Shocked from a tranformer

[be80be]

Like I said You can think what you want I worked with voltages as high as 8800. So I no what is what with power. I was working on switching some phone lines over and I removed the cap to pull the lines my hand slipped and I hit the ground buss with my arm. It pulled my muscles in my arm so hard. It took a week to regain use of that hand.
I was like most of you how can it hurt me ohm law i'm a 1 k resistor as long a it's lower then 1000 volts say 500 I need not worry wrong. watch Ac it can get you.

 

[giftiger_wunsch]

I was working on switching some phone lines over and I removed the cap to pull the lines my hand slipped and I hit the ground buss with my arm.

Ouch, that's rough. A lot of people have heard the phrase 'it's the current that kills, not the voltage', but these are often the same people who don't realise that ohm's law relates the voltage to the current anyway. A 30V, 50A supply won't give you a lethal shock, but a 3000V, 500mA supply certainly can. Confusing the maximum current the power supply can deliver, with the actual current flowing through your body, isn't a mistake worth making.

 

[marcbarker]

marcbarker,
I've never noticed this with small transformers because they have a capacitance of only 100pF or so.

Are you sure they're not powered from switchers? <..> Y rated capacitors .

Yes 'double insulated' SMPS have a leakage to the User (via the Y caps), who can often feel the current.

I'm very sure! Maybe it's not just the transformer, but the other wiring as well. I first noticed it way back in 1978 when some test equipment I was using 'felt strange' when I touched my hand lightly on the insulated painted surface, there was a humming sound as I moved my fingers along the case, it was quite neat! I told one of the older engineers about it, that I thought it wasn't earthed maybe, he said "don't be silly, of course it's earthed, but I'd insisted. He then dismantled the plug and took the cover off the test equipment, showed me the green wire connected in the plug, and the other end connected to a ring tag firmly attached to the chassis. I said I know what I felt was real, so he measured the continuity, it turned out that there was paint under the tag that was insulating it. Then on I became known as the boy who could sense electricity!

 

[BrownOut]

28mA is enough to produce a painful shock and can also cause minor burns if prolonged. A "slight tingle" is more like 1-8mA. 1k is also a very low estimate for resistance through your body. Try tightly gripping the contacts of a multimeter. It is difficult to get a true reading but you'll never get one as low at 1k.

The 1K figure was not an estimate for resistance through your body. If you read my post, I was using a round number to illustrate how ohm's law applies to the old saw about how current, not voltage kills. I was responding to another posters comments.

 

[giftiger_wunsch]

The 1K figure was not an estimate for resistance through your body. If you read my post, I was using a round number to illustrate how ohm's law applies to the old saw about how current, not voltage kills. I was responding to another posters comments.

I did indeed notice that you were using a round-number example. I was simply pointing out that as round numbers go, 1k was a pretty abstract choice. Fiddling with a multimeter should demonstrate that 100k would be closer to a realistic scenario.

 

[nz_electric_specialists]

yea ya get zapped, it trys to ground itself on ya. i just reccomend wrapping it in 1000 layers of insulation tape. ya shuldnt get shocked then

 

[BrownOut]

I did indeed notice that you were using a round-number example. I was simply pointing out that as round numbers go, 1k was a pretty abstract choice. Fiddling with a multimeter should demonstrate that 100k would be closer to a realistic scenario.

Of course it was abstract. The whole example was.

 

[user_88]

I had to write a few lab reports on some AC circuits .... some time ago.
We worked with several voltage levels ...120,240,480 .... mostly 3 phase.
The required procedure was to take 3 individual isolation transformers and hook them up to the main phase connections coming from the power company.

As I recall, with an isolation transformer, the secondary is physically isolated from the primary. Whereas, with a normal transformer, there is actually a physical conductance .... evident with a resistance meter ... between the primary and secondary side. This physical connection is where the shock hazard arises.
Accidental contact with the output of an isolation transformer protected circuit will not produce a fatal event. However, if the circuit is not isolated, the danger level is severely increased.

