US3683923A

US3683923A - Electrosurgery safety circuit

Info

Publication number: US3683923A
Application number: US75358A
Authority: US
Inventors: Robert K Anderson
Original assignee: Valleylab Inc
Current assignee: Valleylab Inc
Priority date: 1970-09-25
Filing date: 1970-09-25
Publication date: 1972-08-15
Anticipated expiration: 1989-08-15

Abstract

A safety circuit suitable for use in electrosurgery apparatus to prevent electrical burns is described herein. A current sensing transformer having a pair of primary windings and a single secondary or sense winding is provided. The primary windings are connected so as to compare the input current applied to an active electrode with the output current from an indifferent plate. When the comparison is unequal, the sense winding generates an error signal that warns the operator of the electrosurgery apparatus of a possible electrical burn situation. Alternatively, the primary windings are connected so as to sense the equality of output currents from two indifferent plates. Lack of equality causes an error signal to be generated to warn the operator.

Description

United States Patent Anderson [54] ELECTROSURGERY SAFETY CIRCUIT [72] Inventor: Robert K. Anderson, Boulder, C010. [73] Assigneez Valleylab, Inc., Boulder, C010.

[22] Filed: Sept. 25, 1970 [211 App]. No.: 75,358

521 U.s.c1. ..l28/303.14,l28/423,317/18 51 1111.01. ..A61b17/36,A6ln 3/00, 110211 1/02 [58] Field 61 Search.l28/2.l, 303.14, 303.17, 303.18

[56] 7 References Cited UNITED STATESPATENTS 1,841,968 1/1932 Lowry ..128/303.14 1,863,118 6/1932 Liebel ..:128/303.18 ux 3,252,052 5/1966 Nash ..317/18 3,402,326 9/1968 Guasco et al. ..317/18 3,495,584 2/1970 Schwalm 128/206 1,945,867 2/1934 Rawl's v.128/303.14

1151 3,683,923 1451 Aug. 15, 1972 Primary Examiner-Channing L. Pace Attorney-Duane C. Burton ABSTRACT A safety circuit suitable for use in electrosurgery apparatus to prevent electrical burns is described herein. A current sensing transformer having a pair of primary windings and a single secondary or sense winding is provided. The primary windings are connected so as to compare the input current applied to an active electrode with the output current from an indifferent plate. When the comparison is unequal, the sense winding generates an error signal that warns the operator of the electrosurgery apparatus of a possible electrical burn situation. Alternatively, the primary windings are connected so as to sense the equality of output currents from two indifferent plates. Lack of equality causes an error signal to be generated to warn the operator.

10 Claims, 4 Drawing Figures Patented Aug. 15, 1972 3,683,923

PRlOR ART INVENTOR ROBERT K. ANDERSON W ZWMU ATTORNEYS 1 ELECTROSURGERY SAFETY CIRCUIT BACKGROUND OF THE INVENTION tive, or other side, of the RF source is connected to a cutting or coagulating electrode. The RF source applies a high density current to the cutting or coagulating electrode at a relatively high voltage (in the range of 1,000 volts). The high density current causes a localized cutting or coagulating action. The current, after flowing through the operation point is returned via the indifferent electrode plate or plates to the RF source. The indifferent'electrode plates contact the patient over a relatively large area so that current density is low at all such contact points. The low'current density prevents'the occurrance of localized electrical burns at point where the indifferent electrode plates contact the patient.

While electrical surgery apparatus of the foregoing nature has been generally satisfactory, all such 'apparatus have one particular disadvantage. More specifically, on occasion patients have been electrically burned when they have been operated on by electrical surgery apparatus. Such burns occur when either:

1. the ground or return cable connecting the indifferent electrode plates to the RF source is broken; or,

2. the patient moves out of contact with the indifferent electrode plates. When either of these conditions occurs and there is another or secondary ground contact to the patient, current will flow through the secondary ground contact and cause localized burning of the patient at the point where the secondary ground contacts the patient. Such secondary ground may be created, for example, by:

l. monitoring electrodes connected to the patient;

2. grounded adjacent metallic equipment; and,

3. vertical supports for supporting ancillary equipment, such as overhead lights. In other words, when the normal ground return is broken or separated from the patient, the electrical energy flowing through the surgical electrode seeks other paths if they exist, Because these other paths usually contact the patient over small areas, the current densities at these areas may be very high. The high current densities cause electrosurgical burns at these contact points.

