just got electrocuted, curious why it didn't seriously hurt

[iandy4]

But can someone explain where this 120v AC is coming from on a laptop that runs on 19v DC?

I bet R.G. can explain this.  I think it has to do with the amp not being properly grounded.  That could explain why there was the full 120vac difference between the "grounds" because the amp was not actually grounded.  I don't know if that's it but hopefully R.G. will chime in again and clarify.

My assumption, by the description of your setup, is that the laptop was not providing 120 volts, the amp was. The laptop was merely in the path of the better ground. Which means you may be lucky some of the more sensitive components of the laptop weren't fried too.

Ah that makes sense!

[greaser_au]

The question it raises is, if the house is wired correctly, what is the point of having three prong outlets?

The process is called MEN (Multiple Earth Neutral). supplies are generally delivered to the local transformer as  a high voltage three-phase (three phases 120 degrees apart) supply, and downconverted to line voltage as 3 phases plus neutral. The neutral is derived in the local transformer, & the number of houses supplied from that transformer are set up to provide roughly equal loads on each phase, so the sum of the currents in the neutral are approximately zero. The neutral is grounded at the transformer (for a known safety reference).   

The question that is really raised is: how do you know the house is wired correctly?  When I was a teenager the supply authority managed to miswire the active & neutral on our run. We found out the hard way when I tried to solder a fixed-wired pump active-side breaker (should have been isolated via the main switch)  with a grounded soldering iron- the pump started and I needed some fresh undies.

The point of the 3-pin plug  is that the safety earth/ground reference on pin 3 goes to the case  of the PC/laptop/powersupply/amp (& thus to the guitar strings via the bridge) regardless of whether the house is wired correctly active/neutral or not.   Picture what would happen on a 2-pin plug, if the bigger prong was wired to the amp case  & then some bozo (sorry 'supply authority') crosswired the active & neutral? That solo would have been 'SMOKIN'!!! (and unfortunately so would the player...  ) Actually this has been done via a capacitor- older amps have a 'hum switch'.

Nice to know the mystery has been solved (capacitive leakage is still likely the culprit ) 

david

[R.G.]

Houses in the USA which are wired to meet the electrical code are wired this way.
- the high voltage transformer at the power pole takes in the high voltage distribution and steps it down to an isolated 240Vac
- the 240Vac is centertapped, 120Vac on each side of center
- the centertap is grounded at the pole with a copper wire which runs down the pole and into the ground; this is actually stapled into a circle on the very bottom of the pole
- three wires come into the house - neutral, which is the centertap that is grounded back at the pole, and two equal-and-opposite 120Vac wires from the ends of the 240 winding
- all three wires go into the distribution box; the grounded-neutral is tied to a wire to a ground rod at the house;
- the two "hot" lines are tied to two different power busses; each is 120Vac with respect to neutral, and there is the same 240Vac between them. The grounded-neutral becomes the white wire to all the power wiring in the house, each of the hot/line 120Vac sides powers various breakers and from the breakers the black/hot side of the power distribution.
- the neutral connection that is tied to a ground rod at the house is distributed as the third-wire safety ground
- each outlet gets a hot/line/black (120Vac) a neutral/cold/white wire, and a green safety ground; 240Vac loads like air conditioners and electric ovens get black and red - black is one 120Vac half, red is the opposite 120Vac half, so there is 240Vac between them.

Found the problem!  I was poking around with the multimeter and it was obvious after a few seconds.  I had two power strips plugged into the same outlet.  The one power strip with the amp and guitar on it had 3 prong outputs but only a 2 prong input to the wall outlet.  The laptop was plugged into the other power strip which went to the same outlet but had a proper 3 prong input to the wall outlet.  When testing the 2 prong power strip I was getting strange voltage readings like 50vac between the ground prong and the smaller slotted prong and 1.2vac between the ground prong and the bigger slotted prong. The wall outlet that it was plugged into gave the expected reading of 123vac between the ground prong and the smaller slotted prong and zero-ish vac between the ground prong and the bigger slotted prong.

The power strip from the amp with only two prongs was the most probable cause. Even perfectly insulated electrical appliances have some capacitance between power lines and any other conductor. The capacitance is so small that the results are generally negligible, but tying the chassis to safety ground cleans this parasitic leakage up by sucking any capacitive leakage down to nothing. Ungrounding the amp's chassis means that any capacitive leakage went to the chassis and ... sat there, waiting to be felt.

