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.
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
I thought all AC allowed you to let go? DC cramps your muscles up.
I call "bollocks". I once picked up a wall socket which I'd used as a trailing socket, and it was wired backwards (earlier on, not by me), so when I'd switched it off, I'd disconnected the neutral and not the live. Hence I got 240V through my hand from live to earth. All it did was warm up my hand. I let go very easily.
Incorrect again. Place a PP3 9V battery on your tongue. That will sting continuously until you remove it.
On Sunday, March 6, 2016 at 6:01:32 PM UTC-5, Mr Macaw wrote:
ailing socket, and it was wired backwards (earlier on, not by me), so when
I'd switched it off, I'd disconnected the neutral and not the live. Hence
I got 240V through my hand from live to earth. All it did was warm up my h
and. I let go very easily.
It also doesn't make sense as written. Volunteers could only tolerate it f
a few seconds? How long does it take to let go?
Back to the main panel disconnect question.
My house always had the meter outside and the main panel inside, with the main disconnect on the panel.
When an upgrade was done to the main panel, an extra breaker was added outside just below the meter. The electrician said this was a code requirement, because wire between the meter and the panel needed to be protected because of the location.
Did this new breaker now become the service disconnect? And if so, is the main panel now noncompliant for having ground and neutral bonded?
Yes they should have pulled in a 4 wire feeder and separated the
neutral and ground.
The issue of how far the SE is running inside the house before it
needs protection is really undefined. Inspectors in the same
jurisdictions may even disagree.
On one extreme some say a few feet, as directly it can be run, using
SE cable, is just fine.
Others say they want a back to back installation with the wire simply
passing through the wall in a short pipe nipple.
The most extreme interpretation pretty much wants an outside
disconnect no matter what.
This is the code
230.70(A)(1) Readily Accessible Location. The service disconnecting
means shall be installed at a readily accessible location either
outside of a building or structure or inside nearest the point of
entrance of the service conductors.
Yeah, it could be something dangerous like 2 volts :-) The wire coming to my house is 300 amps. That thing won't drop much voltage.
Think about it, say it dropped enough to give you a shock (I believe you need 30 volts to even make you feel it) that would mean I'd have 30 volts on neutral with reference to ground. So the voltage drop on the live would be the same. That would mean I'd have 200 volts and 30 volts, a PD of 170 volts. Now they're required by law to provide me with 230 volts +10%/-8%, so anything under 211.6 volts is no good (some equipment wouldn't work, bulbs would be dim etc). 170 is a lot less than 211.6.
I have actually tested the voltage under high load conditions, and it never drops more than about 5 volts (it'll be an equal drop on both conductors) - so I could get a 2.5V shock off neutral - that's less than a lithium torch battery, which I can touch the ends off with wet hands and not even feel it.
Once you've seen one shopping centre, you've seen a mall.
Not sure what you are asking about explain ???
"Mr Macaw" wrote in message wrote:
Yeah, it could be something dangerous like 2 volts :-) The wire coming to
my house is 300 amps. That thing won't drop much voltage.
Think about it, say it dropped enough to give you a shock (I believe you
need 30 volts to even make you feel it) that would mean I'd have 30 volts on
neutral with reference to ground. So the voltage drop on the live would be
the same. That would mean I'd have 200 volts and 30 volts, a PD of 170
volts. Now they're required by law to provide me with 230 volts +10%/-8%,
so anything under 211.6 volts is no good (some equipment wouldn't work,
bulbs would be dim etc). 170 is a lot less than 211.6.
I have actually tested the voltage under high load conditions, and it never
drops more than about 5 volts (it'll be an equal drop on both conductors) -
so I could get a 2.5V shock off neutral - that's less than a lithium torch
battery, which I can touch the ends off with wet hands and not even feel it.
Once you've seen one shopping centre, you've seen a mall.
Don't know what code the electrician was going by, so I can't say for
sure - but IF the neutral and ground were not bonded together at the
outside breaker, the bonded panel definitely meets code and the
outside breaker is not considered to be a "service disconnect"
How much cable is there between the outside breaker and the panel???
On Sunday, March 6, 2016 at 5:52:15 PM UTC-5, email@example.com wrote:
Based on my "inability to let go" experience while in the USCG, I don't
think I agree with this statement:
"...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."
I was learning how to work on power supplies using a "training device"
while attending the USCG electronics school. The training device was a
microwave sized 300VDC power supply which was set up to easily
accept failed components that the students had to find via systematic
trouble shooting steps. It was basically an open box so that all the
components were in full view. It weighed in at about 35 lbs.
One of the troubleshooting steps was to remove the built in load from
the power supply to see if the symptoms changed. The proper way to
remove the load was to shut down the power supply, remove a jumper -
a short cable with banana plugs on both ends - and then turn the power
supply back on.
I was a cocky kid and to save time I figured I would just grab the
jumper in the middle of the loop and just yank it out. Unfortunately,
the load side of the jumper came out, but the supply side stayed in.
I had my left forearm resting on the case and the open end of the jumper
came in contact with my hand. (4 decades later and the scars are still
very visible). At that point my arm became the load for the 300VDC supply
and my brain did not like it. I couldn't move my left arm so my brain
told my right arm to push the case away. As soon as my right arm touched
the case, I was stuck. I grabbed the 35 lb unit and lifted it right off
the table screaming "Turn it off! Turn it off!" I absolutely could not
let go and I absolutely felt the electricity flowing through my arms
and chest. It was no "feeling of warmth", it was in every way the
"sensation of shock".
The lab was set up like a classroom and when I started yelling the
guy in front of me turned around and grabbed the power cord to pull
it out of the power strip on my table. Unfortunately, the power strip
just came up with the cord. The guy next to him slammed the power strip
back down to the table and the cord came out. Once the current stopped
flowing through my chest, I literally threw the power supply down onto
the table. Man, was I pissed.
They took me to the infirmary and did the whole EKG thing. It turned out
that I was OK, other than being pretty shook up and having some bad burns
on both hands. When I went back to class the next day, a couple of things
1 - 2 guys quit electronics school after seeing what happened to me.
2 - All the power strips had been screwed down to the tables. :-)
Anyway, bottom line is that I do not agree that with DC there is "no
sensation of shock" while contact is maintained.
On Sunday, March 6, 2016 at 8:13:13 PM UTC-5, firstname.lastname@example.org wrote:
The lines I object to are not related to the muscle clench. I specifically
quoted the lines related to no sensation of shock while contact is
maintained. The article says that no shock will be felt and I sure as
hell felt the shock during the entire time I maintained contact. Up one
arm, across my chest and back down the other arm.
On Sun, 6 Mar 2016 18:28:47 -0800 (PST), DerbyDad03
I've had ac and dc shocks - and I will say for the same
voltage/current capacity, AC hurts one hell of a lot more than DC. DC
hurts like hell when you get hit, and again when you get off of it. In
between there is pain - but mostly due to constant muscle contraction
- and there is heat.
With AC it just plain hurts like hell - period.. Along with the pain
is the continuous pulsing of the muscle contractions - with 50 or 60
hz AC - a bit different with 400 or higher frequency - and even worse
with something like 25 hz. Lets just say it "hertz"
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.