It will be appreciated by those skilled in the medical profession that electrosurgical burns can be quite severe. While a patient will react to them if he is conscious, he is often unconscious when surgery is being performed. Hence, in most cases electrosurgery burns go unnoticed until the operation is completed. And, because a considerable length of time usually elapses during surgery, the bums are more severe than they would be if the contact time between the secondary ground and the patient were short.

Therefore, it is an object of this invention to provide a safety circuit.

It is a further object of this invention to provide a safety circuit suitable for use with electrosurgery apparatus to prevent electrical burns.

It is yet another object of this invention to provide a safety circuit suitable for use with electrosurgery apparatus to prevent electrical burns at secondary ground points when an indifferent electrode connection to a patient is broken either because of inadequate patient contact to the indifferent electrode or because of a break in the connecting line.

SUMMARY OF THE INVENTION In accordance with principles of this invention a safety circuit suitable for use with electrosurgery apparatus to prevent electrical burns is provided. A current sensing transformer having a pair of primary windings and a single secondary or sense winding is mounted between a radio frequency (RF) source and a patient. The two primary windings are connected so as to sense current flow through predetermined portions of the electrosurgery apparatus. When the current flow is balanced, the sense winding generates no signal. However, when the current flow is unbalanced, the sense winding generates a signal which can be utilized to operate a light, an alarm, or a relay that de-energizes the RF source. The primary windings are connected such that the sense winding only generates a signal when the patient moves out of contact with the indifferent electrodes or if the ground line from the different electrodes to the RF source is broken.

In accordance with further principles of this invention, one of the primary windings is connected between the active side of the RF source and the operative surgical electrode (e.g. coagulating or cutting). The second primary winding is connected between the ground side of the RF source and the indifferent electrode. The primary windings are connected such that current flow through them generates opposing magnetic fields. Hence, when the same amount of current is flowing through both windings, a balance condition occurs. When this situation happens, the sense winding generates no output signal. When a patient moves out of contact with the indifferent electrode or if the ground line connected to the indifferent electrode is broken, an unbalanced condition occurs. When an unbalanced condition occurs, the sense winding generates a signal. The signal can be used to cause the generation of a suitable audible or visual alarm. Alternatively, the signal can operate a relay to de-energize the active electrode.

In accordance with an alternate principle of the invention, the patient is connected to two indifferent electrodes. Each of the indifferent electrode is connected through one of the primary windings of the current sensing transformer, to the ground side of the RF source. Again, the windings are connected such that current flow through them causes opposing magnetic fields. Hence, if either of the electrodes comes out of contact with the patient or if either of the connecting lines is broken, an unbalanced condition occurs. The unbalanced condition causes the sense winding to generate a signal. It will be appreciated that this principle may be extended to electrosurgical systems that use more than two indifferent electrodes to contact the patient.

In accordance with still further principles of this invention a silicon controlled rectifier (SCR) is connected to the sense winding. The SCR is triggered when the sense winding generates a signal. An alarm or other indicator is connected in series with the SCR so that it is energized when the SCR is triggered. I

-It will be appreciated from the foregoing brief summary of the invention that a safety circuit suitable for use in a system that has similar input and output currents, such as electrosurgery apparatus, is provided. The use of a two primary winding, current sensing transformer has various advantages over other systems. For example, inexpensive current sensing transformers can easily handle voltages in the 1,000 volt range while solid state devices that will handle such voltages are expensive. Further, the current sensing transformer isolates the sensing circuitry from the radio frequency source to maintain 60Hz ground protection which is not done by directly connected sensing circuits. In addition, to these advantages, the invention has the additional advantage of being relatively uncomplicated and, therefore, inexpensive to manufacture and maintain. Moreover, the invention can be easily inserted into presently used electrosurgical apparatus without requiring extensive modifications of the apparatus.

