Page 38 - FL-20A Instruction Manual
Model FL-20A Cable Fault Locator
kHz and higher, it can be estimated that the threshold of
startle reaction is approximately equal to 1 mA per kHz of
frequency. For example, if a specific level of reaction from
current at 1 kHz occurs at 1 mA, then a similar level of reac-
tion would occur from 10 mA at 10 kHz. The same level
of reaction would occur from 100 mA at 100 kHz, and so
on. Leakage current measuring instruments, such as those
specified in ANSI C101-1992, take into account the effect
of high frequencies on the body. These instruments pro-
duce readings that are “frequency-weighted,” and indicate
the level of possible physiological effect. The readings corre-
spond to the current magnitude in mA only at low frequen-
cies such as 60 Hz.
Electric current over 5 mA at 60 Hz can cause muscle
tetanization. Tetanization is defined as the state of continu-
ous contraction of a muscle undergoing a series of rapidly
repeated stimuli. A person with tetanized muscles may be
unable to let go of a conductive part, may be immobilized
(frozen), or may be unable to breathe while the current
flows. Tetanization lasts as long as the current flows. When
the current stops, the effect stops, and the muscle returns to
normal function. However, the effect can be fatal if breath-
ing stops long enough. If immersed in water, an immo-
bilized person could drown. In a manner comparable to
perception, tetanization occurs at a higher current threshold
for DC and for higher frequencies.
Ventricular fibrillation is a disorder involving disorga-
nized arrhythmic motion of the heart that affects blood cir-
culation. Unlike muscle tetanization, ventricular fibrillation
can be triggered by a short-duration burst of current of suffi-
cient magnitude. Ventricular fibrillation is not spontaneously
reversible in humans and, if not treated quickly with special
defibrillating equipment, will continue until the person dies
(within a few minutes) from loss of circulation of the blood.
The magnitude of limb-to-limb current sufficient to cause
ventricular fibrillation is greater than that which would cause
muscle tetanization. Therefore, limits for continuous current
(e.g., lasting over five seconds or so) are usually based on
muscle tetanization considerations.
Perception and
Startle Reaction Cont.
Muscle Tentanization
Ventricular Fibrillation
