Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
  • H02H5/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal non-electric working conditions with or without subsequent reconnection
  • H02H5/04—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal non-electric working conditions with or without subsequent reconnection responsive to abnormal temperature
  • H02H5/047—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal non-electric working conditions with or without subsequent reconnection responsive to abnormal temperature using a temperature responsive switch
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01H—ELECTRIC SWITCHES; RELAYS; SELECTORS; EMERGENCY PROTECTIVE DEVICES
  • H01H37/00—Thermally-actuated switches
  • H01H37/74—Switches in which only the opening movement or only the closing movement of a contact is effected by heating or cooling
  • H01H37/76—Contact member actuated by melting of fusible material, actuated due to burning of combustible material or due to explosion of explosive material
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
  • H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
  • H02H3/02—Details
  • H02H3/025—Disconnection after limiting, e.g. when limiting is not sufficient or for facilitating disconnection

Definitions

• This invention relates to the protection of electrical circuit means against potentially damaging changes in operating conditions. These will usually be increases in current, but protection may also be required in certain cases against voltage changes or changes in temperature.
• fuses In household appliances, protection against excessive currents is conventionally provided by fuses. These have the advantages of relatively low cost. Moreover, a fuse has the advantage as a safety device that, when blown, it offers guaranteed and tamper proof isolation. These features can only be provided in mechanical or electromagnetic current isolators with special precautions. With normal fuses, however, there will be uncertainty as to the precise current value which will blow the fuse. This may not be a practical disadvantage when the aim is to protect against the effects of a short circuit. In these circumstances, the current through the fuse will rise rapidly to a level which is considerably in excess of the normal working current and the exact current level at which the fuse blows is therefore not critical. In certain applications, however, this uncertainty can be a substantial disadvantage.
• This invention makes use of well known PTC devices having the property that the resistance of the device increases sharply at a threshold temperature.
• a method for protecting electrical circuit means against currents in excess of a given current threshold by the connection in series of a normally conducting isolator having a fusible element and adapted to break electrical continuity on fusing thereof, the current being passed through a PTC device disposed in thermal communication with said element and being arranged so that above the current threshold, the power output of the PTC device exceeds the rate of heat dissipation therefrom and increases rapidly beyond the level required to fuse said element.
• the cold resistance of the PTC device is very small compared with the resistance of the electrical circuit means to be protected so that, immediately above the current threshold, the power output of the PTC device is generally proportional to its resistance.
• a PTC device can be selected to have a very rapid increase in resistance (typically several orders of magnitude) over a selected short temperature range.
• the power output of the PTC device can thus be arranged to increase very rapidly as the current exceeds the predetermined threshold, hence ensuring fusing of the element even if the melting point is significantly above the nominal figure.
• a device for protecting electrical circuit means against currents in excess of a given current threshold comprising an isolator having a fusible element and adapted to break electrical continuity on fusing thereof and a PTC device arranged in electrical series connection and in thermal contact with the isolator and in thermal contact with a heat sink the PTC device being so arranged that as the current exceeds said threshold, the power output of the PTC device exceeds the rate of heat loss to the sink and increases rapidly beyond the level normally required to fuse said element.
• the present invention consists in a method for isolating electrical circuit means upon potentially damaging changes in operating conditions by the connection series of a normally conducting isolator having a fusable element and being adapted to break electrical continuity on fusing of that element, wherein a PTC device is arranged in thermal communication with the element and with a heat sink and adapted such that in normal operating conditions the rate of heat generation in the PTC device equals the rate of heat loss to the sink, it being further arranged that upon a potentially damaging change in operating conditions the rate of power generation exceeds the rate of heat loss and increases rapidly beyond the level required to fuse said element.
• Figure 1 is a graph illustrating the variation of resistance with temperature for a typical PTC device
• FIG. 2 is a circuit diagram of a typical PTC configuration
• Figure 3 is a graph illustrating the variation of power output with resistance
• Figure 4 is a graph illustrating the variation of power output with resistance and the power dissipation at different current levels.
• Figure 5 is a sectional view of a device according to this invention
• Figure 6 is a graph of working temperature of the device shown in
• Figures 7 and 8 are respectively plan and sectional views of a device according to a further embodiment of this invention.
• Figures 9 and 10 are respectively plan and sectional views of a device according to a still further embodiment of this invention.
• Figure 11 is a graph of power against temperature illustrating the effects of variations in ambient temperature
• Figure 12 is a diagram illustrating a further embodiment of this invention.
• Figure 13 is a sketch view of a still further embodiment.
• a PTC or positive temperature coefficient device has the well known feature that the resistance increases rapidly over a short temperature range. The precise characteristics can be tailored to meet particular applications by controlling the manufacturing process.
