Current limiting device.

Title: Current Limiting Device

The invention relates to a current limiting device for connection between an AC mains electrical power source and a load or loads, the device acting to reduce the flow of current to the load(s) to zero or a small residual cument in the event that the current exceeds a set level.

Known forms of electrical control unit include a simple fuse and electro-mechanical circuit breakers or current limiters. In the case of a fuse, once the circuit has been broken, the fuse has to be replaced, and in the case of an electro-mechanical circuit breaker or current limiter, it must be re-set. These units therefore have to be accessible and as a result are open to abuse. In some electricity supply systems, the normal electricity meter is dispensed with and the charge to the consumer is based upon the maximum current flow allowed by a current limiter. The consumer is allowed to draw the maximum current as determined by the limiter at all times, and it is clear that if the limiter has to be accessible to the consumer for resetting, there is the possibility of fraud either by by-passing the limiter or by interfering with its operation.

The invention provides a current limiting device comprising line input and output terminals for connection in use between one terminal of an AC mains power source and one terminal of a load, a controllable semi-conductor device forming part of a current carrying path between said line input and output terminals, a sensing resistor in said current carrying path, and circuit means responsive to the voltage developed across said sensing resistor, said circuit means acting in the event that the current flow in the load exceeds a predetermined level to turn off said semi-conductor device, said circuit means including delay means so that the current flow can exceed said predetermined value for a limited period of time, the duration of said limited period of time being less for higher excess currents and virtually zero in the event of a short circuit of the load or a sudden or very high current flow. Hence, when the semi-conductor device is in its conducting state, that is, turned on, current flows to the load. However, when the semi-conductor device is turned off and rendered non-conductive,

AP/P/0 t/0 2 1 A8 the current to the load is reduced to zero or at most to a residual current.

The device according to the invention is fully electronic and has no moving parts. By allowing the current flow to exceed a pre-determined value for a period of time, the device according to the invention does not mm off the current flow in the event of surges, due for example, to inrush currents into electrical appliances. Also, the more excessive the current flow, the sooner turn-off occurs.

in the event of a short circuit or excessive current, the device according io the invention switches out rapidly (typically within a few micro-seconds) preventing damage to the device’s electronic circuitry. Tins is important when the device is used as a current limiter m a consumer supply because consumers may attempt to obtain additional electricity by trying to make the limiter fail. Electro-mechanical limiters have been rendered ineffective by repeated shorting of the load. The robustness of the device under short circuit conditions enables it to be installed in very·' low impedance circuits. This in turn means that the device may be installed outside a consumer’s service entrance.

Typically, the device will switch out in less than 10 microseconds in the event that the current exceeds 50 times the predetermined value. Where the current exceeds the predetermined level by a factor of two or three the switch out time may be of the order of 1 second or a few seconds, while at a current 10 times greater than the predetermined value the device may switch out in a time of the order of a few tens of milliseconds

1 he tum-oif speed of the device according to the invention under short circuit or excessive current conditions may be approximately one thousand times faster than the operating time of a fuse or electro-mechanical circuit breaker. It can therefore be used to minimise damage to electrical and electronic equipment under fault conditions and to improve safety, especially when used in flammable or explosive environments.

The device according to the invention generally also comprises a diode rectifier bridge.

AP O 0 1325

The device according to the invention also preferably comprises means for turning on said semi-conductor device after tum off. Timer means may be included, which are operable to turn on the semi-conductor device after a predetermined time has lapsed following tum off. Inhibiting means may also be included to inhibit tum on of the semi-conductor device until the load has been disconnected.

An auto re-setting feature is of benefit when the device is used as a limiter in a consumer supply as it enables the device to be mounted in an inaccessible place, such as on a service connection pole, thereby reducing problems of fraud associated with by-passing and tampering. What is more, an auto re-setting feature avoids the need to have a replaceable element, such as a fuse, and enables the device to be fully sealed, thereby avoiding deterioration due, for example to the operating environment or vermin infestation.

Particularly for use in such applications, auto resetting preferably occurs after a predetermined time delay. Such a delay may be of the order of a few seconds to a few minutes, typically 5 seconds to 60 seconds. In general, the delay should be sufficient to allow a consumer to switch off sufficient load to reduce the current to a level less than the predetermined level at which the semi-conductor device is switched off, without leaving the consumer with no supply for an excessively long period. A delay which may be suitable is approximately 30 seconds. In one practical application, the device may be configured in such a way that it permits the consumer to draw sufficient current to power a specified number of ) 20 electric lights. If the consumer then switches on a further electric light, so that the predetermined current level is exceeded, the semi-conductor device is turned off and the supply of current is stopped. The delay of 30 seconds is then sufficient to enable the consumer to switch off the additional light so that at the end of that delay, when the supply of current is resumed, further tripping does not occur. Should the consumer fail to switch off the additional light, the process of supply current interruption and reconnection is repeated until the load is reduced sufficiently for the current to fall below the predetermined limit.

