U.S. patent number 3,783,340 [Application Number 05/286,864] was granted by the patent office on 1974-01-01 for ground safe system.
This patent grant is currently assigned to Bio-Tek Instrument, Inc.. Invention is credited to Richard S. Becker.
United States Patent |
3,783,340 |
Becker |
January 1, 1974 |
GROUND SAFE SYSTEM
Abstract
The chassis of the equipment which is to be monitored is
connected in series with a closed switch, the secondary winding of
a first transformer, and the primary winding of a second
transformer to a ground terminal such as is commonly found in a
grounded wall outlet. The primary winding of the first transformer
is supplied with a high frequency signal from an oscillator. The
secondary winding of the second transformer is connected to a
transistorized alarm circuit. As long as the equipment is properly
grounded, the high frequency signal will flow through the series
circuit and keep the alarm inoperative. If the ground line of the
hospital equipment is broken or increases greatly in its
resistance, the high frequency signal will be interrupted and this
will activate the alarm circuit. The system includes a redundant
ground connection to the chassis of the equipment.
Inventors: |
Becker; Richard S. (Burlington,
VT) |
Assignee: |
Bio-Tek Instrument, Inc.
(Shelburne, VT)
|
Family
ID: |
23100502 |
Appl.
No.: |
05/286,864 |
Filed: |
September 7, 1972 |
Current U.S.
Class: |
361/50; 324/510;
340/532; 340/649; 340/652 |
Current CPC
Class: |
H02H
11/001 (20130101); G01R 27/18 (20130101) |
Current International
Class: |
G01R
27/16 (20060101); H02H 11/00 (20060101); G01R
27/18 (20060101); H02h 003/16 () |
Field of
Search: |
;340/256 ;324/51
;317/18B,146,9D |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Trammell; James D.
Attorney, Agent or Firm: Gordon K. Lister et al.
Claims
What is claimed is:
1. A system for monitoring the ground lead connection of an
electrically conducting piece of equipment to a ground terminal
comprising a first transformer having a primary winding and at
least one secondary winding, an oscillator connected to the primary
winding of the first transformer for generating an alternating
electrical signal in the primary and secondary windings of the
first transformer at a predetermined, relatively high frequency, a
second transformer having primary and secondary windings, means for
providing an additional connection between the electrically
conducting piece of equipment and the ground terminal in parallel
with the ground lead connection, the additional ground connection
means including means for directly connecting the secondary winding
of the first transformer, the primary winding of the second
transformer, the electrically conducting piece of equipment and the
ground connection in a series circuit, and means connected to the
secondary winding of the second transformer and responsive to the
high frequency signal for providing an indication when the high
frequency signal circulating in the series circuit falls below a
predetermined magnitude.
2. A system for monitoring the conductivity of a grounding
connection from an electrically conductive chassis of a piece of
medical equipment to a ground terminal, the electrically conductive
equipment being of the type which is connected by leads to a power
source, the monitoring system comprising a first transformer having
a primary winding and at least a secondary winding, means connected
to the primary winding of the first transformer for generating an
alternating electrical signal having a high frequency in the range
between 10 KHz and 100 KHz, a second transformer having a primary
winding and a secondary winding, means separate from the power
leads for providing an additional connection between the equipment
chassis and the ground terminal, the additional ground terminal
connection means including means for connecting the secondary
winding of the first transformer and the primary winding of the
second transformer in series between the ground terminal and the
chassis of the equipment to be monitored, detector means connected
to the secondary winding of the second transformer and responsive
to the high frequency signal appearing at the secondary winding of
the second transformer for providing an indication of the decrease
in the conductivity of the grounding connection upon the decrease
by a predetermined amount in the intensity of the high frequency
signal.
3. A monitoring system as recited in claim 2 wherein the means for
connecting the secondary winding of the first transformer and the
primary winding of the second transformer in series with the
equipment to be monitored includes means for selectively increasing
the resistance in the series circuit in order to provide a test of
the monitoring system.