Another item to remember ... Most of the variable transformers .... the adjustable voltage kind, are not isolation transformers. Always obtain a known, confirmed, isolation transformer, and connect it to the AC mains. Then hook the IT output to the variac. This is particularly important when doing TV or radio repair.

.... I realize that isolation transformers are not always available, or practical in the field, but they make any bench or shop type work much easier to deal with.

 

[tunedwolf]

As I recall, with an isolation transformer, the secondary is physically isolated from the primary. Whereas, with a normal transformer, there is actually a physical conductance .... evident with a resistance meter ... between the primary and secondary side. This physical connection is where the shock hazard arises.
Accidental contact with the output of an isolation transformer protected circuit will not produce a fatal event. However, if the circuit is not isolated, the danger level is severely increased.

I would say you recall incorrectly.

There is no conductive path between primary and secondary windings of any transformer unless it is either designed or wired that way, faulty in some way, or has some other influencing factor.

Isolation transformers are simply those where the primary and secondary windings are physically separated and better insulated. Split bobbin types for example. Under worst case fault conditions, i.e where one or other winding burns through, neither can physically come into contact with the other.

The reason why isolation transformers are used on a test bench is to simply remove the potential difference between earth(ground) and any single secondary tap point.

As for not producing a fatal event, you will still be fried if you touch both ends of a winding on a transformer that has sufficient output voltage to overcome body model resistance and can provide sufficient current. And that includes an isolation transformer.

Variac's are designed to produce a variable mains supply and are referenced to the neutral (ground, at the origin of supply), hence do not remove the p.d between earth(ground) and the output.

rgds

 

[goofeedad]

OK, let me get this straight. The reason the secondary windings are "safer" is because the potential ground doesn't need to be to the earth? So the "safe factor" only comes in when you don't touch the earthed ground (ie... if you stand in water while working on a project)?

 

[BrownOut]

Just think of it as an isolated source, not connected to the mains in any way. So even if you were standing in a puddle of water, you're still safer with a transformer, because you're isolated from the current return path through ground that exists in reference to the mains. In other words, the isolation means you're not really reference to the earth as the mains is, and no current will flow through your body to the mains supply.

 

[user_88]

I would say you recall incorrectly.


rgds

I stand corrected .... thanks for the clarification.

 

[goofeedad]

Just think of it as an isolated source, not connected to the mains in any way. So even if you were standing in a puddle of water, you're still safer with a transformer, because you're isolated from the current return path through ground that exists in reference to the mains. In other words, the isolation means you're not really reference to the earth as the mains is, and no current will flow through your body to the mains supply.

OK, that makes sense. Thanks.

 

[giftiger_wunsch]

Of course it was abstract. The whole example was.

Indeed. And suggesting that your bodily resistance is around 1kΩ or that 28mA would only produce a slight tingling sensation is misleading and potentially dangerous. I'm glad we're in agreement

 

[BrownOut]

Indeed. And suggesting that your bodily resistance is around 1kΩ or that 28mA would only produce a slight tingling sensation is misleading and potentially dangerous. I'm glad we're in agreement

I expect people reading to have enough sense to know when I'm making using made up numbers as an example of how to apply ohms law to understand the effects of voltage on the body. You have to use the Look, Read and Understand method. Those who don't have enough sense to understand I was using the numbers as an example shouldn't be fooling around with electricity in the first place. Neither should anyone who isn't willing to look up the actual numbers. Being that the actual resistance is much higher, I am putting absolutely nobody in danger by using 1K as an example. This isn't rocket science.

 

[BaCaRdi]

Wow what a tread! Have to say I am somewhat surprised of the content. I would say, no shock is safe, there could be underlaying conditions you don't know about that could kill you quick. I would say if you looking for a shortcut out of life, give testing what will kill you and what not. There are far too many factors, most we don't know, would all have to be factored in.

It's been a while since I got shocked, of course saying this isn't a good idea, I better be extra careful now.lol

DC is worse, of course that is why our buddy Nikola Tesla formed the basis of modern alternating current. With that being said, it's still electricity. First the current/voltage is not in phase in AC, of course it alternates. DC is Direct, at higher voltages you will not be able to let go.

