DC Current Safety Threshold

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Nardie

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I just recently experienced an issue where a researcher used electrical test leads to a DC power supply, and then to two separate electrical probes. The DC power supply was rated at 10 VDC with an amperage up to 40A. The test leads were normal leads used for a multimeter, so when the researcher increased the amperage the test leads begin smoking and were damaged. From my experience test leads used for multimeters are designed for 10A not 40A, so to me this is a expected condition. My question since I am not very familiar with DC voltages, would 10 VDC with a 40A load pose a safety risk to an individual if they connected any exposed terminal.
 
10 volts is very, very, very unlikely to pose any risk of electrocution, regardless of whether it's AC or DC. A device that dissipates 400 watts and connections that carry 40 amps are likely to cause a thermal burn, again regardless of whether it's AC or DC.
 
Nardie said:
I just recently experienced an issue where a researcher used electrical test leads to a DC power supply, and then to two separate electrical probes. The DC power supply was rated at 10 VDC with an amperage up to 40A. The test leads were normal leads used for a multimeter, so when the researcher increased the amperage the test leads begin smoking and were damaged. From my experience test leads used for multimeters are designed for 10A not 40A, so to me this is a expected condition. My question since I am not very familiar with DC voltages, would 10 VDC with a 40A load pose a safety risk to an individual if they connected any exposed terminal.
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Test leads are very small gauge wire, maybe 20 AWG or smaller - they will not carry 40A for any amount of time before the insulation melts. The voltage isnt the issue, the amperage is - anyone holding onto that wire would most likely get burned.

It's kinda like dropping a Crescent wrench across an automotive battery - you wont get electrocuted, or even shocked - it's still just 12V, but the amperage could be 800 amps or more - exploding battery or molten/hot metal could cause substantial injury.

otoh, you could run a 40A load, AC or DC, across #6 wire indefinitely without problem, provided high ambient temperature and derating for CCC dont come into play.
 
As stated...it's all about current through the body. Higher the voltage, higher the current if you make yourself a circuit. It takes about 0.1 amps to kill, so do the math. But, can you get electrocuted grabbing onto a 12V car battery? YUP! If your hands are sweaty, have acid on 'em and you have a large enough surface area due to the tool you're holding onto. DC is supposedly more dangerous because it is better at sending the heart into fibrillation. All of this is based on our regular safety meetings, though. I'm no Dr. Frankenstein.
 
drcampbell said:
10 volts is very, very, very unlikely to pose any risk of electrocution, regardless of whether it's AC or DC. A device that dissipates 400 watts and connections that carry 40 amps are likely to cause a thermal burn, again regardless of whether it's AC or DC.
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40amps is 40amps voltage doesnt come into it. the leads to test in line amps on a 40amp circuit would have to be #8awg. Which is why rms amp probes are so popular.
 
meternerd said:
As stated...it's all about current through the body. Higher the voltage, higher the current if you make yourself a circuit. It takes about 0.1 amps to kill, so do the math. But, can you get electrocuted grabbing onto a 12V car battery? YUP! If your hands are sweaty, have acid on 'em and you have a large enough surface area due to the tool you're holding onto. DC is supposedly more dangerous because it is better at sending the heart into fibrillation. All of this is based on our regular safety meetings, though. I'm no Dr. Frankenstein.
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Can you show me a single incident where a person was electrocuted by a 12 volt car battery?
 
K8MHZ said:
Those patients have afib and the DC is in measured pulses of around 5ms. That is totally different from the effects of steady current DC on a healthy person.
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The DC supplied by a defibrillator to those patients having a fib to revive them have several thousands volts applied across the heart compared with steady state DC. In spite of that, it can save lives, while the latter can kill.
 
K8MHZ said:
Actually, it's the opposite. DC through the heart is more dangerous than AC.
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Actually, it depends on the frequency. 50-60 Hz is the most dangerous, requiring about half as much current as DC to inflict the same harm. 400 Hz is about equally dangerous as DC. and increasing frequencies are increasingly less dangerous.

K8MHZ said:
Can you show me a single incident where a person was electrocuted by a 12 volt car battery?
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No, but there's a possibility that it has happened with 24 volts. Unfortunately, overlapping jurisdictions and limited resources prevented it from being investigated.