It should be noted that while the preferred use of the invention is with electrosurgical apparatus, it can be also used in other systems. For example, the invention can be utilized to sense a break in the ground line of an arc welding apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing objects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood from the following detailed description when taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a partially pictorial, partially schematic diagram illustrating the dangers involved in the use of present electrosurgical apparatus;

FIG. 2 is a partially schematic, partially pictorial diagram illustrating one embodiment of the invention;

FIG. 3 is a schematic diagram illustrating an altemate embodiment of the invention; and,

FIG. 4 is a schematic diagram of an embodiment of the invention that includes an alarm system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates the coupling between an RF source and the patient of a common prior art system. More specifically, FIG. 1 illustrates the secondary winding 21 of the output transformer of an RF source (not shown). FIG. 1 also illustrates a patient 23 lying on a single indifferent electrode 25. The indifferent electrode 25 is connected via a ground cable 27 to the ground side of the secondary winding 21. The active or other side of the secondary winding 21 is connected via an active cable 29 to a surgical electrode 31. The surgical electrode 31 can be either a cutting electrode or a coagulating electrode. FIG. 1 further illustrates a secondary ground 33 shown as a wire 35 connected through a resistance 37 to ground 39. The secondary ground 33 is illustrated as contacting the patient 23 at a point 41. Also illustrated in FIG. 1 is a break 43 in the ground cable 27.

As will be well understood by those skilled in the medical art, the patient 23 residing on the indifferent electrode 25 may be operated on by the surgical electrode 31 in many different manners. During such operation, current I flows from the secondary winding 21 along the active cable 29 to the surgical probe 31. The dense electrical current at the tip of the surgical probe 31 causes localized cutting or coagulation of the patients tissue. Current flows from the surgical probe through the patient 33 to the indifferentelectrode 25. The current I at the indifferent electrode returns throughthe ground cable 27 to the ground side of the secondary winding 21. While a slight current flows through the secondary ground 33, this current is very small because the major area of ground contact is through the indifferent electrode 25. In other words, I l I for all practical purposes.

When a break 43 occurs in the ground cable 27, current flow through the ground cable 27 stops. However, because a secondary ground 33 exists in this example, a return path remains. This return path or secondary ground now carries the entire return current flow to the ground side of the secondary winding 21 of the RF source. Because only a small area of the secondary ground 33 contacts the patient 23, a high density current passes through this small area and a localized burn occurs at the contact point 41. It will be appreciated that a similar situation will occur if the patient moves off of the indifferent electrode 25 while remaining in contact with the secondary ground 33.

It will be appreciated by those skilled in the art and others that because most patients are under an anesthesia when they are being operated on they are not able to communicate the fact that they are being burned to the operator of the electrosurgery apparatus. Further, because patients are usually almost totally covered during an operation, such a burn is not readily self evident. Because of these two factors in particular, electrical burns causedby electrosurgery apparatus can be quite severe. That is, these two factors allow the electrical burning situation to exist for a long period of time. And, the length of burning time determines the severity of the burn. It is elimination of such burns that is the primary object of this invention.

FIG. 2 illustrates one embodiment of the invention which comprises a transformer 51 having first and second

primary windings

53 and 55 and asecondary or sense winding 57 all wound about a toroidal core 61. The active side of the secondary winding 21 of the output transformer of the RF source is connected through the first primary winding 53 to the surgical electrode 31. The ground side of the secondary winding 21 is connected throughthe second primary winding 55 to the indifferent electrode 25. As before, a patient 23 is illustrated as lying on the indifferent electrode 25 and a secondary ground 33 is illustrated as connected between a point 41 on the patient and ground 39 via a connecting means 35 and a resistor 37. The sense winding 57 is connected to a pair of

output temiinals

59, 59.

Under normal conditions, the surgical electrode 31 applies the radio frequency signal to the patient in the manner previously described and the signal returns via the indifferent electrode 25. Hence, as long as the system is operating correctly, the transformer has no effect. However, if the patient moves off of the indifferent electrode 25 or if a break occurs in the ground v cable 27, the transformer becomes operative and the sense winding generates a signal at the output terminals 59. More specifically, the first and second

primary windings

53 and 55 are wound around the toroidal core 61 of the transformer 51 and connected in such a manner that the magnetic fields generated by currents I and I flowing through these windings are magnetically in opposition. Hence, as long as the input current I is equal to the output current l the sense winding 57 senses an essentially zero magnetic field. However, if a break occurs or if the patient moves off of the indifferent electrode 25, I reduces to zero. Under these conditions, 1 causes a current to flow in the sense winding 57 whereby a voltage is created atthe

output terminals

59, 59. The thusly created voltage can be utilized to operate a visable alarm, an audiable alarm, or a relay which will open circuit the output from the RF source. Or, two or all three of these devices can be operated if desired.

