Simpson 228 - Human Response to Electrical Shock

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In properly designed and installed equipment, leakage current can usually be ignored
because it is limited to safe levels. Excessive leakage current can appear in equipment as
a result of:
• A defective component
• Poor equipment design or installation
• A build up of foreign matter
• Old or deteriorated insulation
The presence of leakage current on any acces-
sible conductive part poses a potential shock
hazard to anyone touching that part (see Figure
1). Severe electrical shocks from leakage cur-
rent can cause burns, serious injury and in some
cases, death. Even when leakage currents are
not high enough to directly injure a person, the
current can be high enough to cause a violent
muscle contraction resulting in accident or
injury.
Leakage current measurement ensures the safety of electrical equipment — not
only for the user but for anyone who comes in contact with the equipment. Leak-
age current measurements should be performed whenever equipment is installed
or repaired.
1.3 Human Response to Electrical Shock
The human body’s reaction to electrical shock depends upon the magnitude of
the current involved and many other variables, including sex, weight, age and
point of contact. Reaction to a shock can range from a harmless tingling sensa-
tion to a more serious response such as violent muscle contractions, massive
burns or death due to heart failure.
Although most electrical or electronic equipment is energized by 60Hz (50Hz)
power circuits, some have solid state control circuits or power conversion circuits
which generate higher frequency currents. This high-frequency current may be a
significant part of the total leakage current. The human body becomes less sensi-
tive to leakage current as the frequency is increased. The 228 takes the frequency
of the leakage current into account when making measurements and displays a
reading that correctly reflects the potential hazard from the leakage current. Be-
cause of this frequency correction, leakage currents are measured in Measure-
ment Indications Units (MIU).
The IEC and other safety agencies have defined four (two are combined) levels
of human body response to electrical shock:
• Perception/Reaction (tingling sensation and involuntary muscle contraction)
• Let-go (the loss of the ability to control muscles and release)
• Electric burns
The 228 includes a separate measurement “network” for each of these response
levels.
Electrical shock due to
excess leakage current
(touch current)
Figure 1.