This is far, far worse on amps which have a real problem letting more leakage through. The "death cap" on two-wire amps deliberately introduces a capacitor from chassis to one or the other of the power wires. This can reduce buzz, but it lets the death cap's leakage through if you get it the wrong way round.

I'm still a little confused because I was going over the wiring of the house with my dad.  He showed me that each circuit in the house has a black, white and bare copper wire.  The copper is ground, the black is going to the smaller slotted prong and the white is going to the bigger slotted prong for each wall outlet in the house.

This is correct.

 He showed me that at the circuit breaker the white and ground connect to the same neutral buss bar and that the black connects to one of the 120vac lines coming in from the power lines outside.  If this is the correct wiring it makes sense to me that appliances can have either 2 prong or 3 prong plugs because one of the two slotted prong is eventually connecting to ground.  The question it raises is, if the house is wired correctly, what is the point of having three prong outlets?

It's that "eventually".  

Three wire grounding is not there for when everything's going right. It's there for when things go wrong. The cold/neutral line carries the return current from the hot/line/black wire. This may be large, and if so it causes voltage to appear at the socket by the current times the wire resistance. The green/safety wire carries no current in the normal case, so its voltage stays at really, no fooling, zero volts. If there is a fault like some of the AC power leaks to the chassis of an appliance, this is carried down the safety wire, and may be enough to trip the breaker or blow a fuse. Ground Fault Interruptors (GFIs) go a step further and take advantage of the fact that if there is any substantial current on the ground wire, something is wrong, so they open the  power to the appliance. Without a third wire ground, a leak from the hot line to the chassis just charges up the chassis. There is no place for it to go, so the voltage just sits there. Waiting for you. This is *exactly* what happened to you. The third wire ground was interrupted by the power strip, so a little leakage to the chassis waited until you made the bridge between the amp and the laptop, which was properly grounded.

I still don't really understand why I was shocked.  It was a difference in voltage between the grounds of the amp+guitar and laptop because in this case the amp wasn't properly grounded?

That's it exactly. In reality, everything leaks a little. This is why fighting hum in an amp or pedal is such a problem - AC power line voltages reach out right through the air. (capacitance, really, but I'm being graphic   ).  It's a very high impedance because the capacitances are small, so only high impedance things like unshielded wiring, amplifier inputs, etc. can pick it up without "shorting out" the hum. Back at Ohm's law, if you have a voltage source (say, 120Vac) and a resistor that goes nowhere, and measure the voltage between the free end of the resistor and the other end of the voltage source, you measure the full 120Vac. The open end of the resistor goes through a phantom resistance, that being the resistance of air between the free end of the resistor and the other end of the voltage source. This may be tera-ohms for dry, clean air, or merely hundreds of megohms for wet, clean air. In any case, the voltage divider of any resistor we can buy  and the resistance of air means that all of the voltage is "dropped" across the air resistance, even if the real resistor is 10M or more. You measure the full 120Vac because the impedance of your meter is much higher than the impedance of the series resistor.

The laptop was tied to real, no-fooling earth ground. The amp chassis was floating, tied to everything in the universe (... not making that up; it's literally accurate) by the small capacitances that physics says are always there. Enter YOU. You look like a 20K resistor (roughly, plus or minus, with dry skin) to the 120Vac on the amp chassis, and when you touch the amp chassis and the laptop, current flows. What kills you or keeps you alive is what the impedance inside the amp is to the 120Vac line. That's effectively all that limits the current. In this case, it was high enough to make you tingle, but not clamp your muscles down and start your heart fibrillating. Mother Nature administered a gentle warning. Natural laws are enforced immediately, and without pity. People often think of Ohm's law as an abstract thing that only applies inside an amp or effects pedal. In this case, your life depended on it.

[iandy4]

Thanks so much R.G. and to everyone for the very detailed explanations! I'm printing this thread and sticking it on the fridge until I can remember everything by heart.

[joegagan]

excellent thread. appreciate all the posts.

extra points to doug for pointing out the werd usage correction. i opened the thread wondering if, and who would have mentioned this.
at least once a year i speak to someone who is a survivor of ' electrocution' and lived to tell about it.

about 14 years ago, in a small club in santa fe, we noticed something wasn't right, so we whipped out our meter and found a full  TWOFORTY volts between mic and guitar string. (yes, two 120 legs somehow converging on one stage). keeping Rg's post about power wiring in mind, my theory is the building was actually being fed from two separate panels, result of combining two residences into one restaurant. the plugs each  read 120 on their own, but combined read 240.