FIG. 3 illustrates an alternative embodiment of the invention which comprises a transformer 71 having first and second primary windings 73 and 75 and a single secondary or sense winding 77. In this case, rather than the patient contacting a single indifferent electrode, the patient (not shown) contacts two

indifferent electrodes

79 and 81. The ground side of the output transformer (not shown) of the RF source is connected to a ground terminal 83. The ground terminal 83 is connected to one side of each of the primary windings 73 and 75. The other side of the first primary winding 73 is connected to the first indifferent electrode 79 and the other side of the second primary winding 75 is connected to second indifferent electrode 81. The sense winding 77 is connected to sense

output terminals

85, 85. Asillustrated in FIG. 3, the first and second primary windings 73 and 75 are wound and connected in series opposition. Consequently, if the patient is equally contacting both of the

indifferent electrodes

79 and 81, as would occur under normal circumstances, the return current I is split and half passes through each primary winding. In other words, I /2 passes through each of the primary windings 73 and 75. Because these currents flow in opposite directions, the magnetic fields generated by the first and second primary windings 73 and 75 are in opposition whereby they tend to cancel one another. Hence, as long as one-half of I is passing through each of the primary windings, the secondary or sense winding generates no output voltage. However, if

one of the return cables is broken or if the patient moves off of one of the indifferent electrodes, the balance or current canceling condition is broken. When either of these circumstances occurs, the sense winding 77 generates a voltage across the

sense terminals

85, 85 caused by thecurrent flow through the other primary winding. This voltage can be used in the .manner previously described to cause an alarm or moves off of one or more indifferent electrodes, a signal is generated that warns the doctor or operator of the system that the patient is probably being burned because there is a return through some other (secondary) ground line. Hence, the doctor can stop the operation or the operation can be automatically stopped through the operation of a relay to prevent further burning of the patient. After the bum situation is corrected, the operation can continue.

While various types of alarm systems can be connected to the sense winding terminals, a preferred form of an uncomplicated alarm system is illustrated in FIG. 4 and hereinafter described. For purposes of illustration, FIG. 4 illustrates a current sensing system of the type illustrated in FIG. 3. However, it will be appreciated by those skilled in the art and others that the alarm system can also be used with a current sensing system of the type illustrated in FIG. 2; More specifically, FIG. 4 illustrates a transformer 91 having first and second

primary windings

93 and 95. One end of the primary winding 93 is connected to a first indifferent electrode 99 and one end of the second primary windings is connected to a second indifferent electrode 101. In addition, a ground terminal 103 is illustrated. The ground terminal 103 is connected to the ground side of the output transformer (not shown) of the RF source.

Rather than the ground terminal being directly directed to the other sides of the first and second

primary windings

93 and 95, it is connected to those sides through a double pole, double throw switch designated S. More specifically, the double pole, double throw switch S is illustrated in FIG. 4 as having a pair of

upper terminals

105 and 107 and a pair of lower terminals 109 and 111 plus an upper common terminal 113 and a lower common terminal 115. The upper common terminal 113 is connected to the other side on the first primary winding 93 and the lower common terminal 115 is connected to the other side of the second primary winding 95. The ground terminal 103 is connected to the

upper terminals

105 and 109 of both the upper and the lower pairs of terminals so that both the first and the second primary windings are connected to the ground terminal 103 when S is in its upper position.

One side of the sense winding 97 is connected to ground. The other side of the sense winding 97 is connected to the anode of a diode designated D. The cathode of D is connected to: a capacitor designated C; a first resistor designated R1; and the cathod of a zener diode designated ZD. The other terminals of R1 and C are connected together and to ground. The other terminal of ZD is connected to the lower terminal 111 of the lower pair of terminals of S and through a second resistor designated R2 to ground. The other terminal of ZD is also connected to the gate of a silicon controlled rectifier designated SCR. The lower terminal 107 of the upper pair of temrinals of S is connected through a voltage source designated V, a normally closed push button switch designated PS and an alarm designated A, all connected in series, to the anode of SCR. The cathode of SCR is connected to ground as is the other side of V.