• a typical plot of the resistance of a PTC against working temperature is shown in Figure 1. It will be seen that over a temperature range of around 5 C the resistance of the device increases by three orders of magnitude.
• PTCs have a wide variety of applications and one particular configuration is shown in Figure 2. Since the resistance R p of the PTC device will vary with working temperature and will thus be dependent upon the power generated by self-heating in the PTC device, it is instructive to consider a plot of power P p against PTC resistance R p and this is shown in Figure 3 « The power P_ generated in the PTC is shown as well as the power P..
• the power P p dissipated in the PTC has a maximum value and it can be shown that this will be reached if and when R p becomes equal to R L
• the operating point will shift to the intersection of the heat dissipation curve with a fresh P/T plot corresponding to the increased current.
• a threshold value will be reached which the P/T curve for that current fails to intersect the heat dissipation curve.
• the rate at which power is generated within the PTC device will then always exceed the rate of heat dissipation.
• the temperature of the device will increase, the resistance will accordingly increase, leading in turn to increased power output. This positive feedback will continue until such time as the resistance R reaches the value of the load resistance R L at the peak value shown in Figure 3 and the power generated in the PTC begins to fall.
• either the power curve will intersect once more the heat dissipation curve with the device re-stabilising or the temperature of the PTC device will reach a level at which the resistance passes its maximum and begins to fall in a thermal runaway condition.
• o e device according to this invention comprises an insulating tube 10 closed at one end by a metal disc 12 carrying an external terminal Ik .
• a PTC pill 16 carrying on its outer surface a terminal 18.
• soldered joint 20 which secures one end of a tension spring 22, the opposite end of which is secured at anchorage 24 to the plate 12.
• the described device is to be used in the high voltage circuit of a microwave appliance.
• a magnitron is powered from the secondary of a high voltage transformer with a diode voltage doubling circuit connected in series to produce the required voltage of around 2.3 V.
• the normal working current may be, for example, 500 milliamps, although failure of a diode may cause the current to rise to, say, 700 milliamps.
• the described device is-connected in series with the magnitron.
• the resistance of the voltage doubling circuit will typically be several kilohms whilst the resistance of the protective device may be, for example, only 5 ohms. Accordingly, the current is effected to a minimal extent by the cold resistance of the PTC pill and it is also the case that the PTC device is not required to withstand the operating voltage of around 2.3 kilovolts.
• the described device In normal operation, the described device is connected in series with the diode circuit and the magnitron, electrical continuity being established through the spring 22, the solder joint 20 and the PTC pill 16.
• the passage of current through the PTC pill will lead to self-heating and the temperature of the pill - and necessarily also that of the solder joint - will rise as before described to an equilibrium temperature at which the power generated through self- heating is balanced by heat dissipation.
• ambient temperature is 60 C.
• the PTC device At the normal working current of 00 milliamps, the PTC device will be in thermal equilibrium at working point Z. It can now be seen that if the current rises in a fault condition to 700 milliamps, the power generation curve fails to intersect the cooling curve and the PTC device will heat rapidly. Within a very short interval, the temperature attained by the PTC pill 16 will exceed the melting point of the solder joint 20. The tension spring 22 will therefore be released and electrical continuity through the device will be broken.
• the protective device may be required to operate over a range of ambient temperatures. In such circumstances the characteristics of the PTC device will be selected so that within the anticipated band of ambient temperatures, there was always thermal stability at any point within the normal current range but that any current in excess of the stated fault current would lead to thermal runaway.
• FIG. 7 There is shown in more detail in Figures 7 and 8 a device according to this invention which is capable of providing isolation at high voltages.
• the device is formed with upper and lower ceramic housing parts 50 and 52.
• the lower housing part 0 defines three internal cavities, being a first contact cavity 54, a spring cavity 6 and a second contact cavity 58.
• recesses 60 which mate with corresponding lugs 62 provided on the upper housing part.
• recesses 64 and 66 which overlie the first and second contact cavities respectively.
• a first contact 68 has a mounting portion 70 positioned in the first contact cavity 54.
• the contact 68 has a neck 72 which is received in the housing aperture 74 and the protecting portion of the contact is formed (in this example) as a spade terminal l ⁇ .
• the mounting portion 70 has a tang 78 which provides an anchorage for one end of a tension spring 80. This tension spring extends between the first and second contact cavities through the spring cavity 56.
• a nose 86 projects integrally from the mounting plate and provides the site for a solder joint 88 between the mounting plate and the corresponding ' end of " the tension spring 80.
• a PTC pill 89 is sandwiched between the mounting plate 82 and a second contact 0 of a configuration generally similar to that of the first contact. That is to say the second contact 90 has a neck 9 received within a housing aperture 9 and a projecting portion formed as a spade terminal 96. Good thermal and electrical contact between the mounting plate 82, the PTC pill 89 and the second contact 90 is ensured by means of a dished spring element 98 received within recess 66.
• the device comprises a housing 100 and a cover 102 of a suitable plastics or ceramic material.
