Electrical shocks ( Was "Silicone Spray into fan in power supply?")


<do_not_spam_me@xxxxxxxxxxx> a écrit dans le message de news:
7f806721-73af-4ef6-98a1-b3d4bb840e24@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Not really. You'd have to be an electrical engineer to truly
understand.

For low frequency current, the human body model is 1500 ohms in
parallel with 0.15uF, and a safe level of current is approximately 10
mA, maximum. At 12V, what's the current flow? What's the current at
120VAC?

I am.
And I can tell that this reply is relevant over all the amnient noise.
However I've to add that the impedance of the body varies with a lot of
parameters: the skin moisture, the voltage, the frequency, the pressure of
the contact, its surface, if there's penetration or not, ...
Some paths are more sensitive than another with regard to the lethality, i.e
generally above waist and especially all what can happen between the two
hands, crossing the heart.

In wet environments (swimming pool, bathroom for example), the secure
voltage (under 50/60 Hz) is 12Vac because of the low impedance coupling to
the shin when wet.
On conductive floors, (i.e. garages, gardens, ...), the voltage generally
used is 24 Vac. In these conditions the skin is considered as dry otherwise
this is the first case that applies. In both cases, even considering the
lowest related impedance of the skin, the lethal current won't be reached.
Anyway the circuits feeding these environments must be differentially(*)
protected, opening in case of electrical schock.

(*) differentially means that, on an insulated line, the sourcing current is
exactlly equal to the return current. Should a failure occur at the end of
the line (or after the differential protection), a part of the current is
derivated to the earth ground, inducing a desequilibrium between the
sourcing and the return currents. This difference is equal to the leakage
current, in our case flowing through the body. When the threshold is
reached, the differential circuit breaker opens the line.

Note as well that there are two kinds of electrical shock: Direct and
Indirect.
Direct is when you touch the live wire, your body being in contact with the
earth ground.
Indirect is when a failure is existing inside an equipment (washing machine
for example), when its connection to earth is not correct (rust, paint, ...)
and when you touch the metallic cabinet, your body, of course being in
contact with the earth.

Concerning the currents: USA people are paranoïd and protect their
installations with Hi-Sensitivity Differential Circuit Brakers. In Europe,
studies have been made proving that 20 mA may be considered as lethal, henst
a margin of two is applied and our Hi-Sensitivity protections are calibrated
at 10 mAmps.
With such a low value lines must be rather short otherwise intempestive
breakings are quite common. With 5 mAmps, for the same reasons, the
protection is normally mobile and plugged into the wall socket. Things may
have evolved since I was no longer involved in this stuff for 18 years.

In a PSU, there's two "class Y" capacitors for EMC protection purpose and
often two "class Y" surge protectors. On their cold side, they're connected
to the PC cabinet. On their opposite side, they are connected to the two
wires of the mains line. Usually the live (phase) and the neutral. It may be
two phases on some installations. Should one of these capacitors be failing,
the PC cabinet's voltage is somewhere between the gnd (if earth connected)
and the voltage line (refer above to Indirect Schock) or the line itself.
The guy who's in contact with something (ground connected) of conductive
(one hand on a central heating pipe or standing bare foot on a tiled floor
or ...) and who touches the cabinet, is crossed by a current (depending on
the voltage difference, his own impedance and the impedance of the failure
inside the PSU). The resulting current may be lethal. This is the reason why
there's an earth wire on every desktop or tower PC; firstly suited to drain
leakages to the earth ground and, secondly, ensuring good EMC suppression
and surge protection. Portable computers are protected by a double
insulation power supply that cannot propagate such a failure towards the
inside electronics.

One word about the insulation classes: Class 2 = Double Insulation. It means
that nothing of metallic that can come in contact with the mains is
accessible for touching. No earth ground is necessary. Roughly plastic
outside, insulation transformer inside.
Class 1 = Single Insulation. Metallic accessible parts may accidentally be
connected to the mains by part failure (Class Y caps) or by short circuit
due to the presence of metallic particles (piece loose wire). The Earth wire
is mandatory.


About the other voltages, the same rules apply. Voltage, source impedance,
body impedance, frequency, ...The higher the frequency, the lower deep
current (skin effect). The current is a consequence of all the above. In any
case it doesn't reach the lethal threshold that is also depending on some
other parameters !
One who have to work on a cathodic monitor has to wait for the complete
discharge of the VHV. Dc current behaves differently than Ac current.

If you have any doubt concerning the special case you're dealing with. Don't
apply generic rules, look for specific directives instead. What I told above
is applicable to 50/60 Hz only.


.

Relevant Pages

  • Re: Soon to arrive my HDTV
    ... Aren't they in direct contact with earth ground? ... plug-in surge suppressors offer NO protection whatsoever? ... made irrelevant by protection already inside all appliances. ... The primary goal is minimizing the voltage between power, ...
    (alt.tv.tech.hdtv)
  • Re: point of use surge protection question
    ... For reliable information on surges and surge protection read a guide from the IEEE at: ... Protection is earth ground. ... The protection of a service panel suppressor is largely keeping the voltage between H-N-G at the panel at a low voltage. ...
    (alt.home.repair)
  • Re: "chain" surge suppressers?
    ... The division of the protection between the suppressors depends on ... Depends on the actual clamp voltage of the MOVs in the 2 units. ...
    (alt.home.repair)
  • Re: House grounding
    ... Lightning seeks earth ground. ... Earthing provides chimney protection. ... wire must connect from breaker box to an earthing electrode that is ...
    (alt.home.repair)
  • Re: Building foundation. Trying to locate ground rod
    ... Lightning seeks earth ground. ... Earthing provides chimney protection. ... wire must connect from breaker box to an earthing electrode that is ...
    (alt.home.repair)
Loading