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#30961 - 11/04/03 03:16 AM Electrical Safty help :)
Ali Offline
Member

Registered: 10/30/03
Posts: 13
Hi there;
I have to do a course work On electrical safty I would appreciate if you guys help me I have listed my questions I would be greatfull if you answers those questions in details


1-What level of electrical voltage would normally be considered safe?

2-An uthomotive battries currently operates at a nominal 12 V d.c, this voltage level is normally considered inherently safe. Explain why the authomotive battery still repersents a safty hazard.

3-Exlain briefly the difference between direct and indirect contact with electrical equipment.


Thanks alot
_________________________
ali[at]frozensky.net

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#30962 - 11/04/03 03:47 AM Re: Electrical Safty help :)
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Hi Ali,

1. This is a difficult question to answer. From an electric-shock point of view, many authorities consider somewhere around 50V to be the limit, and in fact this level figures prominently in many of the wiring rules in my part of the world (United Kingdom).

It's not quite that simple however, as the severity of a shock depends upon many other factors, such as which parts of the body the current flow through, for how long, and the intensity of the current. In extreme circumstances, it has been known for 32V to prove fatal, yet many people have survived contact with 120 or 240V domestic power.

2. At 12V the automotive battery doesn't represent a shock hazard, but the lead-acid battery is capable of delivering a very high current. If you short-circuit a car battery you can quite easily get in excess of 1000A flowing -- Quite enough to melt wiring and start a fire.

The difference between this and a 12V battery for a transistor radio is that the latter has a much higher internal resistance which limits the available current.

3. Direct contact is where somebody touches a part of the equipment which is energized in normal operation. If you were to open a panel cover and grab hold of a live busbar, for example, that would be direct contact (Don't try it! )

Indirect contact is where somebody gets a shock by touching a part which should not normally be live. For example, if you had an electric heater with broken connections inside so that the metal case became hot, then anyone touching the case would get a shock by indirect contact.

Hope this helps.

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#30963 - 11/04/03 04:01 AM Re: Electrical Safty help :)
Ali Offline
Member

Registered: 10/30/03
Posts: 13
Hi Paul;
I'm studying in Coventry University
I appreciate your help but I was wondering If you could explain authomotive battery in more details.

Thanks in advance

[This message has been edited by Ali (edited 11-04-2003).]
_________________________
ali[at]frozensky.net

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#30964 - 11/04/03 06:16 AM Re: Electrical Safty help :)
sabrown Offline
Member

Registered: 12/12/02
Posts: 297
Loc: Ogden, Utah, USA
Another hazard of the automotive battery is the hydrogen gas produced. When working around one of these, a spark can ignite the gas and cause the gas to ignite and blow open the case. At that point, flying sulfuric acid in your eyes will cause blindness after a short contact period.

How I wish that I could teach many of those whose work I see in telecommunications and get it through their heads the dangers of having multiple batteries sitting there on the floor with open terminals.

Shane

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#30965 - 11/04/03 08:15 AM Re: Electrical Safty help :)
Ali Offline
Member

Registered: 10/30/03
Posts: 13
Thank you very much Shabe
can I ask one last question
_________________________
ali[at]frozensky.net

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#30966 - 11/04/03 08:43 AM Re: Electrical Safty help :)
Redsy Offline
Member

Registered: 03/28/01
Posts: 2138
Loc: Bucks County PA
pauluk,
Very quick police work!

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#30967 - 11/04/03 10:39 AM Re: Electrical Safty help :)
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Redsy,
The credit belongs to Bill this time.

Ali,
The circuit to which you connect a battery obeys Ohm's Law:

I = E / R

where I=current, E=voltage, and R=resistance.

The voltage, E, is the EMF supplied by the battery, in this case 12 volts.

The current is then inversely proportional to the resistance of the circuit, i.e. the lower the resistance the higher the current.

Note that the current is determined by the resistance of the whole circuit, which includes not only the resistance of the load and connecting wires but also the internal resistance of the battery.

A typical transistor-radio battery might have an internal resistance of, say, 3 ohms. With 3 ohms accounted for inside the battery itself, the maximum current you can draw is limited to:

I = E / R = 12 / 3 = 4A.

If you connect the radio or a bulb to the battery, then the resistance of that load is added to that 3 ohms and the current will be much less.

But if you put a dead short across the battery terminals, the current is still limited by that 3-ohm internal resistance to a maximum of 4 amps.

A lead-acid battery, on the other hand, has a chemical composition which gives it a much lower internal resistance. maybe in the region of 0.01 ohm.

If you connect a bulb to the battery, the current is still restricted by the complete resistance of the circuit, including that of the bulb.

But if you short the car battery, you have only the internal resistance of 0.01 ohm to limit the current:

I = E / R = 12 / 0.01 = 1200A

So you have two batteries, each supplying the same EMF (voltage), but the difference in internal resistance means that one is capable of supplying much more power than the other:

Radio battery:
P = I x E = 4A x 12V = 48W

Car battery:
P = I x E = 1200A x 12V = 14,400W

In practice, there will be some resistance in whatever object is shorting the battery, so the maximum current will be a little less, but you get the general idea.

That 14kW or so of heat will be partly dissipated by whatever is causing the short (e.g. a wrench, or a bracelet on your wrist!) and partly inside the battery itself.

If the battery gets hot enough, the case can crack and result in the spillage of acid.



[This message has been edited by pauluk (edited 11-04-2003).]

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#30968 - 11/04/03 11:09 AM Re: Electrical Safty help :)
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Neat explanation Paul!

I've measured the short-circuit current of ordinary LR6 (AA?) 1.5V batteries. If I remeber correctly, you get something like 6A. A pack of Ni-Cd cells of the ordinary type used in model cars and planes gave 18A. At first I thought there was something wrong with the meter, since the values seemed so high.

Pauls teoretical demonstration of the power of a car battery can be demonstrated in reality: A friend of a friend put a wrench across the poles, which instantly resulted in melted poles.

It's worth remebering that electric shock is not the only danger from electricity. If one goes through the injuries from electricity reported to the Swedish authorities, one sees that many cases are burns from arcs. These can be very serious or even lethal.

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#30969 - 11/04/03 12:24 PM Re: Electrical Safty help :)
Ali Offline
Member

Registered: 10/30/03
Posts: 13
Hi Pual Thanks so much I just got it...
I hate electrical engineering but i need it so badly for my projects
Thanks veryyyy much
_________________________
ali[at]frozensky.net

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#30970 - 11/04/03 03:59 PM Re: Electrical Safty help :)
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
This is probably not the best place to say that, as we're all pretty much electrical obsessives......

Good luck with your project anyway.

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