BACKGROUND TO THE INVENTION There are numerous items of electrical equipment that are operated by a three-phase power supply and that could become seriously damaged in the event that certain types of irregularities occur in the three-phase power supply. Such irregularities include a significant imbalance in the three phases such as occurs if the voltage of one or more phases deviates outside acceptable limits, of for that matter, if one or more phases is lost, and also the irregularity of à phase becoming'swapped'so that it is no longer of the correct polarity.

There is thus a need for a protection device that can afford protection to the three-phase electrical equipment connected to such a power supply and that can also, in some circumstances, correct the irregularity.

OBJECT OF THE INVENTION It is accordingly an object of this invention to provide a protection device that is aimed at diminishing, if not substantially eliminating, the deleterious effects of such power supply irregularities on the three-phase electrical equipment being operated.

SUGARY OF THE ttEMTtOM In accordance with this invention there is provided a protection device for three-phase electrical equipment characterised in that it comprises a voltage sensor for each phase, a phase sensor for each phase, control circuitry operated by an output from the voltage and phase sensors and itself having a control output adapted to operate switching means in the electrical power supply to an item of three-phase electrical equipment in the event that one or more voltages of the power supply deviates outside predetermined safe parameters or in the event that a phase becomes swapped, or both.

Further features of the invention provide for each voltage sensor to have an output that is fed to a voltage balance monitor, typically in the form of a d. c. sample voltage ; for each phaK sensor to have an output that is fed to a phase comparator, typically in the form of a square wave pulse ; for the outputs from the voltage balance monitor and phase comparator to be inputted to a relay control logic circuit the output of which is operatively connected to a relay typically controlling a three-phase circuit contactor that together constitute said switching means; and for the device to have condition indicator lights indicating the condition of the switching means.

A still further feature of the invention provides for the control circuitry, and in particular the relay control logic circuit, to have a second output operatively controlling a second switch relay for operating a second three-phase circuit

contactor operatively connected into a three-phase power supply to an item of three-phase electrical equipment in a manner adapted to correct a swapped phase.

It will be understood l : hat the device provided Iby this invention may be constructed as a stand-alone device for inclusion in an existing installation or, alternatively, it may be built into an item of three-phase electrical equipment.

The invention therefore also provides an item of three-phase electrical equipment embodying a device according to the invention.

In order that the above and other features of the invention may be more fully understood one embodiment thereof will now be described with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing is a circuit diagram of a device according to the invention connected into a three-phase power supply to an item of three- phase electrical equipment.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWING In the drawing, a three-phase mains power supply is indicated by numeral (1) and has the usual three phase inputs L1, L2, L3, and a neutral N in instances in which a neutral is employed. As a general rule, a neutral may be employed in a five wire system but would not be employed in a four wire system, in each case in an earth wire E being present and forming the additional wire.

The three-phase power supply is connected to the electrical equipment that is indicated by numeral (2) by way of a first contactor (3) that is to isolate the electrical equipment by opening all the phases simultaneously and, where applicable, also the neutral.

In this embodiment of the invention, a second contactor (4) is connected in parallel with the first contactor except that, on the output side, two of the phases, in this case L2 and L3, are reversed in order to correct a swapped phase.

The contactors (3) and (4) are operated by relay switches (5) and (6) respectively in a manner so that both of the contactors cannot be closed at the same time although they can both be open at the same time.

The relay switches are controlled by a relay control logic circuit (7) in the manner that will be discussed further below.

The voltage of each of the phases is sampled by a voltage sampler (8a, 8b, and 8c) that isolates and converts the incoming AC voltage on the relevant phase to a safe and usable level for measurement. Typically, a step-down transformer and bridge rectifier isolates and converts the AC input voltage to DC, where-after a resistor divider network provides a sample voltage output that is fed to a voltage balance monitor (9).

The voltage balance monitor receives the sample voltages from the voltage samplers and checks that all are present and of the correct level (i. e. balanced within predetermined parameters). If 611 of the phase voltages are present and of the correct level, the voltage/balance monitor circuitry outputs a"pass"condition that is fed to the relay control logic circuit (7).

If one or more of the phase voltages are absent or of an incorrect level (out of balance), the circuitry outputs a"fault"condition.

Typically, operational-amplifiers, configured as a window comparator, measure each of the supplied sample voltages, and, if the sample voltage falls between a set upper and lower limit (voltage window determined by a resistor voltage divider network), the comparator oul : puts a logic"1".

