Clinton FL-20A - Page 47

FL-20A Instruction Manual - Page 39
Model FL-20A Cable Fault Locator
A general limit that has been used by UL for a number
of product categories including ground-fault circuit-inter-
rupters is described as I = 20 T -0.7 for bursts of 60-Hz
current down to 20.9 milliseconds. I is in RMS mA cal-
culated over the duration of the current; T is the current
duration in seconds. For durations between four and 20.9
milliseconds, the current is limited to 300 mA. Below four
milliseconds, the current is limited by I = 6.3 T-0.7 . ese
equations represent curves drawn under threshold bril-
lating data points from laboratory experimental work con-
ducted with animal subjects.
For durations shorter than a tenth of a second, the
limits for AC and DC current are the same. For current
lasting only a few milliseconds, a narrow piece of a 60-Hz
sinusoid is not substantially dierent from a rectangular DC
pulse. For durations over a tenth of a second, direct current
has higher limits. Animal test data indicates that for long
duration exposures to combinations of AC and DC, the
parameter of current that is most related to the threshold of
ventricular brillation is the peak-to-peak value of the cur-
rent, if the DC component is low enough so that there is
reversal of the current each cycle. In fact, as long as the cur-
rent reverses, the presence of a DC component is not sig-
nicant with regard to the ventricular brillation threshold.
If the DC component is high enough to preclude reversal
of the current of each cycle of the AC component, then the
occurrence of ventricular brillation is more related to the
peak value of the composite waveform. In no case should
the peak of the composite continuous waveform of AC and
DC exceed the peak-to-peak value of the AC component at
its maximum permitted value. For example, at one second
duration or longer, if the ventricular brillation limit for an
AC sinusoidal current is 20 mA RMS, the corresponding
limit for a direct current would be 40, which is 56.6 mA. If
the duration is between 0.1 and 1.0 second, the equation I
= 56.6 T-0.25 describes a suitable limit for DC current.
Prevention of electrical burns is a very complex subject.
ere are many variables that are dicult to control or esti-
mate. A limit of 70 mA RMS, independent of frequency,
has been used in a number of standards to address burns.
At this current level, it is not likely that a severe burn injury
would occur that would involve an appreciable volume of
Ventricular Fibrillation Cont.
Burns