Touch Current Basics prepared for CTL PTP Workshop May 2010 Ronald Vaickauski Senior Staff Engineer Underwriters Laboratories Inc.
Human Response-Electric Shock • Charles F. Dalziel, University of California Berkeley • W. E Hart, Fluke Corp., IEC 66E Committee
Perception/Reaction Findings IEC TR 60479-5:2007 Table 1
Let-Go Findings IEC TR 60479-5:2007 Table 1 Values for current are in milli-amperes.
Ventricular Fibrillation IEC TR 60479-5:2007 Table 1 Current is in milli-amperes.
Elementary Touch Current Network IEC 60990:1999 Figure 3
Threshold of Perception Figure 1 of IEC TS 60479-2:2007
Perception/Reaction Network IEC 60990:1999 Figure 4
Let-Go Network IEC 60990:1999 Figure 5
Calculated Impedance – Perception/Reaction Network IEC 60990:1999 Table L.2
Calibration of Network IEC 60990:1999 Table L.5
Frequency Range of Design and Calibration • Frequency of Touch Current • 50 and 60 Hertz Sinusoidal • Electronic circuitry • Non-Sinusoidal wave forms • Fourier expansion • Higher frequency components
References • IEC 60990:1999, Methods of measurement of touch current and protective conductor current • IEC/TR 60479-5:2007, Touch voltage threshold values for physiological effects • IEC/TS 60479-1:2005, Effects of current on human beings and livestock- Part 1: General aspects • Measuring Touch Current – Resolving the Controversy About Peak versus RMS; Hart, W.F.,Perkins, P.E., Skuggevig, W., 1997.
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Human Response-Electric Shock: Charles F. Dalziel, a professor of electrical cal engineering at the University of California Berkley, did research on human an response to electric shock and wrote a book, “The Effect of Electric Shock on Ma an.” Much of the requirements in place for touch current are based on his work. Another major contributor was Walter E. H E. Hart, Fluke Corporation who was a member of the IEC 66E committee and IEC IEC TC 74 Working Group 5. 2
Perception/Reaction Findings: Perception/reaction current flowing through rough the body is just enough to cause involuntary muscular contraction to the he person through which it is flowing 3
Let-Go Findings: Let-Go current flowing through the body body is just enough to cause involuntary contraction of a muscle, such as inabilit ility to let go from an a.c. electrode. 4
Ventricular Fibrillation: Ventricular fibrillation current flowing thr through the body is just enough to cause ventricular fibrillation of the heart. heart. Ventricular fibrillation is where the human heart is in danger of failure to funct ction. 5
Elementary Touch Current Network: f the 500 Ω resistor in series with the The three component combination of the parallel combination of the 1500 Ω resis sistor and 0.22 µF capacitor comprises the body impedance model. It represen ents hand-to-hand or hand-to-feet ching a conductive part. The 1500 Ω electrical impedance of a person touchi resistor in parallel with the 0.22 µF capa apacitor represents the combined impedance of the entry and exit skin con contacts (wet skin, not immersed) with a conductive surface of equipment and and ground, or with two equipment contacts. The 500 Ω resistor represents ts the internal body resistance, less the skin, and serves as the current-sensing ing resistor in the measuring circuit since all of the current that flows through ough the body impedance model flows through this resistor. 6
Threshold of Perception: The human bodies response to touch curren rrent varies with frequency. The threshold of e graph shown is figure 1 of IEC TS perception increases with frequency. The g 60479-2:2007. 7
Perception/Reaction Network: The touch current network for perception/reaction i n is a frequency weighted network. This network is the one most often referenced I product safety testin sting standards. The frequency-weighting is done and capacitor connected across the 500 Ω by a voltage-divider consisting of the resistor and current-sensing resistor. The frequency-weight ghting network is essentially a low-pass filter the 500 Ω resistor according to the frequency designed to attenuate the voltage signal from the of the body current. The voltage transfer charac racteristics of the network is designed from Dalziel's data describing human responses for or perception or startle reaction. The frequency weighting network causes the in instrument to have an indication that is related to the expected level of physiological response, se, independent of frequency. A touch current measuring instrument is not a substitute for an an ammeter. The touch current measuring instrument's response is not necessarily equal equal to the number of milli-amperes flowing through