4. A system for separately and simultaneously monitoring the
conductivity of the grounding connections of a plurality of
electrically conductive pieces of medical equipment to a ground
terminal, the equipment being of the type which is connected by
leads to a power source, the system comprising a first transformer
having a primary winding and a plurality of secondary windings,
means for generating an alternating electrical signal having a high
frequency in the range of 10 KHz to 100 KHz in the primary winding
of the first transformer, a plurality of detector circuits
responsive to the high frequency signals appearing at the secondary
windings of the first transformer, each detector circuit including
a transformer having a primary and a secondary winding, means
separate from the power leads for providing separate additional
connections between the chassis of the pieces of equipment and the
ground terminal, the addition ground terminal connection means
including means for separately connecting each of the primary
windings of the detector transformers and a separate one of the
secondary windings of the first transformer in series between the
ground terminal and the chassis of a separate piece of electrical
equipment to be monitored.
5. A monitoring system as recited in claim 4 wherein the means for
generating an alternating signal causes an alternating signal
having a voltage substantially equal to 5 millivolts to flow in the
means for separately connecting the primary windings of the
detector transformers.
Description
BACKGROUND OF THE INVENTION
The present invention relates to systems for monitoring the ground
connection to the chassis of electrical equipment and more
particularly to such systems as used to monitor the ground
connections of hospital equipment.
In some prior ground monitoring systems a direct current is
circulated in a series circuit between the chassis of the equipment
which is being monitored and an alarm system. When this series
circuit is interrupted, as for example when the ground connection
to the chassis is broken, the monitor sounds an alarm. Systems of
this type are commonly used to monitor industrial equipment. One
disadvantage in applying such systems to monitor hospital equipment
is that the D.C. monitoring current poses a potential danger to the
hospital patient who might be connected to the equipment. For
example, extremely low D.C. currents on the order of 10
microamperes may cause fibrillation in the heart of an extremely
ill heart patient in the event that the system fails and the heart
patient becomes the ground connection from the test equipment to
some natural ground terminal such as a water pipe or some properly
grounded equipment.
Another disadvantage of such prior industrial monitoring ground
systems is that they are relatively insensitive to a slight
increase in the resistance of the ground connection to the
equipment which, although not sufficient to impede the monitoring
current, is nevertheless sufficient to allow a voltage potential to
build up between the chassis of the hospital equipment and natural
ground connections within the hospital room so as to present a
hazzard to the seriously ill patient who may come in contact with
the chassis.
Still another disadvantage of some prior monitoring systems is that
while providing an alarm to indicate that a ground failure has
occurred, many such systems do not also provide a redundant ground
connection to the chassis of the equipment to ensure that no danger
is presented to the user of the equipment. Even such systems as do
provide a redundant ground require bulky electro-mechanical
switching equipment to make the ground connection.
The present invention overcomes these and other disadvantages by a
novel design which requires a minimum of parts, construction and
labor, and may be assembled in a relatively small container.
SUMMARY OF THE INVENTION
A preferred embodiment of the present invention comprises a series
circuit including the output winding of a first transformer, the
chassis of the equipment to be monitored, a ground connection such
as the ground terminal in a conventional wall outlet, and the input
winding of a second transformer. The primary winding of the first
transformer is provided with a high frequency signal of a
predetermined frequency from an oscillator, and the secondary
winding of the second transformer is connected to an alarm circuit.
The alarm circuit is activated when the high frequency signal
circulating in the series circuit falls below a predetermined
amplitude level. In one embodiment a switch is provided in the
series circuit in order to test the operation of the ground safe
system.
In the preferred embodiment the oscillator is a stabilized
multi-vibrator which generates a high frequency signal between
10KHz and 100KHz in the primary winding of the first transformer.
The secondary winding of the second transformer is connected
through a diode to a base electrode of a first transistor. The
first transistor is normally in the cut-off region of its operating
characteristics due to the bias provided by the rectified high
frequency signal generated by the oscillator and carried by the
series circuit. The collector electrode of the first transistor is
directly connected to the base electrode of a second transistor
whose output circuit is connected in series with an alarm device
such as an indicator lamp. The second transistor is biased in such
a manner that when the first transistor is in the cut-off region
the second transistor is also operated in the cut-off region. When
the high frequency signal is interrupted, or decreases sufficiently
in its amplitude, the first transistor is then biased into the
conducting region of its operating characteristics which also
causes the bias on the second transistor to operate it in the
conducting region of its operating characteristics, thereby
activating the indicator device. A single-pole, single-throw switch
is provided between the base and emitter electrodes of the first
transistor to turn the circuit off when it is not in use.