In normal operation, S is in the upper position illustrated in FIG. 4 and the system operates in a conventional manner. When a break occurs or a patient moves off of oneof the

indifferent electrodes

99 or 101, the signal across the sense winding 97 is applied through D to ZD. If the voltage level of this signal is sufficiently high, ZD breaks down and SCR is gated on. When this occurs, current flows from V through the push button switch to the alarm A causing a audiable or visual signal to be generated depending upon the nature of the alarm. That is, if desired, the alarm A can be an audible alarm, a visual alarm or a combined alarm. Moreover, the alarm can include a relay that, when energized, prevents the application of further RF power to the surgical electrode. Once SCR is gated on, the alarm continues to operate until PS is opened. When this occurs, the alarm ceases to operate because the cathode-anode terminals of SCR are open circuited.

When S is in its lower position and the indifferent electrodes are joined together via a patient represented by R3, the alarm circuit illustrated in FIG. 4 can be tested. More specifically, when this is done, a voltage fiows from V through the first primary winding 93, the

indifferent electrodes

99 and 101, the second primary winding 95 and R2 to ground. This current flow causes a current in the sense winding 97 which causes the application of a voltage to the gate of SCR. This voltage gates SCR on whereby the alarm A is energized if the alarm circuit is operating satisfactorily. Hence, with S in its lower position, the integrity of all indifferent electrode wires plus patient contact to the indifferent electrodes is proven.

It should be noted that the arrangement illustrated in FIGS. 3 and 4 is intended to insure adequate patient contact,(at a plurality of points) to the, indifferent electrodes whereas the FIG. 2 arrangement is intended to insure that all of the input-current comes out of the indifferent plate.

, It will be appreciated from the foregoing description that the invention provides a system for sensing when an electrosurgical system is not operating in the desired manner. If the system is not so operating, an alarm indicating that such undesirable operation is occurring is provided. Such undesirable operation occurs when the ground return line is broken or when the patient moves off of the indifferent electrode. This mode of operation is undesirable because a secondary ground may be connected to the patient. If so, and the contact area of the secondary ground is small, a burn will occur at the contact point. It is this type of harmful operation that the invention is designed to prevent.

While the invention has been described in an electrosurgical environment, it can be utilized in other environments. That is, it can be utilized in other environments, such as welding, for example, to detect when the input current is not equal to the output current. Further, while one form of an alarm system has been illustrated and described, various other types of alarm systems can be utilized with the invention to carry out its basic objectives. Moreover, while toroidal core transformers have been illustrated, other types of current sensing transformers can be utilized by the invention. Hence, this invention can be practiced otherwise than as specifically described herein.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as fol lows:

l. A safety circuit suitable for use in determining the occurrence of a break in the ground side of an electrical energy source having an active side and a ground side, the active side of said electrical energy source being adapted to apply current to an active electrode and the ground side of said electrical energy source being adapted to receive return current in equal amounts from two indifferent electrodes, said safety circuit comprising:

a current sensing transformer including:

a first primary winding connected between the ground side of said electrical energy source and one of said indifferent electrodes;

a second primary winding connected between the ground side of said electrical source and the other of said indifferent electrodes, said first and second windings being connected so that current passing through them generate opposing magnetic fields; and,

a sense winding connected so as to sense when the magnetic fields generated by said first and second primary windings are unbalanced; and,

an alarm connected to said sense windings so as to provide an indication when said sense winding senses that the magnetic fields generate by said first and second primary windings are unbalanced.

2. A safety circuit as claimed in claim 1 wherein said alarm includes a means for preventing the further application of power from said electrical energy source when said sense winding sense that the magnetic fields generated by said first and second primary windings are unbalanced.

3. In an electrosurgical apparatus wherein electrical energy from a radio frequency source having an active side and a ground side is applied via an active cable to an active electrode, suitable for operating on a patient, and returns via at least one indifferent electrode and a ground cable, aimprovement comprising a safety circuit suitable for sensing a discontinuity in the return portion of said electrosurgical apparatus, said safety circuit being connected between said radio frequency source and said electrodes. I

4. The improvement claimed in claim 3 wherein said safety circuit comprises:

a current sensing transformer having a pair of primary windings connected so as to be balanced when said electrosurgical apparatus is operating in a normal manner and connected so as to be unbalanced when a break occurs in said portion; and,

an alarm connected to said current sensing transformer so as to be activated when said current sensing transformer is unbalanced because of said break in said return portion.

