Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific current responsive fault sensor
Patent
1997-12-22
1998-12-29
Gaffin, Jeffrey
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific current responsive fault sensor
361 23, 361103, 361115, H02H 500
Patent
active
058547312
DESCRIPTION:
BRIEF SUMMARY
This invention relates to electrical apparatus and methods for controlling switching of circuit protection devices.
BACKGROUND OF THE INVENTION
The usual way of protecting electrical equipment and its wiring from current overloads is by means of an electromagnetic relay switch, thermal wire fuse or an electrothermal circuit breaker. An electrothermal circuit breaker uses the current flowing through it to heat a thermal sensor. Typically this consists of a bimetallic strip thermostat heated by a resistive element, which passes the current flowing through the breaker. The thermostat will operate when its temperature is raised beyond a pre-set value, with the consequence that the circuit breaker will open, removing power from the load. The temperature of the thermostat at any one time is a function of the dynamic equilibrium between the heat gain from the resistive element and the heat loss to its surroundings. The resistive element will gain heat at the rate I.sup.2 R, where I is the current flowing through the resistive element and R is its resistance.
These previous arrangements, however, are usually relatively heavy and bulky. They also have a slow response time and can be unreliable in the long term. Because of this there is a move towards using solid state power controllers (SSPC) employing power semiconductors, to switch electrical energy to a load and to interrupt current flow should an overload be detected.
Conventional electromagnetic devices, however, have an advantage not enjoyed by SSPCs in that they have an I.sup.2 t characteristic. That is, the product of the square of current I and the time to reach a critical energy level is a constant so that the time taken to trip out is inversely proportional to the square of the current. This means that the device will trip quickly for high currents but, for low currents, the trip time will be longer. Thus, a moderate excess current may not produce a trip if the current reduces after a short time. This reduces the occurrence of nuisance trips. Also, thermal protection devices have a memory in that a previous non-trip overload will raise the temperature of the device so that, if a second overload current should occur shortly after, it would raise the temperature of the device to its trip level more quickly. This is a useful feature, because the equipment and wiring protected by the device would respond in a similar way.
Various circuit breakers have been proposed previously in GB 2271895, GB 2140633, GB 899354, GB 1527962, GB 1524826, GB 2135146, GB 1124492, EP 5324, EP 326334 and U.S. Pat. No. 4,266,259.
It is an object of the present invention to provide improved apparatus for use in power switching and an improved method of controlling switching of a circuit protection device.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is provided electrical apparatus including a switchable circuit protection device, a current sensor that senses current flow through the circuit protection device, an emulator that receives an input from the current sensor and emulates the thermal sensor of an electrothermal circuit breaker, and means for supplying an output from the emulator to control switching of the circuit protection device.
The emulator may include a multiplier that multiplies the sensed current by itself to derive a voltage signal representative of current squared and a resistor capacitor circuit having a time constant that emulates the thermal sensor, the voltage signal being supplied to the resistor capacitor circuit and the output of the resistor capacitor circuit being connected to the circuit protection device.
Alternatively, the emulator may includes a processor connected to receive an output of the current sensor, the processor calculating the resultant heat gain of the emulated thermal sensor during a short period of time, the processor calculating the resultant heat loss of the emulated thermal sensor during the short period of time, the processor calculating the resultant temperature of the emulated sensor at the
REFERENCES:
patent: 4717985 (1988-01-01), Demeyer
Gaffin Jeffrey
Jackson Stephen
Smiths Industries Public Limited Company
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