One advantage in the use of a high frequency 10KHz - 100KHz, low
voltage (5 millivolt) continuity signal for detecting small changes
in the resistance in the ground connection is that a larger current
may be safely used than if the continuity signal were a direct
current. This result is due to the fact that the human heart does
not respond to high frequency signals as readily as it responds to
low frequency, less than 1000Hz, signals. This is due to the
excitability curve of the heart. Thus a 5 millivolt, 10KHz to
100KHz signal is the physiological equivalent of a direct current
which is less than one microampere. This phenomenon allows the
electronic circuitry to be greatly simplified since less
amplification is needed to detect changes in the resistance of the
ground connection path.
Another advantage of the present system is that a redundant ground
connection is provided through the transformers between the chassis
of the electrical equipment being monitored and the ground terminal
in the wall outlet to eliminate possible leakage current which
might harm the patient connected to the hospital equipment. Still
another advantage is that the use of transformers between the
monitoring equipment and the chassis of the equipment being
monitored provides isolation from the common power line in order to
protect the patient. A still further advantage is that the ground
safe system is operated in a fail safe condition.
The ground safe system of the present invention has many
applications, as will be explained in greater detail hereinafter,
in all areas where class A equipment is prescribed for use in a
hospital. Such equipment might be an electrocardiograph, an
electroencephalograph, or a heart pacemaker. This system can be
incorporated as an integral part of the equipment to monitor the
ground system of that equipment or it can even be installed in the
wall outlet itself.
It is therefore an object of the present invention to provide a
ground monitoring system of small size and simple construction.
It is another object of the invention to provide a ground
monitoring system utilizing a high frequency current which is the
physiological equivalent of a very low magnitude direct
current.
It is still another object of the invention to provide a ground
monitoring system wherein a redundant ground connection is provided
to eliminate possible leakage current from the chassis of the
equipment being monitored .
The foregoing and other objectives, features, and advantages of the
invention will be more readily understood upon consideration of the
following detailed description of certain preferred embodiments of
the invention, taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified schematic diagram of one preferred
embodiment of the invention;
FIG. 2 is a schematic diagram of a modification of the preferred
embodiment of FIG. 1.
DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS
Referring now more particularly to FIG. 1, the ground safe system
is generally designated as 10 and it includes a first transformer
12 having a primary winding 14 and a secondary winding 16 and a
second transformer 18 which has a primary winding 20 and a
secondary winding 22. The primary winding 14 of the transformer 12
is connected to the output of an oscillator 24 which generates a
high frequency signal having a predetermined frequency in the range
of from 10KHz to 100KHz. The secondary winding 16 of the
transformer 12 and the primary winding 20 of the transformer 18 are
connected together in series with a test switch 26, the chassis 28
of the equipment to be monitored, and the ground connection 40 of
the equipment. This series circuit is also connected to one or more
ground terminals 30 of the type normally installed in three prong
power wall outlets 32. The series connected circuit is also
connected by a lead 34 to a natural ground terminal.
A resistance 36, which is of a relatively low value such as 5 ohms,
is connected in parallel with the normally closed switch 26. The
switch 26 provides a test of the operation of the ground safe
system. In normal operation the alternating signal generated by the
oscillator 24 is caused to flow in the series circuit and, through
the transformer 18, is supplied to a detector circuit 38 connected
to the secondary winding 22. As long as the alternating signal
flows with a predetermined amplitude within the series circuit the
detector 38 indicates that the equipment 28 is properly grounded.
If the flow of this alternating signal is interrupted or impeded,
such as by depressing the test switch 26 to cause the current to
flow through the resistance 36, the detector 38 will operate an
alarm to indicate that the resistance in the ground lead connection
of the equipment has increased. It should be noted that the chassis
of the equipment under test 28 is redundantly grounded through the
lead 34 adn the transformers 12 and 18. The chassis 28 of the
equipment under test is normally grounded through its own ground
connection 40 which is typically the ground wire of the power cord
connected to the equipment. The lead 40 is grounded in the ground
terminal of the wall socket 32.
