GB2184303A - Circuit protection arrangement - Google Patents

Abstract

An arrangement for protecting mains operated electrical and electronic apparatus from transients and surges occurring on the mains supply is constructed as a module intended to protect equipment operating on a conventional 13 amp mains supply and is installed into an enlarged 13 amp plug with pins P1, P2 and P3 for fitting in a standard 13 amp socket. The arrangement comprises a fuse F in series with the live conductor, three varistors Z1, Z2 and Z3 respectively connected between live and earth, live and neutral and earth and neutral conductors, and a neon indicator lamp L and ballast resistor R connected between live and neutral conductors. A thermally operated normally open switch S is also connected between the live and neutral conductors and is arranged to be responsive to heat generated by varistor Z3 to close its contacts thereby shorting between live and neutral conductors and causing the fuse F to blow. The circuit is thus made as responsive to a heavy surge or transient between neutral and earth as it is otherwise to a heavy surge or transient between live and neutral or live and earth. <IMAGE>

Description

SPECIFICATION Circuit protection arrangement The present invention relates to a circuit protection arrangement for protecting mains operated electri cal and electronic apparatus from surges and trans- ients occurring on the mains supply. More specifically the invention relates to circuit protection arrangements ofthe type having a voltage dependent device, whose resistance drops as an applied voltage increases, connected between each pair of conductors of an alternating mains supply comprising live, neutral and earth conductors and including afuse element in serieswiththe live conductor which is intended to blow in the eventthat anyof said voltage dependent devices becomes of sufficiently low impedance due to a large surge ortransient occurring on the mains supply.The blowing ofthe fuse may cause an indicator iamp connected between the live and neutral conductors to be extinguished.

Although such circuit protection arrangements operate satisfactorily, i.e. blow the fuse, if either ofthe voltage dependent devices connected between the live and neutral and live and earth conductors be comes of low impedance due to a heavy surge or transient; it has been found that it does not operate satisfactorily in respect ofthe voltage dependent device which is connected between the neutral and earth conductors; insofar as even if thins latter device becomes a low impedance it does not necessarily result in the blowing ofthefuse. In such a case, if afault should exist between the neutral and earth conductors resulting in considerable voltage difference between these conductors, i.e. some tens of volts as can occur in practice, then the voltage dependent device connected between them continues to be subjected to this voltage and hence goes on dissipating heat, withoutthefuse blowing. This is clearly an undesirable state of affairs, since it could result in damage to associated equipment or constitute a hazard to safety.

It is an object of the present invention to provide a circuit arrangement ofthe type abcve described in which the aforementioned disadvantage is avoided.

According to the present invention, in a circuit protection arrangement ofthe type described, a thermally operated normally open switch is connected between the live and neutral conductors and is disposed so as to be responsive to heat generated by the voltage dependent device connected between the neutral and earth conductors, the heat generated effecting closing of the switch and hence causing the fuse in the live conductor to blow.

The thermally operated normally open switch may be a changeover switch which also has a normally closed position completing the connection between the neutral conductor and the said voltage dependent device connected between the neutral and earth conductors.

In a preferred form ofthe invention,thethreevoltage dependent devices comprise solid-state devices, commonly known as varistors, which are arranged side by side as a group orstackwith the device which is connected between the neutral and earth conductors being atone end ofthe group orstack and adjacentthethermally operated switch.

According to a feature ofthe invention,thetherm- ally operated switch also comprises one or more clamp elements which embrace the group orstack and serve to hold the devices together.

The invention will now be further described by way of example, with reference to the accompanying drawings, in which Figure lisa circuit diagram of one embodiment of a circuit protection arrangement according to the invention; Figure2 is a perspective view of a varistor and thermal switch assembly as used in the circuit of Figure 1; Figure3isacircuitdiagram of a furtherembodimentincludingachangeoverswitch; and Figure 4 is a perspective view of a varistor and thermal switch assembly as used in the circuit of Figure 3.

Referring to Figure 1, the circuit protection arran gementillustrated is constructed as a module which is intended to protect equipment operating on a conventional 13 amp mains supply. To this end it is installed into an enlarged 13 amp plug with plug pins P1, P2 and P3forfitting in a standard 13 amp socket; P1 being connected to the live conductor, P2 being connected to the neutral conductor and P3 being connected to the earth conductor. The output from the arrangement is derived via the three standard 13 amp plug terminals T2 and3; which are respectively live, neutral and earth.

The circuit protection arrangement comprises a fuse F in series with the live conductor, three varistors Z1 , Z2 and Z3 respectively connected between live and earth, live and neutral and the earth and neutral conductors, as well as a neon indicator lamp L and ballast resistor R connected between the live and neutral conductors. According to the present invention, a thermally operated switch S is also connected between the live and neutral conductors and is arran ged to be responsive to heat generated by varistor Z3.

In operation, ifa heavy transient or surge occurs between the live and neutral conductors or live and earth conductors,theimpedanceofthevaristorsZl orz2willdropappreciably,therebycausing anex- cessivecurrentthroughthefuseFwhichwillblow and thus protectthe equipment. Atthe sametimethe neon indicator Lwill go out, thus indicating that a faultcondition exists. On the other hand, if a surge or transient occurs between neutral and earth, and a fault causing an appreciable difference in voltage exists between these two conductors, then the imped ance of varistor Z3 will become low and itwill commence dissipating heat without any resulting blowing ofthefuse F.However,thethermallyresponsive normally open switch Swill now respond to the heat generated bythevaristorZ3 and will close its contacts, thereby forming a short circuit between live and neutral conductors and again causing the fuse to blow. In this way the varistor Z3 is caused to respond to a fault arising between the neutral and earth conductors, which under normal conditions should be at approximately the same potential.