If the supplied sample voltage does not fall within this window, then the output is a logic"0". All three outputs are passed to an array of AND gates forming a 3 input AND function. If all three outputs are"1"the output of the AND function is a"1".

If any one of the outputs goes to"0"the output of the AND function goes to "O".

The phase relationship of each of the phases is monitored by a phase sampler (10a, 10b, 10c) that isolates and converts the incoming AC voltage on the relevant phase to square wave pulses. An opto-isolator, connected between the secondary winding of the associated transformer, converts the sinusoidal input voltage to square wave pulses, which are then buffered by a NAND gate.

The three phase sampler outputs are fed to a phase comparator (11) that receives the square wave pulses from each of the phase samplers and compares them to each other to determine whether each is in the correct phase sequence. If the phases are in the correct sequence, the phase comparator outputs a"pass"condition.

If the phases are not in the correct sequence, the phase comparator outputs a"fault"condition. Each of the square wave pulse trains, are 120° out of

phase, ie, phase 2 is 120° behind phase 1, phase 3 is 120° behind phase 2, and phase 1 is in turn 120° behind phase 3.

Typically, in the phase comparator, phase 1 is clocked into a'D'type latch, by phase 2. If the correct phase lag exists, the output of the'D'latch is a logic"1". Phase 2 is clocked into a'D'type latch, by phase 3. If the correct phase lag exists, the output of this'D'latch is also a logic"1". Both of the'D' latch outputs are passed to an AND gate. The output of the AND gate is only a logic"1"if the outputs from the'D'latches are both"1".

If any of the phases are swapped, the clock and data inputs from one of the latches will be out of synchrony, and the output of that latch will be a logic "0"causing a logic"0"at the output of the AND gate. The output from the AND gate is fed to the relay control logic circuit (7).

The relay control logic circuit continuously monitors the outputs from the voltage balance monitor and the phase comparator. If the output from the voltage/phase monitor is in a"fail"condition, then the relay control logic circuit de-energizes (switches off) both of the relay switches (5) and (6) which in turn de-energizes both contactors (3) and (4) thus isolating power from the electrical equipment.

If the output from the voltage/balance monitor is a"pass"condition, and the output from the phase comparator is a"pass"condition, then the first relay switch (5) is energized, which in turn energizes the first contactor (3) to connect the three phase electrical supply to the electrical equipment with the phases arranged as supplied by the electrical mains power supply.

Typically, in order to achieve this, the relay control logic circuit employs an array of AND gates that are used to translate the"1"or"0"output from the voltage balance monitor and the phase comparator.

If the output from the voltage balance monitor circuit is a logic"1" (indicating that all three phases are present and at the correct level) and the output from the phase comparator is a logic"1" (indicating that all three phases are in the correct sequence), an AND function switches on the first relay switch (5) with the second relay switch (6) being de-energized.

If the output from the voltage balance monitor circuit is a logic"0" (indicating that one or more phases are absent or of an incorrect level), the AND function switches off the first relay switch (5), which removes the control voltage to the first contactor (3).

If the output from the voltage balance monitor circuit is a logic"1" (indicating that a) three phases are present and at the correct level) and the output from the phase comparator is a logic"0" (indicating that the phases are not in the correct sequence), an AND/NAND function holds the first relay switch (5) off, and energises the second relay switch (6), which supplies a control voltage to the second contactor (4).

Whilst the first contactor (3) switches the incomihg three phase power supply through to the output in its supplied (original) form, the second contactor (4) swaps two of the incoming phases, thus re-creating the correct phase sequence that is then connected through to the output.

In this embodiment of the invention the device is further provided with indicator lights (12) that are energized by the relay control logic circuitry to provide a visual indication of the current condition of the power suppjy.

Also, auxiliary switg4es 41) Od (14) can be energized for activating other facilities such as an alarm, ; lights, siren, or other monitoring or surveillance devices according to user requirements.

It will be understood that numerous variations may be made to the embodiment of the invention described above without departing from the scope hereof. In particular, the exact nature of the various circuit components can be varied widely according to requirements and available technology and components.

Also, as indicated above, the device can be made as a stand-alone unit to be installed between an electrical mains power supply and three phase electrical equipment or, it may be built into the equipment or, for that matter, built into an electrical power supply reticulation system, typically within a factory or the like.