The present invention relates to electrical risk monitoring devices.
Such devices may be used for the monitoring of equipment supplied from commercial
and industrial a.c. mains power source output wall or floor or other mounted sockets.
Such equipment typically may be industrial process control, medical or information
technology devices including computers and their peripherals.
Where such equipment is in use within a particular area or room in
a commercial or a light industrial environment, there may be a number of electrical
appliances in use either with or without operators for these appliances. In order
to protect both appliances and operators it is desirable to provide some form of
continuous monitoring of the electrical safety of both the appliances themselves
and the a.c. mains power source. This is to ensure that mis-connection or breakdown
of the main supply does not present a safety hazard to the operator or maintenance
staff, and that insulation breakdown within the appliances or their interconnecting
cables does not present a fire hazard to the appliances or any electrical shock
hazard to the operators. In addition it may be desirable to reduce or limit electrical
disturbances appearing on the mains input supply to the appliances from corrupting
data stored in or processed by the appliance.
The a.c. mains power source to the appliances will consist of a Live
and Neutral supply with or without an Earth connection. It has been previously
proposed that the current through the Live and Neutral lines should be continuously
monitored and where an unacceptable difference occurs, a warning may be activated.
This warning desirably should be both visible and audible, and may be displayed
either locally in the region of the appliance or remotely.
It is noted in this context, that the invention is more concerned
with monitoring of devices than with protecting such devices. Thus, a protective
device may have an automatic trip system, whereas a monitoring system warns of
a fault condition without necessarily shutting the equipment down. This may be
important if a computer is running and contains unsaved data which needs to be
saved before the computer can be shut down. In the present invention monitoring
is the main function of the device and protection is an optional secondary feature.
The present invention aims to provide such a monitoring unit which
is easy to install, easy to manufacture and easy to operate.
According to the present invention there is provided an electrical
risk monitor unit for installation between an a.c. mains power source and an a.c.
mains powered appliance comprising a housing, a power inlet connection means,
and at least one power outlet connection means each being mounted in said housing
and being accessible to externally connectable connector means, and there being
directly wired links between inlet and outlet connection means in the form of a
Live to Live connector link, a Neutral to Neutral connector link and an Earth
to Earth connector link, characterised in that said housing incorporates a detector
circuit including an upstanding toroidal transformer, said Live to Live connector
link and said Neutral to Neutral connector link pass through said toroidal transformer
and the detector circuit is arranged to provide an indication when an imbalance
occurs in any currents through said Live to Live and said Neutral to Neutral connector
links, and further characterised in that the power inlet connection means and
the output connection means are arranged adjacently in a wall of said housing so
enabling the connector links other than those passing through said upstanding
toroidal transformer to be direct busbar links.
The indication may be in the form of an audible or visible alarm
which can be displayed either locally or remotely or both, which is triggered when
the imbalance reaches a pre-set limit or it may be a measure of the level of imbalance
giving an increasing indication corrresponding to an increasing level of imbalance.
There can be a plurality of pre-set limits including at least one at a lower level
at which an alarm is triggered and one at a higher level at which the a.c. mains
power supply is arranged to be cut off.
Generally the inlet connection means will be either a three pin panel
mounted connector to receive an a.c. mains power cord set or it will be a fixed
lead and an a.c. mains power plug; and the or each power outlet connection means
will be a panel mounted socket connector. Such plug and socket connectors for personal
computers are usually of a standard configuration, but the a.c. mains power plug
design will vary dependent on the country.
The detector circuit may be incorporated on a printed circuit board
which carries the toroidal transformer in a manner which enables the live and neutral
links to be readily threaded through the centre of the toroidal core during assembly.
To this end, it is preferable that the power inlet and outlet connection means
are mounted on an upstanding wall of the component housing and that the toroidal
core lies in a plane perpendicular to said wall.
The detector circuit can be arranged to perform a number of functions:
- detection of live and neutral connections being reversed at the a.c. mains
- detection of disconnection of the Earth connection at the mains a.c. mains
power socket (unless a secondary Earth path fault exists)
- detection of disconnection of the Neutral connection at the a.c. mains power
- detection of an excess voltage between the Neutral and Earth connections
- detection of an excess leakage current from the attached appliances and their
corresponding cord sets
- disconnection of the Live and Neutral lines within the circuit on the detection
of an Earth leakage current in excess of a level which creates a safety hazard,
for example if Earth leakage current exceeds 30mA. This disconnection feature
is optional dependent on the usage requirement.
