On Sun, 06 Mar 2016 02:25:01 -0500, Micky
wrote:

On Sat, 05 Mar 2016 14:50:19 -0500, wrote:


Over here in the colonies we take that 240v and center tap the
transformer so both ungrounded legs are 120v above ground. That still
gives us the ability to use 240v equipment but most ends up being
120v. I suppose we can blame Thomas Edison for that. He started a fear
campaign against Nick Tesla over AC current, Edison wanted DC and he
said AC was more deadly, to the point of electrocuting an elephant
along with more than a few condemned prisoners ... all with AC.
When he lost the war, the deadly part still stuck and the belief was
that 120 would be safer, still leaving the option of having 240v
equipment.

I thought 240 was indeed more deadly than 120 and that more people
died of shocks, per capita, in the UK than here. How could 240 not
be more deadly than 120?
Something to do with the physiological responce to the shock. Some
voltages make you hang on, others throw you off - making it virtually
impossible to hold on.. I can't remember what my Dad used to say (he
was an electrician) but some of the higher voltages could be safer
than the lower voltages because the lower voltage made you grab the
wire hard and not let go.


The let-go phenomenon for low (600 V) contact

A factor that makes a large difference in the injury sustained in
low-voltage shocks is the inability to let go. The amount of current
in the arm that will cause the hand to involuntarily grip strongly is
referred to as the let-go current.7 If a person's fingers are wrapped
around a large cable or energized vacuum cleaner handle, for example,
most adults will be able to let go with a current of less than 6 mA.
At 22 mA, more than 99% of adults will not be able to let go. The pain
associated with the let-go current is so severe that young, motivated
volunteers could tolerate it for only a few seconds.7 With current
flow in the forearm, the muscles of flexion and extension are both
stimulated. However, the muscles of flexion are stronger, making the
person unable to voluntarily let go. Nearly all cases of inability to
let go involve alternating current. Alternating current repetitively
stimulates nerves and muscles, resulting in a tetanic (sustained)
contraction that lasts as long as the contact is continued. If this
leads to the subject tightening his or her grip on a conductor, the
result is continued electric current flow through the person and
lowered contact resistance.8

With alternating current, there is a feeling of electric shock as long
as contact is made. In contrast, with direct current, there is only a
feeling of shock when the circuit is made or broken. While the contact
is maintained, there is no sensation of shock. Below 300 mA DC rms,
there is no let-go phenomenon because the hand is not involuntarily
clamped. There is a feeling of warmth while the current travels
through the arm. Making or breaking the circuit leads to painful
unpleasant shocks. Above 300 mA, letting go may be impossible.4 The
threshold for ventricular fibrillation for direct current shocks
longer than 2 seconds is 150 mA as compared with 50 mA for 60-Hz
shocks; for shocks shorter than 0.2 seconds, the threshold is the same
as that for 60-HZ shocks, that is, approximately 500 mA.4

Heating power is also increased when a person cannot let go. This is
because a firm grip increases the area of skin effectively in contact
with the conductors. Additionally, highly conductive sweat accumulates
between the skin and conductors over time. Both of these factors lower
the contact resistance, which increases the amount of current flow. In
addition, the heating is greater because the duration of the contact
is often several minutes in comparison with the fraction of a second
that it takes to withdraw from a painful stimulus.

Being unable to let go results in more current for a longer period of
time. This will increase damage due to heating of muscle and nerves.
There will also be an increase in pain and the incidence of
respiratory and cardiac arrest. There can also be shoulder dislocation
with associated tendon and ligament injury, as well as bony fractures
in the area of the shoulders.


Go to:

The let-go phenomenon for high (600 V) contact

Several different outcomes may occur when a person grasps a conductor
giving 10 kV AC hand-to-hand voltage. It takes over 0.5 seconds of
such contact before most of the distal forearm cells are heat damaged.
However, within 10 to 100 milliseconds, muscles in the current path
will strongly contract. The person may be stimulated to grasp the
conductor more tightly, making a stronger mechanical contact. Or, the
person may be propelled away from the contact. Which of these events
occurs depends on the position of the hand relative to the conductor.
Most eyewitnesses report the victims being propelled from the
conductor, possibly because of generalized muscle contractions. The
time of contact is estimated to be about 100 milliseconds or less in
such cases.9(p57