[joegagan]

another possible scenario just occurred to me. if someone wanted to add some 120v plugs, and were abandoning a 240v appliance, they may have simply used each of the 120 hot legs to power up some plugs.

[R.G.]

another possible scenario just occurred to me. if someone wanted to add some 120v plugs, and were abandoning a 240v appliance, they may have simply used each of the 120 hot legs to power up some plugs.

Both are possible. Either one kills you just as dead. 

[PRR]

[rockhorst]

I once got zapped when I was playing guitar and noticed that my tap was dripping (3 prong from amp into 2 prong wall socket...I know I know). Made my arm dance.

[DougH]

another possible scenario just occurred to me. if someone wanted to add some 120v plugs, and were abandoning a 240v appliance, they may have simply used each of the 120 hot legs to power up some plugs.

I was wiring up a ceiling fan on our front porch last year and for a split second was tempted to just use half of the 240v water heater supply line that was conveniently located nearby. Not a good idea... Instead I spent a good part of a day running conduit from the 120v washer supply line and am glad I did.

[Paul Marossy]

Thanks for your input guys.

Found the problem!  I was poking around with the multimeter and it was obvious after a few seconds.  I had two power strips plugged into the same outlet.  The one power strip with the amp and guitar on it had 3 prong outputs but only a 2 prong input to the wall outlet.  The laptop was plugged into the other power strip which went to the same outlet but had a proper 3 prong input to the wall outlet.  When testing the 2 prong power strip I was getting strange voltage readings like 50vac between the ground prong and the smaller slotted prong and 1.2vac between the ground prong and the bigger slotted prong. The wall outlet that it was plugged into gave the expected reading of 123vac between the ground prong and the smaller slotted prong and zero-ish vac between the ground prong and the bigger slotted prong.

That's why you should NEVER defeat the ground on anything. That creates a potential between ground and the piece of equipment with a floating ground that is looking for a path to ground, thru the path of least resistance, which can be our bodies. I've heard of 60 volts difference before, but not 120 volts! This is a good video to demonstrate what happens when you lift the ground of a guitar amp: http://www.youtube.com/watch?v=6ZGokkrsO_Y

Never assume anything when working on the wiring in your house! I replaced an outdoor GFCI receptacle on my house a couple months ago because I was having a problem with my irrigation system somehow power tripping the GFCI outlet in my kitchen. There were two breakers on the electrical panel that were labeled "GFCI". I turned them off thinking that the one outdoor outlet was on these same circuits. When I went to remove that receptacle, I found that it was still active! They had wired it as a sidearm GFCI outlet on the living room circuit instead of where everything goes thru it like (I assume) the ones connected to two breakers on the panel. I instinctively didn't touch anything directly with my hands, and it's a good thing that I didn't. What threw me off is that there are three GFCI outlets in the whole house, but only the ones in the kitchen & bath circuits are actually connected to these two GFCI circuits in the panel. I eventually found my problem, it had to do with a conduit that runs under the patio to another outdoor outlet connected to the same GFCI in the kitchen that supplies power to the irrigation controller. When I disconnected that outlet, my problem went away. Which is fine because I don't ever use it anyway. But I originally thought the problem was with the outdoor GFCI that I replaced, which did need replacing (it was literally corroding/rusting apart on the inside and read zero volts on the DMM), but it had absolutely nothing to do with the problem I was trying to solve. So all that to say that things may appear to have been done correctly or a certain way in your house, but don't take any chances!

Thru all this I learned about how many ways a GFCI outlet can be wired, though. And it's kind of annoying that there are so many ways to do it and they all apparently meet code. It seems to me like there should one standard way to do it, period. But tract home builders would do almost anything to save $20 per house they build.  

[deadastronaut]

ive always found it interesting that in the uk we use 240v

and most other countries use 110v..

is this just to rip us off, bill wise?....

yet, on our building sites (projects)  we use 110v transformers........i know... its a strange country....

[Paul Marossy]

ive always found it interesting that in the uk we use 240v

and most other countries use 110v..

is this just to rip us off, bill wise?....

yet, on our building sites (projects)  we use 110v transformers........i know... its a strange country....

I don't know how they do things in the UK, but in the US, (AFAIK) we are billed according to the amount of current used. So the higher the voltage, the less current is used. That's why people with big buildings prefer to use 208V 3 phase or 460V 3 phase power. It's much cheaper for them to use the higher voltages. Plus when you have a whole basement mechanical room full of chillers cooling a large casino for example, 120V 1 phase won't operate 30HP or 40HP pump motors or the chiller compressors. It would be too impractical and you can only get so much horsepower out of a single phase motor anyway.

