Hello Jan:
MIU = Measurement Indication Unit
This term is an invention (fabrication) of Arnold Smoot,
retired, Underwriters Laboratories research group, Melville,
Long Island, New York.
The MIU is the reading (indication) one gets from the
measuring device specified in the ANSI C101.1 Leakage
Current Standard. I believe MIUI is defined in this
standard.
Smoot reasoned that the circuitry for measuring leakage
current changed the character of the milliameter (or
voltmeter) in the circuit such that the reading was not a
"true" measurement of current. Therefore, the leakage current
measurement was not a measurement of current, but simply a
reading of a particular leg of a circuit. The purpose of
the measurement scheme is to "indicate" the EFFECT of current
passing through the body.
Hence, MIU.
There are two problems in defining "leakage current."
The term "leakage current" was popularized by Karl Geiges,
UL, in his monograph "Measurement of Electric Shock Hazard
in Radio Equipment," Bulletin of Research, Underwriters
Laboratories Inc., July, 1945. Geiges implies that "leakage
current" is a property of the equipment under test.
Geiges was also the inventor of the first standard leakage
current measurement scheme. It, too, is described in his
monograph. Geiges measurement scheme was to "standardize"
the impedance of the current meter because "attempts to
correlate data on leakage had often proved difficult because
resistance values were not connected with the current values
and because widely varying meter resistances were encountered."
Geiges chose 1500 ohms as the impedance of the meter.
"If a resistance comparable with that of the human body under
the most severe conditions normally expected were used, the
values would be directly comparable with endurance test results
and similar measurements on persons."
So, we have two definitions of "leakage current." The first
is a characteristic of the equipment (the source of the leakage
current). The second is a specific measurement.
The irony is that, today, the impedance of current meters is
very low compared to the impedance of the leakage current
source. The "error" due to a 1 ohm or less impedance versus
a 1500 ohm impedance is less than 2% when measuring leakage
currents up to 3.5 mA. This is because the source impedance
must be greater than 68.5 kohms at 240 V for 3.5 mA leakage.
(A 0-ohm current meter would indicate 3.5 mA. A 1500-ohm
current meter would indicate 3.43 mA. The error is 2%
pessimistic!)
Best regards,
Rich
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Richard Nute Quality Department
Hewlett-Packard Company Product Regulations Group
San Diego Division (SDD) Tel : 619 655 3329
16399 West Bernardo Drive FAX : 619 655 4979
San Diego, California 92127 e-mail: [email protected]
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