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10. RMS VS. PEAK
Throughout this manual there have been frequent notices about using the meter
to measure sinusoidal currents only. These warnings are necessary because of
the nature of leakage current and its effect on the body.
Almost all studies investigating electrical shock hazard have shown that the se-
verity of an electric shock is more closely related to peak current than the average
value or RMS value. As a result, the authors of IEC 990 decided that a peak
reading Instrument was the best Instrument for measuring shock hazard. Measur-
ing peak current can be a difficult task in a noisy environment or when the unit
under test is producing high frequency signals. The ANSI C101.1 committee felt
that this difficulty was a significant problem and specified an RMS reading Instru-
ment for measuring leakage current.
The Model 228 attempts to resolve the differences in specifications by providing
a direct reading RMS meter and by providing a buffered output for measuring
peak current. The Instrument provides a peak reading scale on the dial as a
convenience for users who have verified that a sinusoidal current is present.
The output on the Model 228 provides the user with the ability to measure the
waveform that comes out of the measurement network.
When measuring non-sinusoidal currents, take precautions to insure that the
peak levels of current do not overload the internal amplifiers in the meter. The
simplest method of doing this is to connect a peak reading instrument to the
Model 228 ouput. After identifying the appropriate range, select the next highest
range and compare the reading. If the readings match (after adjusting for attenu-
ator scale factor), then the amplifiers are not in overload, and the more sensitive
range may be used. If the readings do not match, the Model 228 is overloaded,
and a higher range must be used.
When high crest factors are present, the difference between the RMS and the
peak readings may be substantial. In those cases, it may be advisable to base
safety on the peak reading even though ANSI C101.1 calls for RMS.
When using the Model 228, it is important to use the correct measurement limits.
Contact the appropriate safety agency or the equipment manufacturer to deter-
mine whether RMS or peak measurements are required.
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