[0001] Electrical circuits that operate at 50V or more are generally considered hazardous. Prior to performing work on electrical installations/equipment that operate at 50V or more, electrically qualified workers are required to verify that the equipment is in an electrically safe state. The process of establishing an electrically safe work condition involves de-energizing the equipment by isolating all energy sources, applying lockout devices, and verifying the absence of voltage. Until proven otherwise, one must assume that the equipment is energized and take all necessary precautions, including utilizing appropriate personal protective equipment (PPE).

[0002] One part of the verification of an electrically safe work condition involves a test for the absence of voltage or the "live-dead-live test." This test is performed by an electrically qualified person using an adequately rated voltage test instrument, usually a portable voltmeter or multimeter. The qualified worker first tests the portable test instrument on a known, energized source to ensure it is working properly. The worker then verifies that voltage is absent in the electrical equipment by metering phase-to-phase and phase-to-ground. Finally, the worker re-tests the portable test instrument on a known, energized source to ensure it is still functioning properly and wasn't damaged during the phase-to-phase and phase-to-ground test. Although voltage verification is an NFPA 70E requirement and considered a best practice, the test itself still presents a hazard because workers are exposed to energized circuits and conductors when using the voltage tester during the "live" verification portions of the test.

[0003] A new approach would be to replace the portable test instrument with an installed device that has the capability to detect, verify, and positively indicate the absence of voltage in a reliable and fail-safe manner from outside the panel without exposing workers to electrical hazards. [0004] One key requirement for such an installed voltage testing system includes a way to isolate the primary voltage on the system being monitored from the circuitry of the installed device that is coupled to the primary power for the purpose of detecting the presence or absence of primary voltage. This isolation technique is important for two reasons: a) to protect the circuitry of the voltage detection, test, and verification system and b) to prevent the possibility of the primary voltage (typically greater than 50V) from being exposed to personnel while energized and presenting a hazard. The primary voltage may be single- or multi-phase AC or DC, while the circuitry of such an installed test system must be separately derived and independent of the primary power and operate at a non-hazardous (less than 50V) level (see Figure 1). The detection circuitry may compare the voltage detected on the primary line(s) to a pre-determined threshold or it may measure the magnitude of any potential on the primary line(s). Multiple techniques to isolate the primary voltage are described in this application.

[0005] When dealing with safety applications, it is crucial that any result or measurement be validated to ensure it is accurate and delivered in a fail-safe manner. There are multiple ways to validate the result. One way to verify the absence of a signal is to perform a check to verify that the detection circuity will recognize a known voltage (sometimes referred to as "test-the-tester," and similar to the "live" portion of the portable voltmeter test). Another step is to verify that the unit is actually measuring the signal and has not registered a no-voltage condition because the device has unknowingly been disconnected (connectivity test). These validation methods are described within this document.

Isolation Techniques

[0006] Detecting and monitoring the voltage on a single or multi-phase power line for applications like power quality or safety applications requires a very reliable and robust isolation technique between the power line under test and the voltage measurement (or detection) circuitry. This isolation is required so as to protect the voltage detection circuitry as well as preventing any possibility of the line voltage from reaching personnel (from a safety perspective). There are a variety of isolation methods that one can employ for this that include optical, thermal, magnetic field and electric field techniques (as shown in Figure 2). These isolation methods can be used to measure (or detect threshold levels) of the voltage level on the power line as well as can be used as a self-test of the detection or measurement circuitry to increase the reliability of the system. The self-test is accomplished by applying a known outward bound voltage signal to the power line; if the circuitry is able to detect this signal it confirms that the detection circuitry is functional and operating as expected. Note that the techniques that will be subsequently described, can be applied to single- and multi-phase AC power systems as well as DC systems. These techniques for detection and self-test, can be homogeneously (all of one type) applied or heterogeneously (mixed types between detection and self-test) applied.