You confused me with the 110V to building sites. Do they run 240 VAC to a step down transformer to make it 110 VAC? What would be the purpose of that?!

[PRR]

> in the uk we use 240v ... and most other countries use 110v.. rip us off...?

No. The earliest wide-spread electric power utility, Edison's DC system, pre-dates good insulation. Voltage was held-down to 100V off ground because Edison observed fewer deaths around <100V machines than >100V machines.

Edison's DC system has serious problems (low voltage means big current thus heavy copper) but became entrenched in Edison's area. Later AC systems had to function with 100V lamps and poor (often no) insulation. Although we've snuck up a few tenths of a volt per year, to 120+V AC, we still owe a lot to Edison's 100V DC system.

The USA and Canada and key trade areas (Mexico, Japan in-part, few others) are on this system.

England once had a VAST array of different local voltages. Also France, Germany, etc. Efforts to rationalize were slow. Some areas clung to odd voltages to protect local lamp factories. But World War II devastated most of Europe's electric infrastructure. By the time of the rebuilding, metals were still in short supply but excellent insulation was available. For the same total power delivered (which is what counts) 230V systems use less copper and more plastic than 120V systems. England also went to Ring Mains which allow even less copper, but is utterly alien to US-type ad-hoc wiring.

Siemens and other giant electrical companies were very effective in pushing 230V systems everywhere in the world *except* USA and Canada. Go to Russia, Estonia, Moldavia, Egypt, Ghana, Argentina, Malaysia, Pakistan, Uzbekistan.... It's all influenced by UK and German practices.

> in the uk we use 240v ... and most other countries use 110v.. is this just to rip us off, bill wise?....

I assume you pay by the Watt-Hour? So same power consumed is the same bill (IF rates were equal).

> US, (AFAIK) we are billed according to the amount of current used.

No. Residential and small-industrial billing meters are WATT(-Hour) meters. There is a coil for current and a coil for voltage. (On 240/120 split, 4 coils total). The meter multiplies V * I * T to get nominal Watt-Hours. Accuracy is normally 2%.

With proper system design, the supply voltage does not affect delivered cost enough to notice.

Even with "poor" design the effect is small. I take 246V/123V from the pole through 400 feet of thin wire. My "120V" appliances are high current and voltage sags to 110V. However the meter is at the house end of the too-long wire. I'm paying for the 110V at this end, not the 123V at the street.... the missing 13V is the company's loss.

> on our building sites (projects)  we use 110v transformers........

Yes, and that is an odd and poorly-documented system (if you don't spend a fortune buying the documents). It appears that it is not even 110V to ground, but either CT with 60V to ground or (with excellent insulation) full-floating. It is probably based on ready availability of US-market 120V hand power tool motors and an over-reaction to the obvious dangers of 230V wires in rough wet conditions.

[Jazznoise]

110 V? As far as I'm aware you can't use anything over 40 V on Irish building sites outside of special circumstances. If you're finding this all abit odd be glad you don't live in Japan, where half the country runs at 50hz and the other half runs at 60hz. Must have lead to extremely uncomfortable viewing with those old CRT's - watching a show filmed at 30 fps at 25 fps sounds like an attempt to induce seizures.

[Paul Marossy]

No. Residential and small-industrial billing meters are WATT(-Hour) meters. There is a coil for current and a coil for voltage. (On 240/120 split, 4 coils total). The meter multiplies V * I * T to get nominal Watt-Hours. Accuracy is normally 2%.

With proper system design, the supply voltage does not affect delivered cost enough to notice.

Even with "poor" design the effect is small. I take 246V/123V from the pole through 400 feet of thin wire. My "120V" appliances are high current and voltage sags to 110V. However the meter is at the house end of the too-long wire. I'm paying for the 110V at this end, not the 123V at the street.... the missing 13V is the company's loss.

I know about watt hours. As far as I am concerned, you pay for the total amount of current (watt/hours) used in a month, for the voltage system you are operating on. That's my simplistic way of looking at it. The more power/current your system draws in a billing cycle, the more you pay. All this energy saving "green" building stuff going on these days to me is a way to reduce the amount of current needed for the power grid so it doesn't have to work as hard. And a side benefit is that the end user theoritically gets to pay less on their power bill. Or at least until successive increases to the power rate negates all the money you spent on "going green".

