Low Voltage/high current

A

Ajnatolly

Member
Location
Johnstown PA
Occupation
Electrical tester
I have a question that I think I know the answer to but talking with other people around me. I’m starting to question myself. I am an electrical tester and my question is about a high current test set that uses very low voltage 7.5 V to be exact, With very high current, usually over 1000 A. I am being told that this is not hazardous and is safe to touch since the voltage level is so low, but my thought was that it’s the current that does the damage and the heat provided from that could cause serious internal damage could someone please clarify for me
 
Safe to touch but hazards from arcing is prevalent. Think about shorting a 12v car battery
 
Ajnatolly said:
Wouldn’t touching this for an extended period essentially cook internal organs? Joules law of heat?
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Are you inserting your body parts into the circuit so that the current can flow through your body? You said safe to touch are you touching one side of the 7.5 volt circuit if so there is no current flow. In the aforementioned car battery whenever you're touching metal parts of you car your touching the negative terminal of the battery and nothing happens.
 
I’m not touching any part of the circuit. Another individual was touching one side of the circuit while leaning up against a grounded metal enclosure. In my mind that gives current a path.
 
Ajnatolly said:
Another individual was touching one side of the circuit while leaning up against a grounded metal enclosure. In my mind that gives current a path.
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Is one side of the 7.5 volt source grounded? If not then how can there be a complete circuit?
 
The amount of current your test set can produce has little to do with the danger of touching it.
The current that flows through is through the coworker is dependent on the resistance of the contact with the test set, the resistance through the workers body, and finally the resistance of the contact to ground.
 
It would take several things happening at the same time for you to feel a shock. First you need a complete circuit. So if you touched both sides of the 7.5 volt supply at the same time you would have a complete circuit but considering the resistance of your body you would not feel a shock.

I know that we've all done it so no need to repeat this test but you could test this with a 9 volt battery. Touch the two terminals with two fingers and you'll feel nothing becuase the resistance is very high making the current flow very low. Now if you touch the battery terminals to your wet tongue you'll feel a shock because the resistance of your tongue is much lower.
 
I apologize, my question was more about the potential heat damage to the body as opposed to shock. If the bodies resistance is very high iand the individual were touching both sides of the 7.5 V circuit at 1000 A would that cause an excessive amount of heat in the person‘s body potentially damaging organs? My question is about Jules first law which says heat equals current squared times resistance times time.
 
Ajnatolly said:
I apologize, my question was more about the potential heat damage to the body as opposed to shock. If the bodies resistance is very high iand the individual were touching both sides of the 7.5 V circuit at 1000 A would that cause an excessive amount of heat in the person‘s body potentially damaging organs? My question is about Jules first law which says heat equals current squared times resistance times time.
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Forget the 1000 amps because it is not germane to the discussion. The 1000 amps is not flowing through the body. The current flowing through the body would be based on the voltage and the resistance of the two points on the body where it is connected to the source of the voltage.

You also asked about touching one side of the power source while grounded, is the power source grounded?
 
FWIW we did a few anodising plants that were 30Vdc 40,000A with no danger. We had a specialist came to our office yearly for a safety risk assessment - it was to keep insurance premiums costs reduced. Anyway the specialist commented that below 70V voltage would not cause a fatality.
 
Ajnatolly said:
I’m not touching any part of the circuit. Another individual was touching one side of the circuit while leaning up against a grounded metal enclosure. In my mind that gives current a path.
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Now do the current calculation based on 7.5 volts and a resistive path of 500 to 1000 ohms.
 
If the 7.5V were connected to a human body through a very low resistance method (say salt water electrodes inserted under the skin), that voltage could certainly be lethal.

But normally skin resistance is high enough that 7.5V is not enough to push hazardous current through the body.
 
@Ajnatolly
I think it will help to understand the higher level concept, namely Ohm's Law (V=IR). If voltage is fixed, then current is inversely proportional to resistance.

Most electrical sources are voltage sources or configured to resemble voltage sources. This means that the voltage stays fixed and the amount current varies with the resistance of the circuit, according to Ohm's law. If it is stipulated that the voltage of the equipment stays at or very near 7.5V then it is a voltage source, and there is no way it will put 1000A through a human body because the human body's resistance is too high for that. Only a higher voltage can put that many amps through a human body.

In order for the test to involve 1000A then at 7.5V the resistance of what is being tested must be quite low, namely 0.0075 ohms. Or, the voltage must actually be able to go higher.
 
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