Figure 2 illustrates one preferred arrangement for the varistors and thermal switch. The varistors Z1, Z2,Z3 are arranged in a stackwith Z3 at the top and adjacent to the neutral and movable contact C1 ofthe thermally operated switch S. The live and fixed con tact C2 ofthe switch is arranged above and spaced from the movable contact Cl. The latter is in theform of a strip like beryllium copper spring member attached to a clamp element E embracing the stack of varistors and holding them together. The spring con tact C1 is normally restrained against closing buy a fusible metal slug (not shown) which secures it to a retaining member M.When the temperature ofthe vdristorZ3 rises dueto its lowimpedance in re sponse to a fault condition,the heat generated is conducted through the clamp element E and the con tact C1 thereby causing the fusible slug to melt and the co ntact C1 to spring away from retaining member M and move into engagement with contact C2, thus closing the switch Sand causing the fuse F to blow as described above.

Figure 3 shows a further embodimentwhich is basicallysimilartotheembodimentofFigure 1,and the corresponding parts bearthe same references.

However, in this embodiment, the thermally respon- sive normally open switch S is replaced by a change overswitch S' having both a normally open and a normally closed position. The normally open posi tion corresponds to that ofthe switch S in Figure 1, and the normallyclosed positioncompletesthecon- nection between the neutral conductor and varistor 23.The switch S' responds to overheating of thevari storZ3 in the same way as in the previous embodi ment, but since it is a changeover switch, when its moving contact changes over to provide a shortcircuit between the live and neutral conductors, it also open-circuits the connection between the varistor Z3 and the neutral conductoratthesametime. Such an arrangement ensures that Z3 is isolated underfault conditions between neutral and earth conditions and isa refinementofthearrangementshown in Figure 1 Figure 4, shows howthearrangementofvaristors and thermai switch as in Figure 2 is modified to pro videforthechangeoverswitchfunction.Ascan be seen, in this embodimentthe movable contact C1 forms the changeover contact and is mounted on a member I of insulating material carried by the clamp element E. The retaining member M now forms one fixed contact and the contact C2 is the otherfixed contact, as previously. The springy movable contact C1 is retained against the fixed contact M bythefus- ible metal slug (not shown) in orderto complete the connection between the varistor Z3 and the neutral conductor. On operation ofthe clangeover switch S' dueto heatingofthevaristorZ3,thefusibleslug melts thereby releasing contact C1 which moves into engagement with fixed contact C2. A short circuit is produced between live and neutral causing fuse Fto blow and varistor Z3 is disconnected from the neutral line.

Whilst particular embodiments have been described it will be understood that various modifications may be made. Thus the thermally responsive switch maytake numerous otherforms andthevoltage dep- endent devices may be other than varistors.

Claims (9)

1. Acircuitprotection arrangementforprotecting mains operated electrical and electronic apparatus from surges and transients occurring on an alternat ing mains supply and ofthe type having a voltage dependent device, whose resistance drops as an applied voltage increases, connected between each pairofthe live, neutral and earth conductors ofthe supplyand includingafuseelementinserieswith the live conductorwhich is intended to blow in the event that any of said voltage dependent devices becomes of sufficiently low impedance due to a large surge ortransientoccurring on the mains supply, wherein a thermally operated normally open switch is connected between the live and neutral conductors and is disposed so as to be responsive to heat generated by the voltage dependent device connected between the neutral and earth conductors, and heat generated effecting closing ofthe switch and hencecausing thefuse in the live conductorto blow.

2. An arrangementas claimed in claim 1,wherein the thermally operated normally open switch is a changeoverswitchwhichalsohasa normally closed position completing the connection between the neutral conductor and the said voltage dependent device connected between the neutral and earth conductors.

3. An arrangement as claimed in claim 1 or2, wherein the three voltage dependent devices comprise solid-state devices which are arranged side by side as a group or stack with the device which is connected between the neutral and earth conductors being atone end ofthe group orstackand adjacent the thermally operated switch.

4. An arrangementasclaimed in claim 3,wherein the solid-state voltage dependent devices are vari- stors.

5. An arrangementas claimed inclaim3or4, wherein the thermally operated switch comprises a resilient movable contact which is normally restrained against movement buy a heatfusibleel- ement.

6. An arrangement as claimed in claim 3, 4 or5, wherein the thermally operated switch also comprises one or more clamp elements which embrace the group orstackand serve to hold the devices together.

7. An arrangement as claimed in any preceding claim,particularlyfora 13ampmainssupply, wherein the said arrangement is incorporated in a 13 amp plug structure.

8. A current protection arrangement, substantially as hereinbefore described with reference to Figure 1, or Figure 3 ofthe accompanying drawings,

9. Acircuit protection arrangement as claimed in claim 8, in combination with an assembly as shown in Figure 2 or Figure 4 of the accompanying drawings.