Each of these functions can be arranged to trip either or both an
audible and a visible alarm circuit, indicating that all appliances connected to
the unit should be progressively shut down and a maintenance engineer called to
assess the type of fault and to correct it, and optionally as mentioned above trip
the Live and Neutral lines if a hazardous condition arises.
Conveniently spike suppression means can be directly wired between
live, neutral and earth within the unit.
The unit can be inserted between the normal a.c. mains power plug
and socket input of the equipment to be monitored and may be attached to that equipment
by commercially available fabric fasteners, or be attached by screws to a convenient
surface or may sit on a horizontal surface via non-slip feet. Thus, the unit has
the advantage that it can be connected to existing equipment without any modification
to that existing equipment being necessary.
An embodiment of the invention will now be described by way of example
with reference to the accompanying diagrammatic drawings in which:
- Figures 1 and 2 show plan and side elevations of a unit in accordance with
- Figures 3 and 4 show alternative forms of the input connections to the unit
in end elevation;
- Figure 5 shows a fragmentary plan view of the Figure 4 arrangement showing
a fixed mains lead;
- Figure 6 shows a plan view of the unit with cover removed;
- Figure 7 shows a side elevation of the unit with cover removed;
- Figure 8 shows a schematic arrangement of the unit as attached between an a.c.
mains power socket and several appliances;
- Figure 9 shows a functional block diagram of the unit;
- Figure 10 shows the actual circuit diagram of the above functional block diagram;
- Figure 11 shows an alternative to the Figure 6 arrangement where the circuit
board is vertical.
Referring to Figures 1 and 2, a housing 1 comprises a series of plastics
mouldings 3, 4, 5 which are snapped and screwed together. These enclose electrical
functions of the unit and ensure a safe working environment for users. A first
of these mouldings 3 includes an upstanding wall which supports a series of output
sockets 6 which enables the unit to be connected to one or more electrical or
electronic appliances by interconnecting cord sets as shown in Figure 8. The same
moulding houses on the upstanding end wall, as one possibility a panel mounted
inlet plug 7 through which power is fed to the unit by an a.c. mains supply cord
through which power is fed to the appliances. Alternatively (see Figures 4 and
5) a fixed mains input power cord 8 may be connected directly within the unit.
Referring back to Figure 1, the unit allows a user to test key electrical
components and related functionality of the unit by depressing a test button 9
on the upper surface of the housing. A mains indicator 11 is fitted to one side
of the test button and a warning indicator 10 is fitted at the other side of the
test button. An outlet for an audible alarm 12 is in the form of a series of holes
or depressions in the top of the moulding.
The housing also contains components which reduce the level of electrical
spikes appearing through the mains connection which can degrade appliance performance.
The unit may be positioned for use in a number of ways. It can be
supported on a horizontal surface via non-slip feet 13, it may be screwed to any
reasonably flat and convenient horizontal or vertical surfaces via screw apertures
14 or it may be mounted on any suitable horizontal or vertical surface via fabric
fasteners such as VELCRO (Registered Trade Mark), 15.
Referring to Figures 6 and 7, the assembly of the components within
the housing can be seen. The three output sockets 6 have their Live Neutral and
Earth connectors strapped in parallel by bus bars 16. Spike suppresors 17 are
connected across each of the respective terminals.
A printed circuit board 18 is mounted to the base of the unit and
therefore is perpendicular to a plane containing the inlet connector 7 and the
outlet connector 6. The printed circuit board contains the test button 9, the
mains "on" indicator 11, and the warning indicator 10, previously mentioned as
well as the audible alarm 12. An alternative arrangement is shown in Figure 11
where the printed circuit board is attached to an end wall and so is parallel
to the upstanding end wall containing the output sockets 6.
Also mounted on the printed circuit board in an upstanding condition
transverse to the plane of the wall carrying the input socket 7 and the output
sockets 6 is a toroidal current transformer 19 including windings connected within
a circuit on the printed circuit board 18 and also two Live and Neutral links 20
carrying the full load current of the circuit between the input plug 7 and the
output sockets 6. They each form one-turn windings of the toroidal transformer.
Also connected between the bus-bars and the printed circuit board are low current
connections 21 providing power to drive the electronic monitoring circuits on the
printed circuit board and serving as connections to enable the correct connections
of the Live, Neutral and Earth a.c. mains power connections to be checked and monitored.
The unit is suitable for operation from the European Standard supply
of 230 V AC single phase (Þ 10% plus a further 10 V drop in building wiring)
and can readily be modified by changes in component values to operate on other
voltages as occur in other countries such as the United States of America. The
unit as described consumes a nominal 4 mA in the quiescent condition and 6 mA in
the alarm mode and a green LED LD1, confirms the flow of this current and constitutes
the mains indicator 10.