The device is preferably installed at a location between the electricity supply main cable and

AP/P/0.1 / 0 2 148

-· ί the consumer’s service entrance, ie at a location which is owned and maintained by the electrics ty supplier rather than the consumer. The device is also preferably fully encapsulated and sealed so that it cannot be tampered with by the consumer.

The device additionally preferably comprises means to ensure that the semi-conductor device is turned on in the region of zero crossing of the AC supply voltage when a Load is connected.

By re-setting close to a voltage zero crossing, the device according to the invention reduces inrush currents and the related stresses in loads, and minimises electro-magnetic interference.

The device according to the invention further preferably includes means operable to turn off said semi-conductor device in the event that the voltage of said supply falls below a predetermined level.

Such under-voltage turn-off helps protect voltage sensitive loads, such as motors, from damage due to operation at too low a voltage and the semi-conductor device from overheating due to excessive on-stale voltage drop as its drive voltage may be reduced because of the low' supply voltage.

The controllable semi-conductor device may be a transistor, especially' an insulated gate bipolar transistor (IGBT). The use of a transistor is preferred as it can be turned off at any point on the mains cycle whereas devices such as a triac or thyristor only turn off at a voltage zero crossing. Ths fast tum-οίϊ of the transistor saves the device in the event of a short circuit which would destroy a tnac or thyristor. The use of an IGBT is particularly preferred as such a transistor is more robust than other forms of transistor under short circuit conditions.

It is also particularly preferred that the transistor should be continuously on during normal operation. T his eliminates radio interference which could occur in a device utilising a triac or thyristor turning oft after eveiy cycle and then back on after a delay.

8<r v its I I 0/J/dV

ΑΡνν 1 4 £ 5

According to another aspect of the invention there is provided a method of limiting the supply of electricity to a consumer’s premises, which method comprises installing a device as described above between the electricity supply main cable and the consumer’s service entrance.

An example of a current limiting device according to the invention will now be described with the reference to the accompanying circuit diagrams, in which

Figure 1 shows the main components of the device,

Figure 2 shows the low voltage power supply, and

Figure 3 is a diagram similar to Figure 1 but of a second embodiment of the device.

With reference to the diagrams, the device comprises line input and output terminals 10,11 for connection in use to the line terminal of the AC power source and the line terminal of the load respectively. The unit is also provided with neutral input and output terminals 12,13 for connection in use to the neutral terminal of the AC power source and the neutral terminal of the load respectively.

The line terminals 10,11 are connected to opposite connection points of a bridge rectifier comprising diodes DI, D2, D3 and D4 whilst the other opposite connection points are connected to a first line 14 and a second line 15 respectively.

A controllable semi-conductor device in the form of an insulated gate bipolar transistor Q3 has its collector connected to line 15 and its emitter connected by way of a current sensing resistor R2, to line 14. In use, providing the transistor Q3 is in a conducting state, in the positive half cycle of the supply, current will flow to the load by way of a diode DI, the transistor Q3, the sensing resistor R2 and the diode D4, and in the negative half cycle, by way of the diode D2, transistor Q3, the resistor R2 and the diode D3. A voltage is developed across the resistor R2 which is proportional to the load current and for the maximum

W continuous operating current the voltage is of the order of 0.1 volt.

AP/P/0 f '0? 1 48

The drive voltage for the gate of the transistor Q3 is obtained from an 18 volt source of DC supply by way of resistors R3 and R4 connected in series and having their junction connected to line 14 by way of the current carrying path of a field effect transistor Ql. The 18 volt source is positive with respect to line 14 and its derivation will be described later. In use.

providing transistor Ql is turned off, a drive voltage .will be applied to the gate of transistor Q3 and current will flow in the load. In normal use, the voltage developed across resistor R2 will have little effect on the effective drive voltage applied to transistor Q3 but in the event of a short circuit in the load, even before transistor Ql is rendered conductive to turn transistor Q3 off by the action of further circuits to be described, the effective drive voltage applied to transistor Q3 will be reduced by the increased voltage drop (typically 10 volts) across the resistor R2. This has the effect of reducing the cument flow in the transistor Q3 and thereby limits the power dissipation therein.

