Method for electromagnetically shielding inductive voltage detectors

(54) METHOD FOR ELECTROMAGNETICALLY SHIELDING INDUCTIVE VOLTAGE DETECTORS

(76) Inventor: Lynn C. Lundquist, 10833 NE.

Russell, Porland, OR (US) 97220

( * ) Notice: Subject to any disclaimer, the term of this patent is extended or adjusted under 35 U.S.C. 154(b) by 0 days.

(21) Appl. No.: 09/325,833

(22) Filed: Jun. 4, 1999

(51) Int. CI.7 G01R 31/02; G01R 1/30;

G01R 19/14; H01H 31/02

(52) U.S. CI 324/72.5; 324/123 R; 324/133;

324/515; 324/555; 324/556; 324/552

(58) Field of Search 324/72.5, 133,

324/123 R, 552, 555, 515, 556, 657

(56) References Cited

U.S. PATENT DOCUMENTS

3,337,801 8/1967 Rinier et al 324/133

3,437,928 4/1969 Baker et al 324/133

3,444,465 * 5/1969 Teixeira 324/72.5

3,525,939 * 8/1970 Cartmell 324/133

3,774,110 11/1973 Roveti 324/133

3,786,468 1/1974 Moffitt 340/515

3,828,256 8/1974 Liu 324/133

3,878,459 4/1975 Hanna 324/555

4,006,409 * 2/1977 Adams 324/510

4,066,953 1/1978 Gold 324/123 R

4,084,134 4/1978 Nagano 324/133

The method of this invention provides a means for altering commonly available inductive voltage detector instruments into dual sensitivity test instruments. By shielding the sensor probe with a metallic material, the probe is made to be unresponsive to energized conductors in near proximity when the shield is grounded by finger contact. A single probe window is provided so that when grounded, the instrument will respond to an energized conductor in only the probe window area allowing testing of single conductors in wire bundles. When ungrounded by the electrician's finger, the instrument reverts to normal sensitivity. Provision is made for safety by isolating the metallic shield from contact with energized conductors. The method may be employed by means of a demountable accessory or incorporated into the test instrument during manufacture.

6 Claims, 2 Drawing Sheets

1

METHOD FOR ELECTROMAGNETICALLY
SHIELDING INDUCTIVE VOLTAGE
DETECTORS

BACKGROUND

1 Field Of The Invention

The method of this invention provides electromagnetic shielding around the sensor probe of an inductive voltage detector so that during testing, the voltage detector can be used to obtain readings on selected conductors in a wire

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bundle. Further, this electromagnetic shielding is constructed so that its shielding effect is selective, allowing the user to employ either the full sensitivity range of the inductive voltage detector or selecting a narrow field of sensitivity for the purpose of testing individual wires in a bundle or testing within an electrical panel with high elec- 15 tromagnetic interference.

BACKGROUND

2 Description Of The Prior Art

Inductive voltage detectors are common electrical test 2o instruments carried by field electricians. In their most common form, they are a pencil shaped meter with a pocket clip. Some manufacturers provide larger hand-held units. In most cases, these instruments have a fixed sensitivity threshold which will activate a flashing light and, or annunciator when ^ the probe of the instrument comes within one or two inches of an alternating current source of a specified voltage intensity. Inasmuch as these inductive voltage detectors are sensitive to the voltage induced electromagnetic field surrounding the conductor, the conductor does not need to be carrying current in order to cause the detector to indicate. 30 For this reason, they are useful test instruments to determine whether an electrical conductor is energized. They sense the presence of live conductors without physically contacting electrically energized metal. In this regard, their unidirectional response to a large field area assures that all conduc- 35 tors in a prescribed vicinity are monitored. In general testing, response to a dispersed electromagnetic field is requisite to assuring the location of all hazardous voltages.

Many manufacturers provide inductive voltage detectors. This includes, but is not limited to, Fluke Corporation of 40 Everett, Wash, (model VoltAlert), A. W. Sperry, Inc. of Hauppauge, N.Y. (model VH-601A), Greenlee Textron, Inc. of Rockford, 111. (models 2010H and 1010) and Tif Instruments, Inc. of Miami, Fla. (models Tic Tracer 100 and Tic Tracer 300). 45

Several manufacturers, namely A. W. Sperry (model VH-601A) and Greenlee Textron (model 2010H), provide inductive voltage detectors which incorporate sensitivity adjustments. In both cases, the adjustment varies the instrument's response threshold. By setting the detector at its 50 lowest operating sensitivity, the instrument may be somewhat successfully used to test a single conductor in a crowded wire bundle or electrical panel with high electromagnetic radiation. However, varying the instrument's threshold does not alter its unidirectional response to a large 55 field area. That is, the entire probe area remains sensitive to any electromagnetic signal. Generally, depending on the design of the test instrument, the probe will read any electromagnetic signal within 360 degrees around the probe or in a cone shaped field from the tip of the probe. Thus, 60 using such an instrument on its lowest sensitivity setting does not assure that a misleading reading from multiple electromagnetic sources cannot occur.

OBJECTIVES OF THIS INVENTION 65

The method of this invention was developed to selectively restrict the field of sensitivity of an inductive voltage detec

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tor. Furthermore, this method was designed with a number of specific objectives considering its potential application.

1. It is the general objective of this method to modify the function of an inductive voltage detector as presently manufactured so that it may selectively test single conductors in a crowded wire bundle or in an electrical panel with high concentrations of electromagnetic interference.

2. Another objective of this method is to equip the inductive voltage detector with an external removable probe shield so that no manufacturing changes are required in the inductive voltage detector itself. The probe shield is not common to the internal circuitry of the instrument.

3. Another objective of this method is to allow the user to selectively alternate between the full sensitivity range of the inductive voltage detector in its normal setting and selectively test single conductors when necessary.

4. Another objective of this method is to provide an inexpensive adaptation to achieve the objective of selectively limiting the inductive voltage detector's field of sensitivity.

5. A final objective of this method is to provide a clip-on probe shield which is electrically safe without introducing hazard to the operating electrician.

These and other objectives and advantages of the present method, and the manner in which they are achieved, will become apparent in the following specifications and claims.

SUMMARY OF THIS INVENTION

All inductive voltage detectors are sensitive in a region defined by an area circumferentially surrounding the probe or radiating from the tip of the probe in the shape of a cone. In practice, because it serves the manufacturers' interests to increase sensitivity, most such instruments will have both fields of sensitivity with one or the other being a predominant area of sensitivity. Though this greater range of sensitivity is useful in general testing, it generally prevents the inductive voltage detector from selective use on a single conductor when that conductor is in a wire bundle or in an electrical panel. The restriction of general field sensitivity can be altered, however, by surrounding the probe with grounded electromagnetic shielding while providing an unshielded window in the single area of desired sensitivity. When the shield is grounded in this manner, the instrument will be sensitive to an electromagnetic field from a conductor only in the unshielded window area. Live conductors which are adjacent to the shielded areas will not activate the instrument. On the other hand, when the shield is ungrounded, the instrument will return to its original unidirectional sensitivity.

In order to achieve selective grounding of the shield, the metallic shield material is made electrically common to a touch pad located on the outside of the plastic shield covering. When the operator touches the touch pad, the shield becomes effectively grounded. When the operator's finger is removed from the touch pad, the shield returns to its ungrounded state.

In order to assure operator safety from electrical hazard, the probe shield is entirely encased in a plastic covering so that it cannot come into electrical contact with energized parts. Provision is also made for a high resistance element between the probe shield and the touch pad which further protects the operator.

The method of this invention can be applied to any manufacturer's inductive voltage detector.