Great thread nonetheless)
-Marc

 

[giftiger_wunsch]

First the current/voltage is not in phase in AC, of course it alternates. DC is Direct, at higher voltages you will not be able to let go.

Indeed; the alternating nature of AC means that it doesn't cause a prolonged contraction in the muscles, so unlike DC it doesn't usually cause the victim to grip the voltage supply.

As for the "no shock is safe" thing, depending on the location and the current of the electricity, some shocks can be perfectly safe. For example, nerve conduction studies involve stimulating a nerve with a small electric shock and measuring the time taken for a reaction at another location along the nerve. These studies take place regularly and as far as I'm aware there have never been any adverse health effects recorded as a result.

On the other hand, an electric shock which travels through the heart or brain may cause a myriad of health problems, even if there initially appears to be no problem. It's partly this reason that, for example, conducting a current to ground from a hand through to a foot, is generally much less likely to cause serious injury than from hand to hand, where the current is very likely to pass through the chest.

 

[giftiger_wunsch]

OK, let me get this straight. The reason the secondary windings are "safer" is because the potential ground doesn't need to be to the earth? So the "safe factor" only comes in when you don't touch the earthed ground (ie... if you stand in water while working on a project)?

While that's true, it's best to be on the safe side when working with potentially dangerous voltages, especially if it's a step-up transformer and you're working at *above* mains voltage rather than below. If you put yourself between the two contacts you could very well end up dead. And depending on the voltage, arcing could also occur.

 

[BaCaRdi]

Yep I had 4 EMG's done,They Suck! I understand there are safe levels, what I am saying is if there were underlaying medical conditions, known or not, it's not safe to play with electricity. Don't get shocked, is what I always went by, it's happened, believe me, got it once from a TV Flyback and a car coil. It's bound to happen, but being safe, learning before you touch should be the way.

-Marc

Indeed; the alternating nature of AC means that it doesn't cause a prolonged contraction in the muscles, so unlike DC it doesn't usually cause the victim to grip the voltage supply.

As for the "no shock is safe" thing, depending on the location and the current of the electricity, some shocks can be perfectly safe. For example, nerve conduction studies involve stimulating a nerve with a small electric shock and measuring the time taken for a reaction at another location along the nerve. These studies take place regularly and as far as I'm aware there have never been any adverse health effects recorded as a result.

On the other hand, an electric shock which travels through the heart or brain may cause a myriad of health problems, even if there initially appears to be no problem. It's partly this reason that, for example, conducting a current to ground from a hand through to a foot, is generally much less likely to cause serious injury than from hand to hand, where the current is very likely to pass through the chest.

 

[crutschow]

DC is worse, of course that is why our buddy Nikola Tesla formed the basis of modern alternating current. With that being said, it's still electricity. First the current/voltage is not in phase in AC, of course it alternates. DC is Direct, at higher voltages you will not be able to let go.

The opposite is true. A small AC current can case the heart to fibrillate and stop its normal beat, leading to death if not defibrillated to restart it. DC just causes a single contraction and then the heart can continue beating when the DC is removed (provided the current wasn't so high as to fry you otherwise). Defibrillator's do their work by applying a pulsed DC (unipolar) shock to the heart.

Tesla and Edison had a large, ongoing feud about whether AC or DC should be used for power transmission, and Edison's main argument was the DC was safer than AC. The reason AC won was because its voltage could be readily changed with transformers to allow more efficient transmission over long distances, not because it was safer.

AC current can cause continuous muscle contractions just as DC can since the muscles don't have time to relax between cycles. AC is about three times more lethal then DC for the same voltage (See https://www.medscape.com/viewarticle/410681_3). I remember hearing of my cousin being stuck to their refrigerator door when it developed a short to the chassis and she grabbed the handle. Her mother had to pull the electrical plug so she could release her grip.

The phase of current and voltage in an AC circuit varies depending upon the type of load. For a pure resistive load the current and voltage are in phase (but of course it alternates between plus and minus voltage/current).