Page 236 - Electrical Safety of Low Voltage Systems

Safety Against Static Electricity and Residual V oltages 219

Needless to say that for static electricity purposes we must only
earth metal objects isolated from ground, or whose resistance-to-
ground is greater than 1 M . We have previously substantiated, in
fact, that by connecting to the main grounding system metal objects
other than these ones, we make them prone to transferred potentials
generated in faulty equipment.

13.5 Residual Voltages
Residual voltages are potentials caused by static charge accumulated
in capacitors within equipment during its normal operation. Residual
voltages may persist after the supply has been turned off, even for
hours, and may expose maintenance personnel to the risk of electro-
cution.
During normal operations of electrical systems, voltages across
capacitors are sinusoidal, and after disconnection of supply they re-
main charged at the value V 0 the sinusoid had at the instant of the
interruption.
Upon direct contact with one, or both terminals, of the capacitor,
the discharge process will initiate and its potential will decay with
exponentiallaw.Suchpotentialwillcausethecirculationofanimpulse
current through the person’s body of duration in the order of a few
milliseconds. If we assume constant the person’s body resistance R B ,
such current, as a function of time, can be expressed by using the
Ohm’s law:

V 0 − t t
i(t) = × e R B C = I peak e − (13.9)
R B
where indicates the time constant of the discharge process.
The r.m.s. value of the discharge current can be calculated as:

I peak V 0
I rms = √ = √ (13.10)
6 R B 6

We can assume that the above impulse current will have practically
transferred almost the whole static energy accumulated in the capac-
itor in a period of time equal to 3 . At this time, the current will be at
5% of its initial value (Fig. 13.4).
The energy E R released during the discharge and dissipated in
the person’s body is the quantity that determines the probability of
ventricular fibrillation.
E R can be so expressed as:

3
∞
2 ∼ 2 ∼ 2
E R = R B i (t)dt = R B i (t)dt = R B I rms 3 (13.11)
0 0