The conduction of Q1 is controlled by a timer which includes an integrated circuit 1C2, a 555 type timer IC, the operation of which is determined by four further circuits including integrated circuits IC1A, IC1B. 1C1C and IC1D respectively. Tne further circuit which includes ICI A provides the maximum current control, that which includes IC IB provides for rapid response to deal with sudden heavy overloads and short circuits, that which includes IC1C ensures that re-setting of tire unit after an overload trip only takes place near zero c rossing of the AC supply (when a load is connected) and that which includes 1C1D ensures that the load is disconnected from the supply in the event that the supply voltage is below a predetermined level.

Forming part of the timer is a resistor R21 and a capacitor C5 which are connected in series between 2. 5 volt positive source of DC supply and the line 14. The junction of the resistor and capacitor are connected to the threshold and discharge terminals of the integrated circuit

IC2. The control voltage terminal of IC2 is connected to the 5 volt source of supply by way cf resistors R19, R20 connected 111 series and the trigger terminal is connected to the 5 volt source of supply by way of a resistor R14.

AP601325

The trigger terminal of the integrated circuit IC2 is connected to the outputs of the integrated circuits IC1A, IC1B and IC1D, the further circuit of which these form part will now be discussed.

Considering firstly integrated circuit IC1 A, this is connected as a comparator having its non5 inverting input connected to the junction of a pair of resistors R9, Rl 0 which are connected to the 5 volt source of supply and line 14 respectively. The inverting input is firstly connected to the junction of the transistor Q3 and the resistor R2, by way of resistors R6 and

R7 connected in series and secondly by way of capacitor C3 to the line 14. The resistors R9 and Rl 0 determine the reference voltage applied to the comparator and therefore the voltage drop across the resistor R2'and the predetermined level of current flow' in the load at which the comparator changes state. When the comparator changes state, the transistor Q1 is turned on and the transistor Q3 is turned off to disconnect the load. However, since in normal use there will be surge currents due for example to motor starts and inrush current into power supplies and lamps etc., which might exceed the predetermined level, it is necessary to prevent disconnection of the load in these circumstances and resistors R6 and R7 together with capacitor C3 provide a time delay function. For higher current surges, such as might cause damage to the transistor Q3 if applied for too long a time, the delay time constant is reduced by the action of a PNP transistor Q2 which switches in resistor R8 to increase the charging rate of the capacitor C3.

The provision of the aforesaid time delay means that there is the possibility of damage to the transistor Q3 in the event of a sudden very high increase in the load current or a short circuit in the load. The circuit incorporating integrated circuit IC1B combined with a current limiting effect due to reduced drive voltage applied to transistor Q3 as described earlier, will prevent damage in these circumstances. IC1B functions as a comparator having its non25 inverting input connected to the junction of a pair of resistors Rl 1 and R12 connected to the 5 volt source of supply and the line 14 respectively. The inverting input is connected by way of resistor R13 to the junction of the transistor Q3 and resistor R2 and a very low value capacitor C4 compared with the value of capacitor C3, is connected between the inverting

APtt tt1425 input and line 14 to act as a high frequency filter and is insufficient to provide a significant delay in response of the comparator to changes in the voltage across resistor R2. The reference voltage applied to the comparator IC1B is substantially higher than that applied to the comparator IC1A and as a result the comparator only changes state to initiate the process of turning off transistor Q3 in the case of very’ high current flow.

The further circuit which includes integrated circuit IC1D is shown m Figure 2 and is associated with the low voltage power supply. Integrated circuit IC1D is again connected as a comparator and its inputs are connected to the junctions of series connected resistors R25, R26 and R27, R31 respectively. The resistors R27 and R31 are connected between the

5 volt supply line 16 and the line 14 and. the resistors R25 and R26 are connected between a 18 volt supply line 17 and the line 14. The values of the resistors are such that in normal use, the voltage at the inverting input of the comparator is 2.5 volts and that at the noninverting input is 3 volts.

Line 17 is connected to the cathode of a diode D8 by way of a resistor R24, the anode of the diode being connected to the neutral terminals 12, 13. The remaining components of the power circuit include a storage capacitor C7 connected between lines 17 and 14, a zener diode D9 also connected between lines 17 and 14 and a resistor R23, and a 5 volt precision voltage reference DI 1 connected in series between the lines 17 and 14. Supply line 16 is connected to die junction of resistor R23 and the cathode of diode DI '1.

In use, the capacitor C7 is charged during the negative half cycle of the supply voltage, to the breakdown voltage (18 volts) of the zener diode D9, the flow of current being by way of the diode D8, the resistor R24 and the diode D3. Precision voltage DI 1 by way of resistor R23 determines the voltage on line 16. in the event of a reduction in the voltage oi the AC supply the voltage on line 17 will start to fall before that on line 16 and when it falls to a value such that the voltage at the non-inverting input is less than that at the inverting input, the comparator IC1D changes state to inhibit the operation ofthe transistor Q3. In this manner, when the supply voltage is low, motors etc., constituting the load and which might be

APO 0 1325 damaged by operating at low voltage, are protected and transistor Q3 is protected from overheating due to operation at too low a voltage.