5. The improvement claimed in claim 4 wherein said current sensing transformer has first and second prim ary windings, said first primary winding being connected between the active side of said radio frequency source and said active electrode, said second primary winding being connected between the ground side of said radio frequency source and said at least one indifferent electrode, said first and second primary windings being connected so that current passing through them generates opposing magnetic fields, said current sensing transformer further including a sense winding connected so as to sense when the magnetic fields generated by said first and second primary windings are unbalanced, said sense winding being connected to said alarm.

6. The improvement claimed in claim wherein said alarm includes means for preventing the application of power from said radio frequency source to said active electrode when said sense winding senses said unbalanced condition.

7. The improvement claimed in claim 6 wherein said alarm includes an alarm device and a triggerable solid state device that is activated to pass current to said alarm device when said sense winding senses said unbalanced condition.

8. The improvement claimed in claim 4 wherein said electrical energy returns via two indifferent electrodes and wherein said transformer includes first and second primary windings, said first primary winding being connected between one of said indifferent electrodes and the ground side of said radio frequency source, said second primary winding being connected between the other of said indifferent electrodes and the ground side of said radio frequency source, said first and second primary windings being connected so that currents passing through them generate opposing magnetic fields, said current sensing transformer further including a sense winding connected so as to sense when the magnetic fields generated by said first and second primary windings are unbalanced, said sense winding being connected to said alarm.

9. The improvement claimed in claim 8-wherein said alarm includes means for preventing the application of power from said radio frequency source to said active electrode when said sense winding senses said unbalanced condition.

10. The improvement claimed in claim 9 wherein said alarm includes an alarm device and a triggerable solid state device that is activated to pass current to said alann device when said sense winding senses said unbalanced condition.

Claims

1. A safety circuit suitable for use in determining the occurrence of a break in the ground side of an electrical energy source having an active side and a ground side, the active side of said electrical energy source being adapted to apply current to an active electrode and the ground side of said electrical energy source being adapted to receive return current in equal amounts from two indifferent electrodes, said safety circuit comprising: a current sensing transformer including: a first primary winding connected between the ground side of said electrical energy source and one of said indifferent electrodes; a second primary winding connected between the ground side of said electrical source and the other of said indifferent electrodes, said first and second windings being connected so that current passing through them generate opposing magnetic fields; and, a sense winding connected so as to sense when the magnetic fields generated by said first and second primary windings are unbalanced; and, an alarm connected to said sense windings so as to provide an indication when said sense winding senses that the magnetic fields generate by said first and second primary windings are unbalanced.

3. In an electrosurgical apparatus wherein electrical energy from a radio frequency source having an active side and a ground side is applied via an active cable to an active electrode, suitable for operating on a patient, and returns via at least one indifferent electrode and a ground cable, a improvement comprising a safety circuit suitable for sensing a discontinuity in the return portion of said electrosurgical apparatus, said safety circuit being connected between said radio frequency source and said electrodes.

4. The improvement claimed in claim 3 wherein said safety circuit comprises: a current sensing transformer having a pair of primary windings connected so as to be balanced when said electrosurgical apparatus is operating in a normal manner and connected so as to be unbalanced when a break occurs in said portion; and, an alarm connected to said current sensing transformer so as to be activated when said current sensing transformer is unbalanced because of said break in said return portion.

5. The improvement claimed in claim 4 wherein said current sensing transformer has first and second primary windings, said first primary winding being connected between the active side of said radio frequency source and said active electrode, said second primary winding being connected between the ground side of said radio frequency source and said at least one indifferent electrodE, said first and second primary windings being connected so that current passing through them generates opposing magnetic fields, said current sensing transformer further including a sense winding connected so as to sense when the magnetic fields generated by said first and second primary windings are unbalanced, said sense winding being connected to said alarm.

6. The improvement claimed in claim 5 wherein said alarm includes means for preventing the application of power from said radio frequency source to said active electrode when said sense winding senses said unbalanced condition.

9. The improvement claimed in claim 8 wherein said alarm includes means for preventing the application of power from said radio frequency source to said active electrode when said sense winding senses said unbalanced condition.

10. The improvement claimed in claim 9 wherein said alarm includes an alarm device and a triggerable solid state device that is activated to pass current to said alarm device when said sense winding senses said unbalanced condition.