Hz current of 50 mA passing through the heart is usually
considered sufficient to cause fibrillation, while about 500 mA
of direct current is required to generate the same effect. The
physiological effects of 50/60 Hz current are shown in Fig. 1.
Fig. 1 Effect of 50/60 Hz current flowing through the body
It’s important to note that low voltage does not mean low
shock hazard; dangerous or fatal electrical injury can occur
even at fairly low voltage. Generally, the severity of an electric
shock depends mainly on:
1. Current path through the body
2. Current magnitude
3. Time duration of current flow through the body
Fig. 2 shows the main current paths through the body:
1. Hand-to-hand passing through the heart
2. Foot-to-foot not passing through the heart
3. Hand-to-foot passing through the heart
Fig. 2 Main current paths through the human body
According to IEC 60479-1 [6], the effects of electric current
on humans can be divided into four zones according to
duration and current level, as shown in Fig. 3. Currents in
region AC-1 (up to Line A at 0.5 mA) are harmless and usually
not even felt. Currents in region AC-2 (up to Line B) can be
felt but still may not do any harm, depending on how long they
last. Currents in region AC-3 can cause involuntary muscular
movements (including self-injury from striking objects, or the
inability to let go) and, if they continue for more than two
seconds, breathing difficulty. Currents in region AC-4 can
cause heart fibrillation, stopped breathing, and severe burns.
The longer the current lasts the greater the injury.
Fig. 3 Effect of AC current and time on human beings
III. INDUSTRIAL GFCIS
Residential Class A GFCIs are defined by UL 943 which is
a harmonized North American standard; therefore the same
requirements apply in Canada and Mexico. As mentioned
previously, Class A GFCIs are not suitable for most industrial
applications. UL 943C defines new GFCI classes more
appropriate for such industrial applications; Class C, D, and E.
Class C is for use in circuits with a maximum line-to-ground
voltage of 300 V (i.e. on system voltages up to 480 V) where
reliable equipment grounding or double insulation is provided;
Class D is for use in circuits with a line-to-ground voltage
higher than 300 V (i.e. on system voltages up to 600 V) with
oversized grounding to prevent the voltage across the body
during a fault from exceeding 150 volts; and Class E covers
systems similar to Class D, but with high-speed tripping
required, therefore the oversized ground of Class D is not
required. Fig. 4 shows how the class of a device for a
particular application is determined.
Fig. 4 UL-listed GFCI Classes
Classes C, D, and E use the same inverse-time
characteristics defined by UL 943 standard for Class A GFCIs
and defined by (1) up to 300 mA of ground-fault current: