How dangerous is dc compared with ac

Smokin said:

One will kill you and the other one will too.

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Nah, the other one will just make you dead.

Not only will they kill you.... It will hurt the entire time you are dying.

Grandma said AC/DC was the devil. I've been on a highway to hell since then.

Adding injury to insult, many Chinese HF All-In-One HF inverters do not isolate AC output from PV inputs. There may be high voltage AC output riding on HF inverter's PV inputs relative to AC grounded neutral. Check PV input lines to ground with AC voltmeter.

If you accidentally touch one of the PV inputs to ground while inverter is active, and PV is producing power, it can blow out inverter.

This is one reason they could never be U.L. approved. They have multiple ways to kill you.

RCinFLA said:

Adding injury to insult, many Chinese HF All-In-One HF inverters do not isolate AC output from PV inputs. There may be AC output riding on HF inverter's PV inputs relative to AC grounded neutral. Check PV input lines to ground with AC voltmeter.

If you accidentally touch one of the PV inputs to ground while inverter is active it can blow out inverter.

This is one reason they could never be U.L. approved. They have multiple ways to kill you.

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Can you name some of these specifically? Are there any you know of that don't?

RCinFLA said:

Adding injury to insult, many Chinese HF All-In-One HF inverters do not isolate AC output from PV inputs. There may be AC output riding on HF inverter's PV inputs relative to AC grounded neutral. Check PV input lines to ground with AC voltmeter.

If you accidentally touch one of the PV inputs to ground while inverter is active, and PV is producing power, it can blow out inverter.

This is one reason they could never be U.L. approved. They have multiple ways to kill you.

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Could you elaborate on this a bit more I have an 120v AIO unit and I would like to check it for this. I have done fairly extensive testing of the ac input and outputs on mine and there’s no connection between the neutral and the ground.

Bobert said:

Could you elaborate on this a bit more I have an 120v AIO unit and I would like to check it for this. I have done fairly extensive testing of the ac input and outputs on mine and there’s no connection between the neutral and the ground.

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You should always have a neutral to ground connection.

Many inverters have a relay that bonds neutral to ground inside inverter when there is no AC input applied. When there is an AC input voltage detected, the relay disconnects the neutral to ground relay connection. The responsibility of neutral-ground bonding is then turned over to the grid main breaker panel which should have a neutral-ground bonding at service entrance breaker box neutral bus bar.

This can be a problem if totally off grid and using a generator connected to AC input. If inverter disconnects its neutral-ground bonding when AC input applied then generator must be grounded and generator ground wire connected to inverter ground.

There should only be one neutral to ground bonding point. Multiple neutral-ground bonding points will create neutral current to be shared between neutral wire and ground wire. There should normally be no current flowing in the ground wire. If you have GFI breakers they will not like having current leakage to ground wire. GFI's break the circuit if hot and neutral lines do not have the same amount of current.

One factor that has not been discussed much here is the fact that the conductivity of human skin is not constant nor is it the same person to person. Most of the shock situations I have heard of from lower voltage under 60v have occurred when the “victim” is wet or touched a live surface with an open wound. My grandfather ran a junkyard and used car dealership when I was a teenager he and I both spent many hours diagnosing engine problems. The most common engine problem was a skipping cylinder to diagnose this you would start the engine and pull the spark plug wire of of each spark plug until you found the cylinder that did not affect how the engine ran. The voltage to a spark plug is somewhere 20,000 60,000 volts but only a small fraction of an amp. I was constantly getting shocked. I found that the trick was to get your body in such a position that you didn’t touch any portion of the car except the plug wire. One time I got into such a position but my head was to close to the hood latch pin and I completed the circuit through my head ouch. My grandfather in his 60s at the time) never got shocked and never took any such precautions he could lean over a wet car and never complete the circuit. He had worked all his life hauling hay and working on cars the callus on his hands was 1/4” thick he wore bib overalls rubber galoshes to keep his shoes dry. He was a walking insulator. I was always careful around any of his corded tools or extension cords because he never replaced them because he never got shocked by 110. Voltage can affect children much easier than may adults so covering connections where children are around is very important.