The incoming supply 7 is connected to the live (L), neutral (N),
and earth (E) terminals on the printed circuit board and, in addition, as already
described the pair of Live and Neutral conductors pass through the centre of the
toroidal current transformer 19 to feed the load output sockets 6 of the unit.
Half-wave rectification of the supply to the circuitry is provided
by diode D1, with diode D3 providing continuity to the neutral supply terminal.
Resistor R4 provides a nominal voltage drop before regulation to 15 V is provided
by transistor Q1 and zener diode ZD1,
Warning of unequal live and neutral currents (indicating the existence
of an earth leakage current) in the attached appliances and their interconnection
cables is provided by the pair of conductors passing through the current transformer,
TR1, whose 40-turn secondary winding feeds the non-inverting input of integrated
circuit 1C1A. The gain of this lC is set by resistor R10 to provide the level
of alarm threshold required (20mA unbalance in the conductors passing through TR1).
The output from the current transformer feeds the virtual earth provided
by the inverting input of the operational amplifier, whose output is rectified
by transistor Q2 providing an output which pumps down the voltage on diode D4
to switch the multivibrator 1C1B. This multivibrator determines the cadence of
the audible alarm 12 in the form of a piezo-electric transducer (PZT) incorporating
an internal audio oscillator) and the warning indicator 11 is a red light emitting
diode (LED)producing both audible and visual warning of abnormal conditions.
Additional circuits detect reversal of live and neutral supply conductors,
a disconnected Earth connection, or disconnected Neutral connection. These are
fed to a 180-turn tertiary winding on the current transformer 19, any current
being transferred to the secondary winding and raising alarms as previously described.
Reversal of Live and Neutral is electrically the same condition as connecting
the Earth terminal to the Live one, applying the supply voltage between the Earth
and Neutral terminals. Zener diode ZD1 limits the Earth to Neutral voltage at
which current will flow in TR1, so that Earth to Neutral voltages less than a set
value will have no effect.
Finally, the test button 9 is used to apply a half-wave ac signal
to the 180-turn winding of TR1 to simulate fault conditions for reassurance of
The unit alarm circuit is triggered by an input current exceeding
20 mA rms to the single-turn winding on the current transformer TR1 or by 110 óA
to the 180-turn winding. The single-turn winding is provided, in practice, by
the "live and neutral pair" feeding the output whose leakage-current is being monitored.
The 180-turn winding links any abnormality in the "Live", "Neutral" and "Earth"
connections to the monitoring circuit through the 40 turn winding.
The normal "earth to earth" supply circuit will present an input
of up to 40 volts from an impedance of up to 1600 ohms and this will be isolated
from TR1 by the diodes D2 and Zener diode ZD1. If these values are exceeded, current
will flow through R1 into TR1 and the alarm will be triggered. A consequence of
this circuit arrangement is a residual earth leakage current through resistor
R1 (1 MΩ). Reversal of the "Live" and "Neutral" lines will also create alarm
status, since this is effectively the same as presenting < 40 V at < 1600
ohms between Live & Earth, and the unit will "see" a high voltage between
its true Earth and Neutral terminals.
Finally, if the "Neutral" line becomes disconnected, the return current
of the unit flows to the "Earth" terminal, passing through the 180-turn winding
of the current transformer, hence creating the alarm condition.
For the purpose of meeting the European Directives and FCC Regulations
on electromaqnetic compatibility, the printed circuit board is partitioned in a
particular manner, one part of the board being produced as a single sided PCB
and the remainder as a double sided PCB. The circuit diagram Figure 10 shows the
partitioning of the circuit, with components and connections to the right of the
centre of the current transformer being connected to a reference plane which covers
that portion of the printed board over which the said components are mounted.
Components as necessary are connected directly to the reference plane, thereby
reducing the liability of the circuit to pick up radiated emissions in the bands
from 26MHz to 2GHz and conducted emissions from 100kHz to 30 MHz. Bypass and decoupling
capacitors are applied as necessary, being specifically capacitors C9, C10, C11,
C12, C13, C14 and C15. In addition a ferrite bead FB1 is connected between the
output of the secondary winding on the current transformer and the non-inverting
input of the operational amplifier 1C1A.
Although not shown in the described embodiment, a higher level discriminator
circuit can be provided where an excess level of Earth leakage current is monitored.
In this event, for example at an Earth leakage current greater than 40 ma, the
Live and Neutral lines can be arranged to be tripped automatically.