The further circuit which includes integrated circuit IC1C comprises a pair of resistors R16, R17 connected in series between line 14 and the 5 volt supply line 16, the junction of the two resistors R16, R17 being connected to the inverting input of integrated circuit IC1C. The non-inverting input is connected to the base of an NPN transistor Q4 by way of a resistor Rl 8 and to the line input terminal 10 by way of resistor R15. The emitter of the transistor is connected to the output of IC1C and the collector to the junction of resistors R19, R20. Moreover, series connected diodes D5 and D6 are connected between line 14 and the 5 volt supply line and are poled in the non-conducting sense relative to the 5 volt supply. The junction of the diodes, D5, D6 is connected to the non-inverting input of integrated circuit IC1C. The purpose of the diodes D5, D6 is to control the voltage at the non-inverting input to within the safe operational limits of integrated circuit IC1C which have a considerably smaller range than the AC supply voltage.

In normal operation, transistor Q1 is off because the output of IC2 is low, and therefore transistor Q3 is in a conducting state so that current can flow to the load. If for the reasons mentioned earlier, any one of the comparators IC1A, IC1B or IC1D changes state, the voltage at the trigger input of integrated circuit IC2 is lowered and the output of integrated circuit IC2 goes high thereby turning on transistor Ql. At the same time, capacitor C5 is ·' 20 discharged and then recharged by way of resistor R21 and when the voltage across C5 reaches the predetermined control voltage, IC2 is re-set so that transistor Ql is rendered nonconductive and transistor Q3 conductive.

If it is required that automatic re-setting should not occur unless the load is disconnected, the modified form of device shown in Figure 3 is used. This differs from the circuit of Figure

1 in the provision of transistor Q5 and resistor 30. With transistor Q3 turned off a very small current will continue to flow in the load by way of resistors R29 and R30. This flow of current causes a voltage drop across resistor R30 which turns transistor Q5 on thereby

AP/P/0 1/0? 1 ⁴⁸ preventing the recharging of capacitor C5. Thus once an overcurrent or a high current surge or low supply voltage has been detected, transistor Q3 is rendered non-conductive and will remain so until the load is disconnected. With the load disconnected transistor Q5 is rendered non-conductive and the capacitor C5 is recharged.

The nee Hgible residual current which will continue to flow through the load despite it having been disconnected will be due not only to the continued flow through resistors R29, R30 but also very small leakage currents through circuit elements such as Cl and Q3.

The circuit could be modified so that in the event of transistor Q3 being turned off due to a short-circuit, overcurrent or undervoltage trip, turn on is prevented until a switch is manually operated. The manual reset feature could be applied to all or just some of the trips and could be a single switch for all trips or individual switches for each trip.

In order to minimise high current surges and electromagnetic interference when tr ansistor Q3 is rendered conductive, it is aaanged that this can only take place near the zero crossing point of the supply voltage when a load is connected. This function is provided by integrated circuit 1C1C and the associated components. The output of integrated circuit IC1C is low when tiie supply voltage is below the threshold value determined by the relative values of resistors R16 and Rl 7. Moreover, only when the supply voltage is above approximately 0,7 volts is the transistor Q4 in a conductive state. Between these two supply voltages therefore, the control voltage applied to IC2 is reduced, hence ensuring that only at close to a supply zero crossing is transistor Q1 turned off, allowing current flow to the load through transistor Q3. As a result the surge current and electromagnetic interference are minimised as resetting of the unit takes place.

The zero crossing switch on only occurs if there is load connected to the output of the device. To ensure that the transistor is always switched on at a zero crossing irrespective of the load connected, a small current should be passed between L_0UT and N by means of a high value resisior

V I 01I o Zd/dV

ΑΡν ύ 1 4 2 5

The automatic re-setting feature and the fact that it contains no moving parts mean that the circuit components can be encapsulated and in use mounted in a physically inaccessible position. The control unit responds very quickly to large overloads and short circuits and thereby is able to minimise damage which might otherwise occur to the load or wiring.

Good accuracy can be achieved by using precision components and, unlike in the case of fuses and thermal circuit breakers, operation of the control unit does not rely upon thermal heating and therefore good accuracy can be achieved over a wide ambient temperature range. The control unit may be easily customised for different requirements by changing resistor and capacitor values.