Referring now more particularly to FIG. 2 the circuit details of a
modified embodiment of the ground safe system are illustrated.
Power is supplied to the system through a standard three prong plug
42, the ground terminal 44 of which is connected to the chassis of
the ground safe system 10. The contacts of the plug 42 are
connected to the primary winding 46 of a transformer 48. An
indicator light 50 is connected in series with a resistance 52
across the leads of the primary winding 46. The indicator light 50
indicates that the monitoring system is in operation.
The secondary winding 50 of the power transformer 48 is connected
across a diode bridge 52. The positive terminal of the diode bridge
is connected to a voltage regulator 54. The negative terminal of
the diode bridge 52 is connected to the chassis ground of the
monitoring system 10. A filtering capacitor 56 is connected between
the positive and negative terminals of the bridge 52. The output
from the voltage regulator 54 is a DC voltage, such as 8 volts for
the embodiment depicted in FIG. 2, and this voltage is supplied to
the oscillator 24 and the detector 38. It should be apparent that
the voltage supply system discussed above could be replaced in
other embodiments by batteries or could be used in parallel with
batteries which would allow the system to continue operation in the
event of a power failure. This latter feature is most advantageous
when the system is used to monitor equipment having its own
auxiliary power source.
The output of the voltage regulator 54 is connected to the
oscillator 24 at a junction 56. The oscillator 24 is basically a
multivibrator circuit and includes a first transistor 58 and a
second transistor 60. The junction 56 is connected to the base
electrode of the NPN transistor 58 by a bias resistance 62 and to
the collector electrode of the transistor 58 by a bias resistance
64. The junction 56 is also connected to the base electrode of the
NPN transistor 60 by a bias resistor 66 and to its collector
electrode through the primary winding 14' of the transformer 12'
which correspond, respectively, to the winding 14 and the
transformer 12 of the embodiment depicted in FIG. 1. A capacitor 68
is connected in parallel with the winding 14'. The capacitor 68 and
the transformer winding 14' together constitute a LC circuit which
determines the frequency of oscillation of the oscillator 24.
The base of the transistor 60 is connected to the collector
electrode of the transistor 58 through a capacitor 70. The emitter
of the transistor 58 and the emitter of the transistor 60 are
connected to the chassis ground through the resistors 72 and 74,
respectively. The collector electrode of the transistor 60 is
connected to the base electrode of the transistor 58 through a
capacitor 76. In the preferred embodiment illustrated in FIG. 2,
the oscillator 24 produces an alternating signal of a predetermined
frequency within the range of from 10KHz to 100KHz in the primary
winding 14'. However, in other embodiments other oscillator
circuits may be utilized.
In the embodiment depicted in FIG. 2 the transformer 12' has a
plurality of secondary windings 16a, 16b, 16c and 16d which
correspond to the single winding 16 in the embodiment depicted in
FIG. 1. Each of these windings is connected through a separate
series circuit for checking the ground connection of a separate
piece of hospital equipment. In the following description only the
circuit including the secondary winding 16a will be described in
detail as it will be understood that the remaining circuits operate
in a substantially identical manner.
One lead of the secondary winding 16a is connected in series with a
test switch 26a to the chassis of the equipment under test (not
shown) such as the device 28 in FIG. 1. A resistor 36a is connected
in parallel with the switch 26a. The switch 26a and the resistor
36a correspond to the switch 26 and resistor 36, respectively,
previously described in reference to FIG. 1. In a similar fashion
one lead of each of the windings 16b, 16c and 16d is connected to
parallel switch-resistor combinations 26b-36b, 26c-36c and 26d-36d,
respectively.
The other leads of the windings 16a, 16b, 16c and 16d are each
connected to one lead of the primary windings 20a, 20b, 20c and
20d, respectively, of the transformers 18a, 18b, 18c and 18d. The
transformers 18a, 18b, 18c and 18d each correspond to the
transformer 18 described in reference to FIG. 1. Each of the
transformers 18a, 18b, 18c and 18d has a secondary winding 22a,
22b, 22c and 22d, respectively, and one lead of each secondary
winding is separately connected to the input to a detector circuit
38a, 38b, 38c and 38d, respectively. The remaining leads of the
windings 20a, 22a, 20b, 22b, 20c, 22c, 20d and 22d are connected to
the circuit ground. A detailed description will only be given of
the detector circuit 38a since it will be understood that the
circuits 38b, 38c and 38d operate in a substantially identical
manner. Corresponding reference designations have been given to
corresponding circuit elements.