the body impedance model. The readi eading of a touch current measuring instrument is adjusted by the frequency-weighting network fo for variations in human response due to frequency. Therefore, the reading can be compa pared to the limit value in the requirements without knowing the frequency, and allowing a s a single numerical value for the touch current limit for the physiological effect to be addressed. ed. 500 Ω resistor relates to electrical burns. Actual RMS current measured through the 500 Weighted current indicated by the voltage after ter the reaction is related to the body muscular response to the touch current independent of fr f frequency. Weighted current is useful to evaluate current comprised of combinations of of frequencies, including non-sinusoidal wave shapes. The weighted current is referenced to t to the 50/60-Hz current that produces a certain physiological effect. 8
Let-Go Network: s also a frequency weighted network. The The touch current network for “let-go” is a voltage transfer characteristics of the ne network is designed to emulate human responses for the “let-go” response. The frequency weighting network cause auses the instrument to have an indication that is related to the expected ted level of physiological response, independent of frequency. The reading ng of a touch current measuring instrument is adjusted by the frequency cy-weighting network for variations in human response due to frequency. The herefore, the reading can be compared to the limit value in the requirements wi without knowing the frequency, and allowing a single numerical value for the the touch current limit. 9
Calculated Impedance Perception/Reacti ction Network: The performance of the Perception/Rea eaction Network is checked by passing variable frequency sinusoidal current th t through the input of the instrument, test terminals A and B. The input current ( I) ( I), input voltage ( U) and output voltage ( U2) are measured at various f s frequencies. using the same voltmeter. Measured ratios of input voltage to inpu nput current (input impedance) and output voltage to input current (transfer fer impedance or network response) are compared with ideal values calculated f ed from the nominal component values specific. In building the instrument, care are must be taken in the arrangement of the circuitry so that inter-component capa capacitance, lead inductance and characteristics of the voltage measuring ing instrument do not significantly affect the voltage-current ratios. A guard band indicating the uncertainty nty of measurement at various frequencies can be specified for the ins instrument. 10
Calibration: Each instrument that is used to determi mine acceptability for the purpose of certification shall be routinely calibrated ted in a confirmation system to ensure that no drift of its performance outside t de the limits of permissible error has occurred. Calibration in a confirmation system is c is carried out in two steps. Measurement of input resistance The d.c. input resistance is measured and ed and its value is checked against the ideal value, 2 000 ohms. Measurement of instrument performa rmance The input voltage and the output voltage age (or milli-amperes as indicated on the meter) are measured at various frequen requencies and the ratios compared to the data in tables as appropriate. The inpu he input voltages used should be such as to produce output indications in the range ange of the TOUCH CURRENT values for which the measuring instrument is in s intended. 11
Frequency Range of Calibration: In the distant past, touch current was ty typically sinusoidal 50 or 60-Hz current driven by line voltage through linear impedan impedance. With the introduction of power electronics, switching power supp upplies and other electronic circuitry, the touch current available from even the the most ordinary products has become non-sinusoidal. Fourier expansion of non-sinusoidal tou touch current wave forms with fundamental frequencies of 50 and 60 H 60 Hz, results in many significant higher order frequency components that must st be measured. Therefore, it is important to ensure that touch current m t measuring instruments can accurately measure current not only the fundamen ental power line frequency but also at the higher frequency components conta ontained in the non-sinusoidal wave form. Depending upon the electronic circuitry try in the product under test, it is also possible to generate lower frequencies es than the mains power frequency. Therefore, capability to measure lower er frequency touch currents is also important. 12
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