Bobert said:

I got to thinking today about the extremely high voltage some solar arrays are running these days and wondering if the dc voltage they are producing could potentially be more dangerous than ac. So I googled it and after wading through a bunch of opinions and reasons for Certain osha standards I stumbled upon this article. https://www.electricaltechnology.org/2019/08/ac-dc-which-is-more-dangerous.html

Apparently they are both dangerous but in different ways. I found the reference to secondary injury from dc to be paticlarly interesting. What do you think about this article and safety of dc vs ac. I think understanding the dangers and the differences helps me to practice the oxymoron of being more cautious and less afraid at the same time.

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A very interesting article.

I never knew about defibrillators using dc to reverse ac shock induced heart failure…

The bottom line is… bare voltage passing through the skin is deadly… respect it.

I can confirm working around various voltages etc… 24V ac hurts like hell… in the summer when sweaty… cannot feel it with dry hands. Dry skin DC voltage up to 52V and it is Rare to feel it at all… I have no experience with higher dc voltage and contact with my skin.

AC bites and will either cause you to flail and injure yourself, or grab you and keep you from letting go… been there done that, do not want the t-shirt…

RCinFLA said:

You should always have a neutral to ground connection.

Many inverters have a relay that bonds neutral to ground inside inverter when there is no AC input applied. When there is an AC input voltage detected, the relay disconnects the neutral to ground relay connection. The responsibility of neutral-ground bonding is then turned over to the grid main breaker panel which should have a neutral-ground bonding at service entrance breaker box neutral bus bar.

This can be a problem if totally off grid and using a generator connected to AC input. If inverter disconnects its neutral-ground bonding when AC input applied then generator must be grounded and generator ground wire connected to inverter ground.

There should only be one neutral to ground bonding point. Multiple neutral-ground bonding points will create neutral current to be shared between neutral wire and ground wire. There should normally be no current flowing in the ground wire. If you have GFI breakers they will not like having current leakage to ground wire. GFI's break the circuit if hot and neutral lines do not have the same amount of current.

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Mine has no neutral to ground bond unless I am connected to shore power. No bonding in the inverter itself. My generator has no bonding either.

RCinFLA said:

Adding injury to insult, many Chinese HF All-In-One HF inverters do not isolate AC output from PV inputs. There may be high voltage AC output riding on HF inverter's PV inputs relative to AC grounded neutral. Check PV input lines to ground with AC voltmeter.

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DerpsyDoodler said:

Can you name some of these specifically? Are there any you know of that don't?

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SMA.

Page 14:


"Grounding the PV modules
This Sunny Boy is a transformerless inverter. That is why it has no galvanic isolation. Do not ground the DC electric circuits of the PV modules connected to the Sunny Boy. Only ground the mounting frame of the PV modules. If you connect grounded modules to the Sunny Boy, the error message F-Riso is displayed. The DC electric circuit is connected to the AC grid during operation."

Connected to 120/240V split-phase, PV+ and PV- are biased steadily above and below ground.
Connected to 208V (two legs of 120/208Y), there is 104Vrms AC at midpoint of PV string, and PV+/PV- DC ride on top of AC.

Page 13:

"PV modules with large capacities relative to ground, such as thin-film PV modules, may only be used if their coupling capacity does not exceed 2 µF. During feed-in to the grid, a leakage current flows from the cells to ground. The amount of current depends on how the modules are installed and on the weather. This leakage current may not exceed 50 mA since otherwise the inverter will automatically disconnect from the grid."

Page 39:

"Equip all string cables for DC+ and DC − connected to the DC Disconnect with a fuse."


SMA is German, of course.