Thermal Isolation

[0007] The thermal isolation technique utilizes a resistor (or voltage to thermal converter device such as infrared detector) across the line voltage being monitored and a temperature sensitive resistor (or thermal sensor) within the detection circuitry. The two are coupled via a thermally conductive substrate. Hence, when the line voltage has a voltage higher than a particular threshold the resistor will heat up and this heat will transfer to the thermal sensor and indicate to the system that there is unsafe line voltage present. If the line voltage is less than a particular threshold, the resistor heat will not be high enough to indicate an unsafe line voltage (Figure 3). Resistive isolation technique.

[0008] The resistor isolation technique to provide primary line voltage to detection circuitry isolation in prior art uses Ohm's Law with series resistors to perform a voltage divider. Figure 4 shows two resistors in series but a series of several resistors may also be used. Multiple resistors are often used to reduce the physical size and power consumption in a single resistor. The voltage divider ratio must be such that for the maximum line voltage, the maximum output or sensed voltage is in the non-hazardous, or safe, operating range and in a predetermined operating range suitable to make an accurate voltage measurement. The relationship from input to output is linear, as seen in the prior art voltage plot (Figure 4). The example shows the dynamic range required by the measurement when the line voltage range is very large and the maximum is much higher than the measured voltage to support the measurement accuracy. The large dynamic range and the measurement accuracy requires the ADC (Analog to Digital Converter) used by the microprocessor to have more bits per operating range.

[0009] The new art technique for resistor isolation uses a PTC (Positive Temperature Coefficient) resistor in the series resistor voltage divider chain. The PTC resistor's characteristics are such that it operates as if it has two resistor values, for simplicity the Figure 4 shows a low R and a high R by the red line in the graph. When the line voltage is low the PTC resistor behaves as a small value resistor, then as the line voltage is increased and the power increases in the device, it passes a threshold region and the resistance of the device also increases, limiting the output power. The resulting output voltage of the PTC being implemented in the resistor chain is shown in Figure 4(A) by the blue line. Describing the new art technique' s operation by the blue line in Figure 4(B) is a follows: Moving along the Vi _n axis from left to right as the line or Vi„ voltage increases from zero volts the linear relationship to the V _ou t voltage is present just as in the prior art example, the PTC resistor is in the low R value operating range. Then in this example when the Vi _n voltage reaches about 100 V, the PTC resistor transitions to the high R operating range shown by the drop in the Vout value and a more gradual rise in the Vout value to its maximum. The benefit of the new art voltage measurement technique is that the dynamic range of the Vout value is much lower than in the prior art technique. If the voltage value is converted to a digital value by an ADC, a less precision ADC can be used. The new art technique is beneficial when it is only necessary to know when the output voltage (representing the line voltage) is above a certain threshold.

Optical Isolation (through the use of an LED and a PTC resistor) technique

[0010] Another technique to isolate the line voltage from the voltage detection circuitry is to employ optical isolation. In this technique the line voltage, on the power lines being monitored, generates an optical output signal, for example via an LED, which transmits to a detector within the receiver circuitry in the detection circuitry. Figure 5 describes a few ways to implement this optical isolation technique. Figure 5a utilizes a transistor to detect the line voltage. As the line voltage increases, the current through Ri increases which will result in a higher base current into the transistor. The gain of the transistor will amplify the increased base current and increase the collector current which is the current through the optical device, such as an LED. The increase in current through the LED will increase the light output power. This increase in light output power will result in an increase in detection current in the receiver, and hence indicate an increase in the power line's voltage. There is a voltage divider setup between the transistor's base-emitter and the series resistance formed by Ri and the PTC resistor. The minimum detectible voltage (due to the typical minimum base-emitter voltage of the transistor of approximately 0.7 volts) is:

Minimum detectable voltage on power line = 0.7*[PTC-R\ \R3 + Ri + Ri] / Ri [0011] Utilizing the example values as shown in Figure 5b results in a minimum detectible voltage of 7 V. Also note that if the power line voltage increases to 500V rms, the power dissipated without the use of the PTC resistor would be: on R ₃ = 274W, R ₂ = 2W and Ri = 18W. The PTC resistor aids in the large reduction of power required on the R ₃ resistor.

