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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Jonathan Johnson on June 14, 2016, 12:13:29 PM
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Here and there on the Interwebs I see the claim often repeated that "120V is safer than 240V." I say that's hogwash.
I don't have any sources I can cite, but I've heard that actual rates of electrocution show little difference between 100–120V and 220–240V circuits. That’s likely because practically speaking, both can be just as deadly.
While many people have been shocked by contacting both 120V and 240V lines and survived with no ill effects, other factors are more determinate for lethality than simply voltage. How well a person is connected to earth/ground/common (are they barefoot? Wearing shoes?), the resistance of their skin (dry/damp/salty), surface area of contact, and other factors all play a greater part in lethality than does voltage. This resistance determines current flow through the body. 30 mA is enough to cause muscle contraction which can lead to cardiac arrest.
Another factor is source impedance. Utility distribution is a very low-impedance source, which means that voltage is only minimally diminished as a load is applied. An electric fence (used on farms to contain cattle) operates around 5000V, but has a relatively high impedance (some newer fence chargers have a lower impedance). That means that once an animal — or a human — contacts the fence, the voltage across the body can drop dramatically, to a thousand volts or less.
A third important factor is duration. While the current can cause muscle contraction and cardiac arrest, the shorter the duration of the shock the less likely there is to be damage to the body. The electric fence operates in a pulsed manner; each voltage pulse is measured in a few milliseconds. That’s too short to cause direct injury, long enough to be unpleasant, and the pulsed manner ensures that a person won't become "locked" onto the fence due to muscle contraction.
Saying that 120V is less dangerous than 240V is like saying it’s better to be crushed by a 1000 kg weight than a 2000 kg weight. Given the right circumstances, it may be less dangerous, but in every case of electrocution or severe injury at either voltage, the circumstances weren’t just right. It’s not worth the risk to assume that a lower voltage is safer.
Regardless of the voltage in use, always take electrical safety seriously. Don't allow equipment with damaged power cords to be connected on your stage. Any indication of shock is a danger sign and should result in an immediate shutdown of the show until the cause can be identified, isolated, and eliminated. Use GFCI/RCD wherever possible. Wear proper protective gear when working with electrical power, especially around exposed conductors. Power off loads before connecting or disconnecting them. Remember that electric arcs can spray molten metal -- don't operate switchgear with the cover open.
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Saying that 120V is less dangerous than 240V is like saying it’s better to be crushed by a 1000 kg weight than a 2000 kg weight.
All very good points but I really like the quoted text :)
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If we were on 110/120, instead of 220, I would worry a little less about my electronics catching fire ?
I don't think my amps handle 440 very well.
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More and more US 120V circuits are getting GFCI protection. These are sensitive protectors as they trip at 5 or 6 mA. Meanwhile in the UK, their newer 230/240V circuits a protected by a RCD that trips at 30 mA.
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240 is nice because it means half the current for the same work, and less voltage drop. Because the voltage is higher and appears more scary, the connectors have been better designed to stop people touching it for years. People don't seem to worry about the prospect of a 110V shock, and some accept it as almost normal.
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I haven't seen any such claim. Logic suggests all else equal less is less harmful.
A 9v battery shoved deep into you heart will stop it, it's all relative.
JR
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I was once told that 240 is actually safer because you stick to 120 and get blown off 240. If you survive the initial contact then you've got a greater chance of overall survival. I was told at the same time that high voltage line workers usually die from the fall not the initial contact (which isn't to say that the initial contact won't do serious damage).
This may all be untrue. A quick google didn't turn up anything useful pro or con on this but I also didn't try to hard on the searching either. :) I'd love to know if anyone can speak authoritatively on this. I also don't know if that 240 referred to US style 120 to 120 or european style 240 to neutral.
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I was once told that 240 is actually safer because you stick to 120 and get blown off 240. If you survive the initial contact then you've got a greater chance of overall survival. I was told at the same time that high voltage line workers usually die from the fall not the initial contact (which isn't to say that the initial contact won't do serious damage).
This may all be untrue. A quick google didn't turn up anything useful pro or con on this but I also didn't try to hard on the searching either. :) I'd love to know if anyone can speak authoritatively on this. I also don't know if that 240 referred to US style 120 to 120 or european style 240 to neutral.
Getting stuck due to muscle contraction is a function of current above a certain threshold. Logically 240v is twice as easy to get stuck to as 120V.
I would speculate that high enough voltage/current to burn the skin contact area might eventually lead to reduced resistance and release, but I don't expect 240V to immediately do that.
JR
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I am amazed at how many people will seek my services saying, "I'll do 120 wiring, but won't mess with 220/240". I doubt stats exist, but I would venture to say that the VAST majority of shock incidents and electrocutions in the US are line-ground. Relatively few will be line to neutral or line to line. A line-line shock (assuming a 120/240 system) is the only way to get a 240 volt shock. If you are using a 240 volt piece of equipment and get a line-ground shock, it is no different than getting a line- ground shock from a 120 volt piece of equipment.
Of course, if you are working with a 277/480 system, then you are looking at 277 volts to ground- but that would be uncommon for most people.
I am hearing rumors that the 2017 NEC has requiremnts for 240 volt GFCIs- both single and 3 phase. Just a couple of months till we learn about the truthfulness of those rumors.
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If we were on 110/120, instead of 220, I would worry a little less about my electronics catching fire ?
I don't think my amps handle 440 very well.
