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
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•
•
•
•
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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.
1
Human Response-Electric Shock:
Charles F. Dalziel, a professor of electrical engineering at the University of
California Berkley, did research on human response to electric shock and wrote a
book, “The Effect of Electric Shock on Man.” Much of the requirements in place
for touch current are based on his work.
Another major contributor was Walter E. Hart, Fluke Corporation who was a
member of the IEC 66E committee and IEC TC 74 Working Group 5.
2
Perception/Reaction Findings:
Perception/reaction current flowing through the body is just enough to cause
involuntary muscular contraction to the person through which it is flowing
3
Let-Go Findings:
Let-Go current flowing through the body is just enough to cause involuntary
contraction of a muscle, such as inability to let go
from an a.c. electrode.
4
Ventricular Fibrillation:
Ventricular fibrillation current flowing through the body is just enough to
cause ventricular fibrillation of the heart. Ventricular fibrillation is where the
human heart is in danger of failure to function.
5
Elementary Touch Current Network:
The three component combination of the 500 Ω resistor in series with the
parallel combination of the 1500Ω resistor and 0.22 µF capacitor comprises
the body impedance model. It represents hand-to-hand
hand
or hand-to-feet
electrical impedance of a person touching a conductive part. The 1500 Ω
resistor in parallel with the 0.22 µF
F capacitor represents the combined
impedance of the entry and exit skin contacts (wet skin, not immersed) with
a conductive surface of equipment and ground, or with two equipment
contacts. The 500Ω resistor represents the internal body resistance, less the
skin, and serves as the current-sensing
sensing resistor in the measuring circuit
since all of the current that flows through the body impedance model flows
through this resistor.
6
Threshold of Perception:
The human bodies response to touch current varies with frequency. The threshold of
perception increases with frequency. The graph shown is figure 1 of IEC TS
60479-2:2007.
7
Perception/Reaction Network:
The touch current network for perception/reaction is a frequency weighted network. This network is
the one most often referenced I product safety testing standards. The frequency-weighting is done
by a voltage-divider
divider consisting of the resistor and capacitor connected across the 500 Ω
current-sensing resistor. The frequency-weighting
weighting network is essentially a low-pass
low
filter
designed to attenuate the voltage signal from the 500 Ω resistor according to the frequency
of the body current. The voltage transfer characteristics of the network is designed from
Dalziel's data describing human responses for perception or startle reaction.
The frequency weighting network causes the instrument to have an indication that is related
to the expected level of physiological response, independent of frequency. A touch current
measuring instrument is not a substitute for an ammeter. The touch current measuring
instrument's response is not necessarily equal to the number of milli-amperes
milli
flowing
through the body impedance model. The reading of a touch current measuring instrument is
adjusted by the frequency-weighting
weighting network for variations in human response due to
frequency. Therefore, the reading can be compared to the limit value in the requirements
without knowing the frequency, and allowing a single numerical value for the touch current
limit for the physiological effect to be addressed.
Actual RMS current measured through the 500 Ω resistor relates to electrical burns.
Weighted current indicated by the voltage after the reaction is related to the body muscular
response to the touch current independent of frequency. Weighted current is useful to
evaluate current comprised of combinations of frequencies, including non-sinusoidal
non
wave
shapes. The weighted current is referenced to the 50/60-Hz
50/60
current that produces a certain
physiological effect.
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Let-Go Network:
The touch current network for “let-go”
go” is also a frequency weighted network. The
voltage transfer characteristics of the network is designed to emulate human
responses for the “let-go” response.
The frequency weighting network causes the instrument to have an
indication that is related to the expected level of physiological response,
independent of frequency. The reading of a touch current measuring
instrument is adjusted by the frequency-weighting
frequency
network for variations in
human response due to frequency. Therefore, the reading can be compared
to the limit value in the requirements without knowing the frequency, and
allowing a single numerical value for the touch current limit.
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Calculated Impedance Perception/Reaction Network:
The performance of the Perception/Reaction Network is checked by passing
variable frequency sinusoidal current through the input of the instrument, test
terminals A and B. The input current ( I), input voltage ( U) and output
voltage ( U2) are measured at various frequencies. using the same
voltmeter.
Measured ratios of input voltage to input current (input impedance) and
output voltage to input current (transfer impedance or network response) are
compared with ideal values calculated from the nominal component values
specific. In building the instrument, care must be taken in the arrangement of
the circuitry so that inter-component
component capacitance, lead inductance and
characteristics of the voltage measuring instrument do not significantly affect
the voltage-current ratios.
A guard band indicating the uncertainty of measurement at various
frequencies can be specified for the instrument.
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Calibration:
Each instrument that is used to determine acceptability for the purpose of
certification shall be routinely calibrated in a confirmation system to ensure
that no drift of its performance outside the limits of permissible error has
occurred.
Calibration in a confirmation system is carried out in two steps.
Measurement of input resistance
The d.c. input resistance is measured and its value is checked against the
ideal value, 2 000 ohms.
Measurement of instrument performance
The input voltage and the output voltage (or milli-amperes
milli
as indicated on
the meter) are measured at various frequencies and the ratios compared to
the data in tables as appropriate. The input voltages used should be such as
to produce output indications in the range of the TOUCH CURRENT values
for which the measuring instrument is intended.
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Frequency Range of Calibration:
In the distant past, touch current was typically sinusoidal 50 or 60-Hz
60
current
driven by line voltage through linear impedance. With the introduction of
power electronics, switching power supplies and other electronic circuitry,
the touch current available from even the most ordinary products has
become non-sinusoidal.
sinusoidal touch current wave forms with
Fourier expansion of non-sinusoidal
fundamental frequencies of 50 and 60 Hz, results in many significant higher
order frequency components that must be measured. Therefore, it is
important to ensure that touch current measuring instruments can accurately
measure current not only the fundamental power line frequency but also at
the higher frequency components contained in the non-sinusoidal
non
wave
form.
Depending upon the electronic circuitry in the product under test, it is also
possible to generate lower frequencies than the mains power frequency.
Therefore, capability to measure lower frequency touch currents is also
important.
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References:
Shown on the slide are three of the main references used to prepare this
presentation. Additionally, I visited a large number of Internet Sites too numerous
to tabulate here.
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