[0012] Figure 5c, utilizes an OpAmp in front of the transistor so that a smaller power line voltage can be detected. The OpAmp circuitry will be able to detect power line voltages well under 1 V, and with the PTC resistor it will minimize on the amount of power required to be dissipated in R ₃ . This hence is a preferred configuration.

Magnetic Isolation

[0013] Another technique to isolate the line voltage from the voltage detection circuitry is to employ magnetic field isolation. In this technique the line voltage generates a magnetic signal which will couple to a magnetic receiver in the voltage detection circuitry. Figures 6 and 7 show various ways to implement this magnetic isolation technique.

Magnetic isolation with PTC and resistor technique.

[0014] The magnetic isolation technique provides isolation between the primary line voltage side and the detection circuitry side circuitry on the secondary side of the transformer. In prior art circuitry, a resistor in series with a transformer is used where the high voltage windings are isolated from the low voltage windings. The Ri series resistor (shown in Figure 6) limits the current in the transformer primary windings. The current through the transformer's primary windings and the turn's ratio of the transformer determines the output voltage range on the secondary side of the transformer. The voltage transfer function (defined by the input line voltage to the output voltage on the secondary side of the transformer) must be such that for the maximum line voltage, the maximum output or sensed voltage is in the non-hazardous or safe operating range and in a predetermined operating range suitable to make an accurate voltage measurement. The relationship between the input current (Itrsf) through the transformer to the output voltage is linear, as seen in the prior art voltage plot (Figure 6). The example shows the dynamic range required by the measurement when the line voltage range is very large and the maximum is much higher than the measured voltage to support the measurement accuracy. The large dynamic range and the measurement accuracy requires the ADC (Analog to Digital Converter) used by the microprocessor to have more bits per operating range.

[0015] The new art technique for magnetic isolation uses a PTC (Positive Temperature Coefficient) resistor in the series with the resistor, Ri, and in series with the line voltage side of the transformer (see Figure 6). The PTC resistor's characteristics are such that it operates as if it has two resistor values, for simplicity the Figure shows a low R and a high R by the red line in the graph (Figure 6). When the line voltage is low the PTC resistor behaves as a small value resistor then as the line voltage is increased and the power increases in the device, it passes a threshold region causing the resistance of the device to increase limiting the output power. The resulting output current of the PTC being implemented in series with the primary line voltage transformer winding is shown in the graph (Figure 6) by the blue line.

[0016] Describing the new art technique's operation by the blue line in the chart (Figure 6) is a follows: Moving along the Vi„ axis from left to right as the line voltage increases from zero the linear relationship to the transformer current is present just as in the prior art example, the PTC resistor is in the low R value operating range. Then in this example when the Vi _n voltage reaches about 100 V, the PTC resistor transitions to the high R operating range shown by the drop in the Itrsf value and a more gradual rise in the Itrsf value to its maximum. The benefit of the new art voltage measurement technique is that the dynamic range of the rsf value is much lower than in the prior art technique. If the voltage value on the secondary side of the transformer is converted to a digital value by an ADC (Analog to Digital Converter) in the micro-processor a less precise ADC can be used. The new art technique is beneficial when it is only necessary to know when the output voltage (representing the line voltage) is above a certain threshold.

Hall Effect technique.

Magnetic isolation (through the use of Hall Effect devices and PTC resistors) technique.

[0017] The magnetic isolation technique (that utilizes a Hall Effect device and PTC resistors) provides isolation between the line voltage to the voltage detection circuitry is shown in the Figure 7. As shown, in the prior art (Figure 7a), a resistor is used in series with a Hall Effect device which provides the isolation to the low voltage detection circuitry. The Ri series resistor (shown in Figure 7) limits the current to within the Hall Effect device's specification. The current produces a magnetic field that the Hall Effect device converts to an output voltage. The output voltage must be such that for the maximum line voltage, the maximum output or sensed voltage is in the non- hazardous or safe operating range and in a predetermined operating range suitable to make an accurate voltage measurement. The relationship from the input current through the Hall Effect device to the output voltage is linear, as seen in the prior art voltage plot (Figure 7a). The example shows the dynamic range required by the measurement when the line voltage range is very large and the maximum is much higher than the measured voltage to support the measurement accuracy. The large dynamic range and the measurement accuracy requires the ADC (Analog to Digital Converter) used by the micro-processor to have more bits per operating range.

