducting devices thereafter inhibit current flow through the winding of the relay whenever an associated series-connected contact is opened.

9. A single closed electrical loop as claimed in claim 8 including normally-open parallel-connected contacts each arranged to provide intruder protection at a respective specific location and to be closed by an intruder and at least one of said parallel-connected contacts having a unidirectional current-conducting device connected in series therewith and connected such that when the current flows through the loop in one direction closing of an associated contact fails to interrupt the current flow through the winding of the relay, and when said switching means changes the direction of current flow through the loop closing of a contact thereafter interrupts current flow through the winding of the relay.

10. A protective system as claimed in claim 1 wherein the loop includes switching means for connecting the loop alternately to a DC and an AC voltage source, the loop also including a plurality of series-connected contacts, selected ones of which having capacitor means connected in parallel therewith, and a relay having contacts associated with an alarm circuit, the arrangement being such that when the loop is connected to the AC source, the current through the loop is maintained at a sufficient level to retain the winding of said relay energized despite the actuation of any series-connected contact shunted by said capacitor means, and when the loop is connected to said DC voltage source, the current through the loop is interrupted in response to the actuation of any series-connected contact, such as to change the state of said relay.

11. A protective system as claimed in claim 1 wherein the loop includes a plurality of series-connected contacts shunted by impedance means and a relay having contacts associated with an alarm circuit, the voltage level of the voltage source being sufficient to maintain the state of the relay despite actuation of any series-connected contact shunted by impedance means and means for reducing the voltage such that the voltage across the relay is insufficient to maintain the stare thereof whenever a series-connected contact is actuated.

12. A protective system as claimed in claim 1 wherein the loop includes a plurality of series-connected contacts and a relay having associated contacts connected in an alarm circuit, and a capacitor connected across said relay winding, such that whenever a series-connected contact is actuated, the capacitor maintains the state of the relay, and switching means for removing the capacitor from the loop such that actuation of a series-connected contact effects a change of state of the relay.

13. A protective system as claimed in claim 2 wherein each unidirectional current-conducting device is incorporated within a switch unit containing the respective contacts.

14. A protective system as claimed in claim 13 wherein the contacts are of the magnetic reed type.

15. A protective system as claimed in claim 2 wherein each unidirectional current-conducting device is incorporated within a switch unit containing the respective contacts; the said unidirectional device and an impedance means being connected in series with or across the contacts of the switch.

16. A protective system as claimed in claim 1 wherein a two wire loop is terminated by impedance means first relay means arranged to be actuated whenever a whole or partial short circuit or open circuit condition occurs in the loop, second relay means operable to provide second channel operation and interlocking means associated with the two relay means to prevent operation of the second relay means should such a whole or partial short circuit or open circuit condition occur.