GB2271895A

GB2271895A - Relay operating system.

Info

Publication number: GB2271895A
Application number: GB9320900A
Authority: GB
Inventors: Andrew Ceri Davies
Original assignee: Smiths Group PLC
Current assignee: Smiths Group PLC
Priority date: 1992-10-23
Filing date: 1993-10-11
Publication date: 1994-04-27
Also published as: GB9320900D0; DE4335421A1; GB9222308D0; FR2697383B1; FR2697383A1

Abstract

A circuit breaker has a current sensor 3 responsive to current flow along a conductor 2, and a relay 1 connected in series with the conductor. The current sensor is connected to a control circuit 4 having a network of resistors 43, 44, 45 and capacitors 40, 41, 42, the output of which provides one input to a comparator 46. The other input of the comparator 46 is connected to a reference voltage 48. The circuit 4 has a time function proportional to the product of time and the square of current, so that it is dependent on the amount of excess energy flowing on the conductor 2. Switching of the relay 1 is controlled by the circuit 4. The relay may be electromagnetic or electronic, eg, a switching transistor. The circuit 4 may also include inductors. <IMAGE>

Description

1 Electrical Systems This invention relates to electrical systems.

The invention is more particularly concerned with electrical relay systems.

2271895 Mechanical circuit breakers are used extensively for protecting electrical installations from excessive power dissipation when a fault occurs. Some of these devices have what is known as an '12C trip characteristic, that is, the device trips only after a time t, having let through a particular level of excess energy. This can be a useful feature because it ensures that the trip time is shorter for more severe faults, thus minimizing damage. It can also reduce the number of nuisance trips.

In some applications where greater accuracy and remote control is required, electronic systems comprising a relay and an electronic control circuit can be used instead of the mechanical circuit breaker. Such systems, however, respond to excessive current levels but do not vary the trip time according to the energy passed, as in the mechanical circuit breaker.

It is an object of the present invention to provide an improved form of electrical system including a relay.

According to one aspect of the present invention there is provided an electrical system for interrupting current flow along a conductor, the system including a relay connected in series with the conductor, means for sensing current flow along the conductor or in associated equipment and for providing a voltage output in accordance therewith, and a control circuit arranged to receive the voltage output from the current sensing means and to control operation of the relay in accordance therewith, the control circuit having a time function such that the trip time of the relay is dependent on the amount of excess energy flowing along the conductor.

The control circuit preferably has a time function that is proportional to the product of time and the square of current flowing along the conductor. The control circuit may include a network of resistors and capacitors and a comparator having an input connected to the network. The network may include a first resistor connected in series with the current sensor, a series connection of a resistor and capacitor connected to the first resistor, and a second capacitor connected in parallel with the series connection. The comparator may have a second input connected to a reference voltage and the relay may be an electromagnetic relay.

An electrical system according to the present invention will now be described, by way of example, with reference to the accompanying drawing, in which:

Figure 1 shows the system schematically; and Figure 2 is a graph illustrating operation of the system.

The system comprises a relay 1 connected in a power supply line 2, and a current sensor 3 which is responsive to current flow along the power supply fine and supplies a signal to a control circuit 4 by which the relay is operated.

The relay 1 is of conventional construction and may be of the electromagnetic kind in which a switch element is displaced by the action of an electromagnet. More particularly, the switch element may be held in a closed state bridging two contacts by the magnetic field produced by the electromagnet, against the action of a spring urging the switch element away from the contacts to an open position. When power to the electromagnet is removed, the magnetic field falls below what is necessary to maintain the switch element in a closed state so that it is moved open by the spring. This has the effect of preventing power supply along line 2. The relay could be of various different forms. For example, the natural state of the switch element could be closed so that it is diplaced open by a magnetic field from the electromagnet.

3 In another form, the relay could be purely electronic, the switching being provided by, for example, a switching transistor.

The current sensor 3 may be of any conventional kind that provides an output with a voltage that increases in proportion to current flow along line 2.

The circuit 4 comprises an RC network of three capacitors 40 to 42 and three resistors 43 to 45, and a comparator 46. The input of the network is connected to the current sensor 3 and its output is connected to one input of the comparator 46. The output of the current sensor 3 is applied, via a first of the resistors 43, across a series connection of a resistor 44 and capacitor 40. A second series connection of a resistor 45 and capacitor 41 is connected in parallel across the resistor 44 and capacitor 40. The third capacitor 42 is connected in parallel across the second series connection of resistor 45 and capacitor 41. Typical values for the components in the RC network are as follows: resistor 43, 470M; resistor 44, 30OM; resistor 45, 47Kn; capacitor 44, 5 gF; capacitor 4 1, 1 gF; and capacitor 42, 1 0OnE The RC network has a time-lag function, producing an attenuation of the input voltage supplied to the circuit 4 that is similar to mechanical circuit breakers, and having an output voltage on line 47 that is approximately proportional to Ft.

The output on line 47 is supplied to one input of the comparator 46 the other input of which receives a threshold voltage from a reference source 48. When the voltage on line 47 rises above that from the source 48, the comparator 46 produces a trip output to the relay 1 causing the relay to open and thereby preventing flow of current along line 2. With this system, the trip time varies in the fashion illustrated in Figure 2 showing that the trip time reduces considerably for increasing current. On this graph, one line illustrates the ideal current/ time trip curve and the other line shows the approximate function achieved by the system of the present invention.

4 It will be appreciated that various modifications to the system are possible. For example, instead of having an RC network to produce the desired trip characteristic, the system could have a network including inductors. The current sensor need not respond to current flowing along the conductor in which the relay is connected but could instead respond to current flow in some associated piece of equipment supplied by the conductor.

Claims

Claims

An electrical system for interrupting current flow along a conductor, wherein the system includes a relay connected in series with the conductor, means for sensing current flow along the conductor or in associated equipment and for providing a voltage output in accordance therewith, and a control circuit arranged to receive the voltage output from the current sensing means and to control operation of the relay in accordance therewith, and wherein the control circuit has a time function such that the trip time of the relay is dependent on the amount of excess energy flowing along the conductor.
2. An electrical circuit according to Claim 1, wherein the control circuit has a time function that is proportional to the product of time and the square of current flowing along the conductor.
An electrical circuit according to Claim 1 or 2, wherein the control circuit includes a network of resistors and capacitors and a comparator having an input connected to the network.
4. An electrical circuit according to Claim 3, wherein the network includes a first resistor connected in series with the current sensor, a series connection of a resistor and capacitor connected to the first resistor, and a second capacitor connected in parallel with the series connection.
5. An electrical circuit according to Claim 3 or 4, wherein the comparator has a second input connected to a reference voltage.

6. An electrical circuit according to any one of the preceding claims, wherein the relay is an electromagnetic relay.
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7. An electrical circuit substantially as hereinbefore described with reference to the accompanying drawing.
8. Any novel feature or combination of features as hereinbefore described.

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