[0018] The new art technique for magnetic isolation uses a PTC (Positive Temperature Coefficient) resistor in parallel with a resistor R ₂ both in series with the resistor, Ri and the Hall Effect device which all are directly across the line voltage (see Figure 7b). The PTC resistor's characteristics are such that it operates as if it has two resistor values, for simplicity the Figure shows a low R and a high R by the red line in the graph (Figure 7b). When the line voltage is low, the PTC resistor behaves as a small value resistor then as the line voltage is increased and the power increases in the device it passes a threshold region and the resistance of the device also increases, limiting the output power. The resulting output current through the PTC being implemented across the line voltage is shown in the graph (Figure 7b) by the blue line.

[0019] Describing the new art technique's operation by the blue line in the chart (Figure 7b) is as follows: Moving along the Vi„ axis from left to right as the line voltage increases from zero, the linear relationship to the Hall Effect Device current is present and the PTC resistor is in the low R value operating range. Then in this example when the Vi„ voltage reaches about 100 V, the PTC resistor transitions to the high R operating range shown by the drop in the Inaii value and a more gradual rise in the ½/ value to its maximum. The benefit of the new art voltage measurement technique is that the dynamic range of the Inaii value is much lower than in the prior art technique. If the voltage value from the Hall Effect device is converted to a digital value by an ADC in the micro-processor a less precision ADC can be used. The new art technique is beneficial when it is only needed to know when the output voltage (representing the line voltage) is above a certain threshold.

Electric Isolation

[0020] Another technique to isolate the line voltage from the voltage detection circuitry is to employ electric field isolation. In this technique, the primary line voltage generates an electric field signal that can be coupled to an electric field receiver within the voltage detection circuitry. Figures 8 through 11 show various ways to implement this using electric field isolation techniques. Frequency modulation (FM) technique.

[0021] This method can be accomplished by the use of a varactor diode (essentially a reverse biased diode), who's capacitance varies with applied voltage. Figure 8 describes a direct frequency modulator utilizing a varactor diode.

[0022] This circuit deviates the crystal oscillator controlled frequency by varying the capacitance (controlled by the input voltage) across the varactor diodes. The voltage across the varactor diodes follows the input AC or DC line voltage, but limited by the zenor protection diode. The function of the resistors are to limit power to and to protect the detection circuitry. The voltage across the varactor diode influences the frequency of the oscillations. As the input voltage increases, the reverse bias increases resulting in a decrease of the diode capacitance and thus increasing the oscillation frequency. The varactor diodes are placed in back-to-back mode such that the circuit behavior is symmetric whether the input voltage is positive or negative. The use of a crystal oscillator means that the output waveform is very stable, but this is only the case if the frequency deviations are kept very small.

Phase-shift detection technique.

[0023] Phase-shift technique is a method in which the phase of a RF (radio frequency) signal can be varied to convey the voltage level to which the circuit is being exposed. There are several methods that can be used to accomplish this approach to detect the presence of voltage. Figure 9 illustrates an example for the utilizing this Modulation method to detect the line voltage. The Resistor-Capacitor Network whose output voltage "leads" or "lags" the input voltage by some angle less than 90 degrees. Change capacitive value of the varactors result in change in Phase shift of the RF signal. A differential amplifiers output voltage will reflect the amount of phase change relative to a reference phase signal. Pulse width modulation (PWM) using discharge time constant technique.