It is 'less' of an issue for GT inverters that do not have grounded-neutral involvement.

GT inverter can be galvanically isolated between AC and PV but it usually cost a little degradation in inverter efficiency. GT inverters that allow PV voltage input above and below peak AC voltage level are more likely to be galvanically isolated.

120V or 220V Alternating current will lock up your muscles and you may not let go until you are dead. Very high voltage ac will toss you a distance while you are dead. The industrial high voltage ac will through a sustained arc and kill you. It will travel through the ground some distance and kill you. There are many drownings in freshwater marinas caused by bad earth grounds between boats and land. The real cause is electrocution by ac.
A six volt lead acid battery will send 1000s of amperes through a short circuited jewelry item that could burn off a finger. Think what a higher voltage LiFePo will do.
Improperly handled. AC will kill you. DC will only kill you.

When I was a little boy working electrical... under a muddy house...
I was stripping the conductors to hook up to a furnace...
My knees were in the mud.
I was leaning over the furnace stripping the wires.
My shoulder bumped the switch on, mid stripping the hot leg...

I could not move anything between the hand and my knees... I could feel the pulsing of the phase swap cycles... it felt like an eternity, pulse, pulse, pulse...

I realized I could move my feet, and my calf muscles... so I extended them and pushed myself off the wire...

My knife had cut deep into my thumb from the squeezing action, but I lived.


I'm more cautious around wiring now...

Thanks for sharing this. I don't think anything could better hit home. Glad you're still around.

DerpsyDoodler said:

I believe code defines high voltage as 50+ volts and I believe it does so because that's when it becomes blatantly lethal.

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Typically low voltage is 1000 vac and below. Medium voltage is 1000 vac to 100,000 vac. High voltage is 100k to 345k vac . There are also Extra high voltage 345k - 765k and Ultra high voltage 745k + systems.

LywWyr said:

Typically low voltage is 1000 vac and below. Medium voltage is 1000 vac to 100,000 vac. High voltage is 100k to 345k vac . There are also Extra high voltage 345k - 765k and Ultra high voltage 745k + systems.

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Cool story. Got a reference or context for that information?

DerpsyDoodler said:

Cool story. Got a reference or context for that information?

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The context is that 50vac is not high voltage , not even close. The reference is the last 30 yrs as a working electrician.

LywWyr said:

The context is that 50vac is not high voltage , not even close. The reference is the last 30 yrs as a working electrician.

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Can we reclassify in terms of lethal voltage? Happy to soak up any knowledge you will impart.

Lethal voltage is what can overcome the skin's natural resistance. It doesn't take many milliampere. Wet? Sweat? Injuries?

LywWyr said:

Typically low voltage is 1000 vac and below. Medium voltage is 1000 vac to 100,000 vac. High voltage is 100k to 345k vac . There are also Extra high voltage 345k - 765k and Ultra high voltage 745k + systems.

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you sound like an outside wireman.

LywWyr said:

Typically low voltage is 1000 vac and below. Medium voltage is 1000 vac to 100,000 vac. High voltage is 100k to 345k vac . There are also Extra high voltage 345k - 765k and Ultra high voltage 745k + systems.

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As an electrician myself, low voltage is considered less than 600v and high voltage is industrial above 600V

What many are here citing as High Voltage is the safety threshold in ac voltage… but it really is 24V ac… dc is different…

AC is more conductive in the human body at low voltages than DC. The reason is that the human body is not a pure resistor, but instead can be modeled as a set of resistors and capacitors (look up the circuit model of the body). Because this impedance isn't purely resistive, AC (for which a capacitor is a short circuit) has much more effect than DC, especially at lower frequencies. If you look at the bodily effects of both DC and AC currents, you'll see that AC has similar effects as DC, but at lower currents. This is also why the voltage safety limits are higher for DC than AC. For example IEC 60364-4-41 says exposed live parts can be up to 25V AC or 60V DC.