• a recess 104 in the housing accommodates a PTC pill 106 abutting on one circular face a heat collector plate 108 and on the opposite face a connection disc 110.
• a U-shaped spring clip 112 acts between the side of the recess 104 and the connection disc 110 to ensure that the PTC pill is correctly positioned and in proper thermal and electrical contact, notwithstanding minor variations in dimension These may occur- as a result-of differential thermal expansion or because of manufacturing tolerances.
• a spade terminal 114 is rigidly secured to the connection disc 110 and extends outwardly of the housing through channel 116. This channel 116 is slightly oversize to accommodate variations in thickness of the PTC pill and the fact that the spade terminal 114 is disposed orthogonally minimises the surface opening.
• a coiled torsion spring 118 is located within a spring cavity 120 in the housing and has opposed limbs 122 and 124 which are mutually inclined at an obtuse angle.
• Limb 122 has an end portion 126 folded in two directions which is received in a hook 128 pressed out of a spade terminal 130.
• the opposite torsion spring limb is soldered to a step-132 pressed out of the heat collector plate 108.
• the described device is adapted to be plugged into a mounting having appropriate sockets for receiving the spade terminals 114 and 130.
• electrical continuity between the terminals is provided by the spring 118, the heat collector plate 108, the PTC pill 106 and the connection disc 110.
• the PTC pill is designed, as explained above, to increase power output and rise in temperature to effect melting of the solder joint enabling the torsion spring limb 124 to move to the position shown in dotted outline. It will be seen that in this way a well defined clearance gap is guaranteed.
• Devices as previously described for use as current protection can accordingly also be used to protect against temperature increases. At lower voltages, the size of the device can of course be reduced.
• the parameters of a device may be selected so that it provides both temperature and current protection. That is to say either a current increase or an ambient temperature increase will cause the PTC device to heat rapidly and fuse the element.
• thermal cutout One particular application of such a thermal cutout is to protect against the dangers of plugging a household appliance into house wiring which through deterioration or for other reasons is not capable of carrying the required current. In such circumstances, the appliance itself will operate normally and conventional fuses will not offer protection. It is likely, however, that overheating will occur at the socket and it is proposed to locate a thermal cutout according to this invention in the appliance plug in a position to respond to any overheating of the socket.
• the fusible element may. take forms other than the above described use of solder.
• FIG. 12 a further embodiment is illustrated with the PTC pill being omitted for the sake of clarity.
• a heat collector plate 150 which abuts the PTC pill provides a central mounting point for a pawl 152 which can pivot with respect to the axis of the PTC pill.
• One arm of the pawl is formed as a tang 154 which opposes a block 1 6 secured optionally to the heat collector disc or to the body of the device.
• a thermal fuse pellet 158 is bonded between the tang 154 and the block 156. This pellet 158 takes the form of a spring compressed within a body of wax material.
• the opposite arm of the pawl 1 2 is formed as a tooth l60 which engages beneath one limb of a torsion spring 140, the heat collector plate 150 having lugs 162 which restrain sideways movement of the torsion spring.
• electrical continuity is achieved through the collector plate 150, the pawl 152 and the torsion spring 140. If the temperature of the heat collector plate exceeds the melting point of the wax in the thermal fuse pellet 158, the compression spring trapped within the pellet is released to urge the tang 4 in the clockwise direction thus causing tooth 160 to move away from and in turn to release the torsion spring 140.
• the fusible element need not form part of the normally conducting path of the isolator.
• Figure 13 in a diagrammatic form a device for protecting against earth current leakage.
• Contact pairs 100 and 102 in the live and return rails of a power supply are biased open and held closed by respective wax pellets 104, 105 mounted on a PTC pill 106 connected electrically in the earth rail.
• a current passing to earth will cause rapid heating in the PTC device melting the wax pellets and opening both live and return circuits.
• a fusible element in general provides the described devices with certain recognised advantages of conventional fuses, that is to say mechanical simplicity, reliability and tamper-proof isolation.
• the invention provides the further advantages of very accurate response and the ability to operate reliably at high voltages.
• the fusible element could take a variety.of forms, the choice being determined by such factors as the temperature at which fusing occurs and the available heat outputv "Solder has-the advantage that the melting point can be controlled by variation of the solder composition with a comparatively low temperature limit. It will be possible, however, to use other fusible alloys. Similarly a variety of plastics materials will be suitable.
• the fusible element need not form part of the conducting path of the isolator and other mechanisms can be employed for ensuring that isolation takes place on fusing of the element.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Combustion & Propulsion (AREA)
• Thermistors And Varistors (AREA)
• Emergency Protection Circuit Devices (AREA)
• Fuses (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=10587621

Family Applications (1)

Country Status (5)

Families Citing this family (10)

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• 1985
  • 1985-11-01 GB GB858527024A patent/GB8527024D0/en active Pending
• 1986
  • 1986-10-30 WO PCT/GB1986/000671 patent/WO1987002835A1/en not_active Application Discontinuation
  • 1986-10-30 EP EP19860906401 patent/EP0247080A1/de not_active Withdrawn
  • 1986-10-30 AU AU65924/86A patent/AU6592486A/en not_active Abandoned
  • 1986-11-03 ES ES8602881A patent/ES2005836A6/es not_active Expired

Non-Patent Citations (1)

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