Hi Bob:
Electric shock requires both voltage AND current. Most
safety standards specify conditions for electric shock
when the voltage is 30 volts rms or more AND the current
is either 0.25, 0.5, or 3.5 mA rms or more.
Think of it as an "AND" gate. Voltage exceeding 30 V rms
AND current exceeding 3.5 mA rms equals conditions for
electric shock.
Often, this is not clearly stated in the standard. This
is because the typical supply exceeding 30 V rms also
exceeds 3.5 mA rms.
Conversely, electric shock is not possible if ONE of the
ingredients, voltage or current, is less than the specified
limit values.
The most common examples are ELV and SELV. If the voltage
does not exceed 30 V rms, then the voltage is safe to
touch regardless of the current. (I am not discussing
here the special case of SELV, but simply that low voltage
in itself is a safeguard against electric shock.)
You cite the uncommon example of "limited current." If
the current does not exceed 0.5 mA rms (in your case, 23
mA rms at 33 kHz), then the current is safe to touch
regardless of the voltage. (Try it! You will feel
nothing! Use one hand only between ground and the source.)
(Unfortunately, 1100 V rms at 33 kHz may arc to your skin
and burn a small hole in your skin if you touch a point;
touch a large flat surface.)
Actually, every mains-connected product in the world uses
this concept of protection. It is the phenomenon of
leakage current. The current is safe to touch even though
the voltage exceeds the limits of ELV. (Half the mains
voltage appears on accessible conductive parts of ungrounded
products.)
It is difficult to believe, but a limited current circuit
such as you describe needs no safety insulation whatsoever.
It is equivalent to an SELV circuit. It can be touched
just as an SELV circuit can be touched.
Of course, it must remain limited current under single-
fault conditions. This would be a single fault in the
insulation from the 5 V dc circuit to the limited current
circuit. This should change it from limited current
circuit to an ELV (or SELV) circuit. Or, it may not do
anything, depending on the circuit configuration.
Another fault is to maximize the voltage to the oscillator,
if this is a possible 5-volt power supply fault (e.g.,
short the 5-volt regulator). Increased oscillator voltage
may increase the available current to more than the
allowed 23 kHz (but beware the frequency may change, which
will change the allowable current).
I understand this sort of circuit is commonly used for
the backlight of laptop computers. This is why there is
no need for an extensive insulation system for laptops.
We use a variation of this circuit for the lamp power in
many of our scanner products. We rely on limited current
for protection of the customer from electric shock.
Best regards,
Rich
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Richard Nute Quality Department
Hewlett-Packard Company Product Regulations Group
San Diego Division (SDD) Tel : 619 655 3329
16399 West Bernardo Drive FAX : 619 655 4979
San Diego, California 92127 e-mail: [email protected]
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"The future has a way of arriving unannounced."
-- George F. Will