The ungrounded lead of the winding 22a of the transformer 18a is
connected to the cathode terminal of a diode 78a. The anode
terminal of the diode 78a is connected to the base of an NPN
transistor 80a through a resistor 82a. The anode terminal of the
diode 78a is also connected to the circuit ground through a
capacitor 84a.
The output from the voltage regulator 54 is connected to a junction
86 common to all of the detector circuits. The base of the
transistor 80a is connected through a resistor 88a to the junction
86. The collector electrode of the transistor 80a is connected
through a resistor 90a to the junction 86. The emitter electrode of
the transistor 80a is connected to the circuit ground. The base
electrode of the transistor 80a is connected through a normally
open switch 92a to the circuit ground.
The collector electrode of the transistor 80a is also connected to
the base electrode of a PNP transistor 94a. The emitter electrode
of the transistor 94a is connected directly to the junction 86. The
collector electrode of the transistor 94a is connected through an
indicating device 96a, such as an alarm, to the circuit ground.
In operation, the oscillator 24 generates an alternating signal
which flows through the series circuits including the windings 16a,
20a, the chassis of the equipment under test and its ground
connection. The alternating signal appears at the leads of the
winding 22a and is rectified by the diode 78a. The resistors 88a
and 82a, together with the diode 78a, constitute a voltage divider
biasing network which normally biases the base electrode of the
transistor 80a such that the transistor is in the conducting region
of its operating characteristics. However, the alternating signal
rectified by the diode 78a produces an opposing bias which forces
the transistor 80a into its cut-off region. As long as the
alternating signal is present with a sufficient amplitude at the
output lead of the winding 22a, the transistor 80a is biased to be
essentially non-conducting and its collector electrode has a
relatively high positive voltage potential. Since this voltage
potential is applied directly to the base of the transistor 94a,
transistor 94a is also operated in the cut-off or non-conducting
region of its operating characteristics and no power is supplied to
the indicating device 96a.
When the series circuit which includes the windings 16a or 20a is
interrupted by a break in the ground lead of the monitored
equipment or when its resistance is increased sufficiently, the
output signal from the winding 22a is diminished in amplitude and
the bias supplied by the resistors 82a and 88a causes the
transistor 80a to become conducting. The bias supplied to the base
of the transistor 94a is thereby substantially reduced in potential
which causes the transistor 94a to become conducting and to
activate the alarm device 96, thereby indicating that the ground
connection of the equipment under test is faulty.
Although the embodiments of FIGS. 1 and 2 have been described as
utilizing transistors, in other embodiments integrated circuits can
be utilized so that the ground safe system according to the
invention can be in a sub-miniature form. It should also be
apparent that while it is preferable to provide a separate detector
circuit for each piece of equipment which is to be grounded in some
embodiments it is also possible to connect the chassis of several
pieces of equipment in series with a single ground connection.
Furthermore although only four detector circuits are depicted in
FIG. 2, in other embodiments a larger number of detector circuits
can be provided for each piece of equipment which is to be
monitored.
The ground safe system of the present invention may be used to
monitor all electrically conductive objects around the patient's
room as well as hospital electrical equipment. By grounding every
metal object in the patient's room to a common point, the patient
is given a greater measure of safety. Certain devices, such as
electric beds, electric heaters, and fluorescent lights can be
fatal if ungrounded (i.e., not connected in common with all the
metal objects in the room.)
Such precautions are especially necessary in situations where the
patient is internally connected to the equipment, such as by a
catheter applied directly to his heart. Typical devices which might
be monitored by a system according to the invention are E.C.G.
monitors, electric beds, bed lamps, call buzzers, electric heaters,
telephone housings, metal furniture, suction equipment and infusion
pumps.
The terms and expressions which have been employed here are used as
terms of description and not of limitation, and there is no
intention, in the use of such terms and expressions, of excluding
equivalents of the features shown and described, or portions
thereof, it being recognized that various modifications are
possible within the scope of the invention claimed.
* * * * *