Anyway, in the summertime here where I live, the power at the outlet is often 125V. I've measured it a few times. So the voltage coming out of the transformer might be 135VAC I guess, assuming some voltage drop between transformer and electrical service to the house.

110 V? As far as I'm aware you can't use anything over 40 V on Irish building sites outside of special circumstances. If you're finding this all abit odd be glad you don't live in Japan, where half the country runs at 50hz and the other half runs at 60hz. Must have lead to extremely uncomfortable viewing with those old CRT's - watching a show filmed at 30 fps at 25 fps sounds like an attempt to induce seizures.

It seems as though they picked 50Hz because it's not as dangerous to humans, according to something I just read. Or maybe that's just a conincidence? Anyway, how in the world can you live on a 40V power system? Nothing in my house would even function at all on that voltage. Except for maybe light bulbs, which would be very dim...

[deadastronaut]

You confused me with the 110V to building sites. Do they run 240 VAC to a step down transformer to make it 110 VAC? What would be the purpose of that?!

yep, transformers, i think its mostly for outside/power tools/lighting  etc...

no idea why...   i'm still getting used to 9v....

[Jazznoise]

It seems as though they picked 50Hz because it's not as dangerous to humans, according to something I just read. Or maybe that's just a conincidence? Anyway, how in the world can you live on a 40V power system? Nothing in my house would even function at all on that voltage. Except for maybe light bulbs, which would be very dim...

Well they did and didn't pick 50 hz, it's just there's a border going down the country where they have to convert it back to DC and then back to AC again. What a waste of power, eh? As for the advantages, hard to say. I'd assume 60hz would be more efficent but only by a fractional amount. I've never been sure why either frequency was picked.

The 40V is exclusively used during construction - ie: Drills, Electric Saws. The idea being that 40 V isn't really high enough to spark much of a distance/ arc over a long distance. I should've mentioned the 40 V also has to be isolated from the mains - either via a transformer or by using DC generators (Much more common due to wiring going into houses pretty much last). The equipment is designed to work with this and I'm fairly sure they step up the voltage internally, it's more so cutting a cable doesn't cause severe heart or muscle contractions - these may not be so dangerous themselves until you're falling over mith a bansaw in your hand. 

Lighting on any building site I've seen has used a generator, though generally once it's that dark they just call the end of the day. They wont make enough to pay for the fuel that the generators will use.

[Paul Marossy]

It seems as though they picked 50Hz because it's not as dangerous to humans, according to something I just read. Or maybe that's just a conincidence? Anyway, how in the world can you live on a 40V power system? Nothing in my house would even function at all on that voltage. Except for maybe light bulbs, which would be very dim...

Well they did and didn't pick 50 hz, it's just there's a border going down the country where they have to convert it back to DC and then back to AC again. What a waste of power, eh? As for the advantages, hard to say. I'd assume 60hz would be more efficent but only by a fractional amount. I've never been sure why either frequency was picked.

The 40V is exclusively used during construction - ie: Drills, Electric Saws. The idea being that 40 V isn't really high enough to spark much of a distance/ arc over a long distance. I should've mentioned the 40 V also has to be isolated from the mains - either via a transformer or by using DC generators (Much more common due to wiring going into houses pretty much last). The equipment is designed to work with this and I'm fairly sure they step up the voltage internally, it's more so cutting a cable doesn't cause severe heart or muscle contractions - these may not be so dangerous themselves until you're falling over mith a bansaw in your hand. 

Lighting on any building site I've seen has used a generator, though generally once it's that dark they just call the end of the day. They wont make enough to pay for the fuel that the generators will use.

Oh I see, 40V for construction purposes. Still kind of weird to me living in the USA, though.

I've wondered why there was 50Hz in some places in the world and 60Hz in others. I guess it's because of how things started in those areas. Too late to globally standardize I suppose? 

[bobodechimp]

one week back in school i was having a really bad gremlin problem with my gear, mind you it was all old second hand stuff but still. my cab when kaput and while it was in the shop my head went kaput. so while i was saving up for a new head my sound guy lent me a Frankensteined  bass rig of sorts. a dod crossover for a pre, a 1977 Yamaha stereo 300 as and amp and a 1x18 peavey  sub. it was afoul. well the amp was only working on the left side and the ride side was going bad  at lest that what he said. well i was setting up one night plugged into the wrong side and zap. when i came to  i was speaking jibe-rage and i  pissed all over my self. isn't electricity cool.