It went down like this:
- Late at night, a woman went to the garage to coax a guy to quit tinkering with his beloved extreme car stereos and come to bed. She found him wedged in the trunk and experiencing tetanic muscle spasms, and called for help.
- Paramedics arrived and declined to begin treatment due to the clear & present electrical danger. They called for additional help.
- Firefighters arrived and cut cables until the tetany ceased.
- Paramedics examined the guy and deemed him "not a viable candidate for resuscitation".
- The coroner ruled it "accidental". There being no sign of foul play, nobody investigated anything, despite the possibility that it might have been a precedent-setting electrical-trauma incident. 24 volts isn't usually fatal, but being wedged into a confined space might have resulted in electrical contact for an extended period of time.

Or there could have been a 120-volt drop light in the trunk with him. Like I said, nobody investigated anything.
 
drcampbell said:
Actually, it depends on the frequency. 50-60 Hz is the most dangerous, requiring about half as much current as DC to inflict the same harm. 400 Hz is about equally dangerous as DC. and increasing frequencies are increasingly less dangerous.
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May be high frequency current flows along skin due to 'skin effect' not passing through internal organs, making it less dangerous?
 
Wrong test equipment?

Wrong test equipment?

Most meters have a fuse for 10 amps on the current range, I have never seen one that will carry 40 amps. The fuse on the meter should of blown before the lead heated up. Is some part of the story is missing?
 
just the cowboy said:
Most meters have a fuse for 10 amps on the current range, I have never seen one that will carry 40 amps. The fuse on the meter should of blown before the lead heated up. Is some part of the story is missing?
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I wondered about that too...
We wouldn't normally measure direct current directly with a meter in series. In the old days we'd have a shunt. Now it is mostly Hall effect transducers.
 
Edison invented the electric chair in order to prove to the public how much more dangerous AC was (in his mind) than DC. But the thing is, Edison's demonstrations, first on an elephant, then on condemned prisoners, never included a side by side comparison to a DC powered electric chair. The effects might have been different, the results likely the same. But that wasn't his point...

The only electrocution I have ever witnessed was on a DC bus of a large drive at a steel mill. I've witnessed, and experienced, several AC shocks, resulting in no deaths or even serious injuries. I'm not saying AC is safer, nor DC. It's all about the extenuating circumstances, which are so varied and complex that nobody is qualified to pass judgement in advance as to what is or is not 100% safe. So no matter what, both should be taken seriously.
 
Jraef said:
The only electrocution I have ever witnessed was on a DC bus of a large drive at a steel mill. I've witnessed, and experienced, several AC shocks, resulting in no deaths or even serious injuries. I'm not saying AC is safer, nor DC. It's all about the extenuating circumstances, which are so varied and complex that nobody is qualified to pass judgement in advance as to what is or is not 100% safe. So no matter what, both should be taken seriously.
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I've had to sit through various electrical safety seminars (including those from Eaton and Cooper Controls). The lowest voltage that might cause a fatality was said to be 70V dc or ac. However, it is as you say, dependent on actual circumstances.

Somewhat related to your steel mill example............
We did an upgrade of a rolling mill that previously had a Ward Leonard system with a floating DC bus. This was replaced with SCR dc drives. The existing dc motors were retained.

A technician used to regularly inspect the brushgear and check spring tension - by hand while the motors were running!
With the old floating system, the risk was minimal. He tried it with the new SCR system - just the once..........
 
Besoeker said:
I've had to sit through various electrical safety seminars (including those from Eaton and Cooper Controls). The lowest voltage that might cause a fatality was said to be 70V dc or ac. However, it is as you say, dependent on actual circumstances.

Somewhat related to your steel mill example............
We did an upgrade of a rolling mill that previously had a Ward Leonard system with a floating DC bus. This was replaced with SCR dc drives. The existing dc motors were retained.

A technician used to regularly inspect the brushgear and check spring tension - by hand while the motors were running!
With the old floating system, the risk was minimal. He tried it with the new SCR system - just the once..........
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Ouch.

My guy was doing the commissioning of some big ABB DC rolling mill drives and was taking measurements with the doors open. He was squatting on his haunches and for some reason lost his balance, leaned in and put his forearm across one DC bus. Then what we THINK he did (it was all so fast) was a reflex to push off with his other hand, but made contact with the other bus. So he locked on and we had to push him off with a board, but by then it was too late. He was still alive for a few minutes waiting for the ambulance, but didn't make it to the hospital.
 
Besoeker said:
I wondered about that too...
We wouldn't normally measure direct current directly with a meter in series. In the old days we'd have a shunt. Now it is mostly Hall effect transducers.
Click to expand...
The researcher was not measuring current, he was using the test leads as jumpers between the power supply and the test probes. There was no fuse installed, only exposed terminals.
 
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