If your amps have universal power supplies, I'd venture that it's less likely that they'd catch fire at 220/240V, as the current flow in the primary side of the power supply will be half of what it would be at 110/120V. Since heating is more a function of current flow than voltage, the heat would be less of a problem at the higher voltage/lower current option.
Now if you plug that amp into 440, things will probably blow up and catch fire because the power supply insulation and components are not rated for that voltage. Something is liable to short out.
At least that's what happened the time I inadvertently connected a 120V computer power supply (non-universal) to a 208V circuit. It didn't like it and it let me know with a loud pop and a release of the magic smoke. Thankfully, it was not a critical system.
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Here's some research on the relative safety:
https://www.quora.com/Does-110V-give-any-advantages-to-the-US/answer/Paul-Davey-6
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Of course, if you are working with a 277/480 system, then you are looking at 277 volts to ground- but that would be uncommon for most people.
Steve is 100% correct. It's almost impossible for a consumer to get a line-to-line shock in the USA, so most shocks are limted to 120 volts. However, in industrial situations with 208/120-volt high-leg delta or 480/277 -volt Wye distribution, it's pretty easy to get between 208 or 277 volts to ground. And JR is correct that 2x the voltage equals 2x the current. And at these relatively low voltages, current is what causes heart fibrillation problems.
As far as the physiological problems from shock, at somewhere around 20 mA of current, the muscles in your hand that make a fist are stronger than the ones that open up your fingers. That 20 mA is enough to overwhelm your nervous system, making ALL of your muscles contract. So there's a tug of war happening with your muscles, and the ones that make a fist win. So when you grab onto something like an aluminum ladder that's electrified, you can't let go. Secondly, it appears that 60 Hz is just the right frequency to interrupt your hearth rhythm somewhere around 30 mA across your chest cavity. Of course, your mileage will vary depending on age, heart condition, etc... But if you figure that the human body (meat puppet) has a hand-to-hand resistance somewhere around 1,000 to 1,500 ohms, at 120 volts you can easily get 100 mA fault current though your chest cavity (and your heart) with a hand-to-hand or hand-to-foot shock. The last part of the equation is that your skin acts like a non-linear resistor. Once you get somewhere around 60 volts or so, the voltage will punch through the surface of your skin and more current will flow. So 120-volts produces way more than 10 times the current of a 12-volt shock. That is, at low voltages dry skin is relatively high in resistance so you don't feel a shock when touching a 9-volt battery. But touch it to your wet tongue and you'll get a real wake up shock.
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Secondly, it appears that 60 Hz is just the right frequency to interrupt your hearth rhythm somewhere around 30 mA across your chest cavity.
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For our friends elsewhere in the world where 50 Hz is the norm, is that 30 mA threshold any different, or is it comme ci, comme ca?
frank
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For our friends elsewhere in the world where 50 Hz is the norm, is that 30 mA threshold any different, or is it comme ci, comme ca?
frank
I've often wondered that, but there's not enough data out there for a comparison. And the 30 mA threshold is time dependent, so a few cycles of 60 Hz at 30 mA may not cause your heart to go into fibrillation. But a few minutes of it would probably be life threatening.
What bothers me the most is that in many cases of home or recreational electrocution, someone witnessed the shock, but yet the victims still died. Just calling 911 and alerting the emergency responders, then starting compression only CPR immediately would would probably save some lives.
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I've often wondered that, but there's not enough data out there for a comparison. And the 30 mA threshold is time dependent, so a few cycles of 60 Hz at 30 mA may not cause your heart to go into fibrillation. But a few minutes of it would probably be life threatening.
What bothers me the most is that in many cases of home or recreational electrocution, someone witnessed the shock, but yet the victims still died. Just calling 911 and alerting the emergency responders, then starting compression only CPR immediately would would probably save some lives.
Thresholds can vary widely from person to person (and situation, like how sweaty etc). I am not aware of any significant difference between 50Hz and 60Hz AC, but AC and DC are likely different.
The scary thing about electrical shock events, is you can get stuck to the same electrical fault, trying to save a friend. Think a few seconds before just grabbing somebody in obvious distress.
JR
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The scary thing about electrical shock events, is you can get stuck to the same electrical fault, trying to save a friend. Think a few seconds before just grabbing somebody in obvious distress.
Best practice is to turn off the circuit breaker first, THEN get the victim clear of the fault. However, if that's not possible, then use a piece of dry wood if it's available to knock the person clear of the electrical fault.
Possibly the worst case of multiple electrocutions was at a ditch with an irrigation pump that wasn't properly grounded. A woman went into the ditch to rescue a dog that had been shocked, she was electrocuted. A few hours after she was reported missing a man found her body and went into the ditch to rescue her, and HE was electrocuted. Then a second man tried to rescue the first two victims, and HE was electrocuted. So, three people were killed after the first one tried to rescue the dog without turning off the power first. https://www.youtube.com/watch?v=RDs8a_ym3Zc
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For our friends elsewhere in the world where 50 Hz is the norm, is that 30 mA threshold any different, or is it comme ci, comme ca?
No idea, but there used to be a published statement in the UK that no-one had ever died of electric shock from a source being protected by a 30mA RCD. Japan would be the place to ask, as they have 50Hz on one side, and 60Hz on the other.
Here in New Zealand, we have the 30mA RCDs, except in patient treatment areas, 10mA RCDs are required. I think 30mA has been settled on internationally in 220/230/240-ville. And for some years now, its been mandatory to have home socket outlets RCD protected. There's loads of old houses, of course, so its far from universal, but if more sockets are added in a house, then RCD protection has to be fitted to the feeds to the new stuff, which generally protects (some of the) old.