[0024] Figure 10 illustrates the PWM with discharging time constant technique. The RC time constant, T, is the time constant of an RC circuit and is equal to the product of the circuit resistance and the circuit capacitance:

T= R * C

It is the time takes to charge the capacitor across the resistor by 63.2 percent of driver voltage value. The voltage that charges the capacitor versus time is given by formula:

V (t) = Vo (l-e ^{_t/ T} ) (Vo is the initial voltage on the capacitor)

[0025] As shown in Figure 10, by using a fixed resistor and fixed voltage threshold one can sense the capacitance value of the Varactors diodes. The Varactors diode's capacitance value is a function of the applied voltage (DC or low frequency signal such as 60/50 Hz). Using a comparator circuit, the change in rise time can be transformed to a PWM (pulse wide modulation) signal by comparison of the modified signal to the initial input signal and the disparity can be translated back to an analog signal which will indicate the applied low frequency voltage on the Varactors. Figure 11 illustrates the effect of the varactor's capacitance which is a function of the applied voltage (V(t)for C _va ractor-2 and V(t) for C _V aractor-i)th&t changes the charging time on PWM signal's duty cycle as shown in the figure.

Validation Methods

[0026] When the isolation and detection techniques described within this document, or other similar measurement techniques, are applied to safety applications, such as for the purpose of verifying the absence of voltage, it becomes critical to validate the accuracy of the result so that any subsequent indication can be provided in a failsafe manner. This validation step can be accomplished using various methods including the use of redundant circuitry, test methods, or some combination of these methods.

Validation via Redundancy

[0027] One way to increase confidence in the output of the detection circuitry used to determine a safe state is to employ redundancy in detection circuitry that performs these critical functions (Figure 12). In this method, at least two groups of circuitry are required and confidence in the resulting output is increased because the likelihood of independent circuits failing in the same manner at the same time is relatively remote.

[0028] To validate the result using this method, each group of detection circuitry functions independently and the results are then compared. If each group of circuitry produces the same result, it is likely that the result is valid. If there is a discrepancy between results, the result cannot be determined and it can be reported that the monitored input is in an unknown state. Similarly, if there is a failure of one method or more of the redundant circuits to produce a result, any reported results would be considered invalidated and an unknown or failsafe state would be reported.

[0029] This method can be used with two or more independent sets of detection circuitry and works with any of the detection techniques described within this document. The additional, redundant circuitry may be identical to the original detection circuitry or a combination of other techniques. For example, this method could be demonstrated by using two FM modulation circuits; one FM modulation circuit and one AM modulation circuit; one FM modulation circuit and one Hall effect sensor; three FM modulation circuits, etc.

Validation via Test Methods

[0030] Another alternative to achieve confidence in the output of the detection circuitry used to determine a safe state is to create and inject a known reference signal onto the line and confirm that the detection circuitry accurately interprets the result. This method could be thought of as a self-test or "test-the-tester" type function and is illustrated in Figures 13. This method, to test the detection circuitry, can be employed before, after, or both before and after the detection circuitry analyzes the input from the source being monitored to establish confidence that the detection circuitry is performing as intended. In the case of the voltage detector, where indicating the absence of a signal must be a guaranteed safe state, it would make sense to test both before and after to ensure that the detection circuitry was not damaged while performing it' s primary function. However, in less critical applications, it may be sufficient to confirm the detection circuitry is functioning after or before performing the primary function.

[0031] A variety of techniques can be used to create the known reference including the electrical, optical, thermal, and magnetic techniques described within this document for detection purposes. The major difference is that when the technique is implemented for the purpose of a validation or test function, it must be generated from a separately derived source that is independent of the source being monitored. This is critical to ensure that the test and subsequent validation can be performed when there is absence of signal on the line being monitored.

[0032] The techniques used for detection and test can be mixed and matched. For example, if a thermal detection technique is used, the complimentary test circuitry could be based on the same thermal technique, an alternate thermal technique, or a non-thermal technique. Certain combinations may be more desirable than others from a cost standpoint or performance sensitivity. The optimum combination will be determined by environmental and application constraints.

Hybrid Approach to Validation

[0033] Finally, the validation methods described herein can be combined into a hybrid method using various combinations of redundancy and test techniques. A generic description of this approach is depicted in Figure 14. When this approach is used, the results from redundant circuitry must match in addition to receiving the expected result from the known reference, in order for the result to be validated. This logic applies regardless of the individual techniques implemented.