Hi Tom:
> So, for voltage up to 450V d.c. (i.e. up to 318V a.c.), capacitor
> up to 0.1uF will become a Limited Current Circuit, hence the voltage
> is not Hazardous Voltage (1.2.8.4) - no additional condition would
> be required for the capacitor connected to the primary circuit.
Electric shock (or electrically-stimulated
sensation) is a function of BOTH voltage
and current. For an electric shock to
occur, the source must exceed, say, 30 V
rms and 0.5 mA rms.
As a general rule, we say that any voltage
not exceeding 30 V rms is not hazardous,
regardless of current. We identify this
voltage as ELV or SELV.
Likewise, we say that any current not
exceeding 0.5 mA rms is not hazardous,
regardless of voltage. We identify this
current as "Limited Current."
Furthermore, "Limited Current" addresses
capacitors charged to voltages exceeding
42.4 V dc. In this case, we control the
capacitance, or the charge, or the stored
energy, and deem the transient discharge
current equivalent to a steady state
current.
In the case of an across-the-line
capacitor installed on the supply side of
the power switch, the capacitor can be
charged to the peak of the line voltage
if the plug is disconnected at the peak
of the line voltage.
As has already been noted in this forum,
the discharge is not a pleasant sensation,
and may very well result in an involuntary
reaction, depending on the individual.
By comparison, the sensation from 3.5 mA
steady-state leakage current (allowed by
some standards for Class I equipment) is
also not a pleasant sensation and may
result in an involuntary reaction.
The initial discharge current of a
charged capacitor is limited only by the
body resistance. Regardless of capacitance
value, the initial current is the same.
The effect of capacitance value is the
duration of the current. If the current
has a short duration, the body will not
sense it. Assuming all bodies have the
same resistances, then the duration of the
current is a function of the value of the
capacitance.
The standards happen to draw the line at
0.1 uF. Any value less than 0.1 uF is
deemed acceptable. Any value greater than
0.1 uF is deemed unacceptable.
Consequently, for capacitors less than 0.1
uF, the discharge time is short and the
body is less likely to sense the current.
If the capacitor is more than 0.1 uF, the
discharge time is long, and body is likely
to sense the current.
Another factor, of course, is the magnitude
of the voltage to which the capacitor is
charged. If the capacitor is charged to
the peak of the 230 V mains, the discharge
is almost always sensed. The same
capacitor charged to the peak of a 120 V
mains is barely detectable. The lower the
voltage, the lower the initial current.
The fact that the initial discharge
current is limited only by body resistance
also applies to leakage current through
Y capacitors. At the moment the body is
inserted into the circuit, the initial
current is limited only by the body
resistance. Thereafter, the current is
the steady-state current due to the
capacitive reactance. However, the Y
capacitors are much smaller in value,
and therefore the discharge is much
shorter in time. (You can test this by
putting a switch in series with the leakage
current; if the switch closes at the peak
of the line voltage, you will feel an
initial sharp sensation.)
By the way, the sensation of leakage
current from a Y capacitor is greater
than the sensation of leakage current
from a resistor whose value is equal to
the value of capacitive reactance. This
is due to the same phenomenon, namely the
initial discharge of the capacitance.
Best regards,
Rich
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