Shocked from a tranformer

[giftiger_wunsch]

It's bound to happen, but being safe, learning before you touch should be the way.

I agree; personally I have a severe dislike for electric shocks and I usually take perhaps too extreme precautions when there is a known risk. I'm also quite new to electronics so until I have experienced exactly what's safe and what is not, I'm still going to avoid getting too close to mains transformers while plugged in, despite the heavy insulation and the fact that the secondary winding of the one I was testing is only 10V

 

[giftiger_wunsch]

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.

You misunderstand; we were discussing the way DC voltage causes a sustained contraction in skeletal muscle, which can often result in a prolonged shock as the victim is forced to grip the power source. The effect of AC on the heart never came up

Anyway, there are various 'advantages' and disadvantages to getting shocked by DC over AC and vice verca, but it may take a while to go through them all

 

[be80be]

Ac is more likely to stop your heart

Other issues affecting lethality are frequency, which is an issue in causing cardiac arrest or muscular spasms, and pathway—if the current passes through the chest or head there is an increased chance of death. From a main circuit or power distribution panel the damage is more likely to be internal, leading to cardiac arrest.

The comparison between the dangers of alternating current and direct current has been a subject of debate ever since the War of Currents in the 1880s. DC tends to cause continuous muscular contractions that make the victim hold on to a live conductor, thereby increasing the risk of deep tissue burns. On the other hand, mains-magnitude AC tends to interfere more with the heart's electrical pacemaker, leading to an increased risk of fibrillation. AC at higher frequencies holds a different mixture of hazards, such as RF burns and the possibility of tissue damage with no immediate sensation of pain. Generally, higher frequency AC current tends to run along the skin[citation needed] rather than penetrating and touching vital organs such as the heart. While there will be severe burn damage at higher voltages, it is normally not fatal.

It is sometimes suggested that human lethality is most common with alternating current at 100–250 volts; however, death has occurred below this range, with supplies as low as 32 volts. Danger increase dramatically with voltages over 250 volts.[citation needed] Shocks above 3300 volts are usually fatal, and those above 11000 being mostly fatal.

Electrical discharge from lightning tends to travel over the surface of the body[citation needed] causing burns and may cause respiratory arrest.

[edit] Skin Resistance

The voltage necessary for electrocution depends on the current through the body and the duration of the current. Using Ohm's law, Voltage = Current × Resistance, we see that the current drawn depends on the resistance of the body. The resistance of our skin varies from person to person and fluctuates between different times of day. In general, dry skin is a poor conductor that may have a resistance of around 100,000 Ω, while broken or wet skin may have a resistance of around 1,000 Ω[4].

 

[goofeedad]

Wow! When I originally posted this thread my main concern was with someone other than me getting shocked. I am about to test a devise in a store and even though my electronics is enclosed and store employees can't easily access the electronics I wanted to know how I could reduce the risk of shock even more.

I've bee told useing a switching supply would probably be safer but at the time I needed it I couldn't find one that had the output or the price I wanted.

At this time I'm pretty happy with the degree of safety I have.

Thanks everyone for all the responces, I will keep on reading all posts as I'm sure others will too.

 

[Hero999]

This debate about which is more deadly AC or DC, surfaces every now and then.

The fact is that AC is more deadly. I think the myth that AC is safer is very dangerous and wouldn't be surprised if some people have been killed because they believed it. Before you even consider the physiological effects, just looking at the numbers; common sense tells us that AC is more deadly because the peak voltage is Vrms√2, in fact I think its shocking effect is equivalent to the peak-to-peak voltage.

Another myth is that the skin effect protects one from high frequency (100kHz to 1MHz) current; it doesn't. Human flesh has a higher resistance than copper so the skin effect is less pronounced, allowing the current to penetrate deep into the body causing nasty RF burns. In reality it's the fact that one's nervous system doesn't respond to high frequency current (>20kHz) so it doesn't have the same shocking as power line frequency AC or DC.

EDIT:

I've bee told useing a switching supply would probably be safer

I disagree with that. I would consider a transformer to be safer than a SMPS because there's not voltage regulation circuit to fail (although this shouln't give deadly voltages) and there are no Y rated capacitors from the primary to secondary.

I consider a transformer (this includes a transformer wall plug) to be the mose reliable and safest way to power a hobby project.

 

[BrownOut]

Alot of what your're reading on here is fluff. It really doesn't matter if AC or DC is safer. The bottom line is to take all safety percautions. The suggestions on here are intended to lower the risk of shock. It can not be completely eliminated, and so it's good to know how to reduce it to a managable leve. If you use a grounded plug, insulate well and isolate the electronics with an isolated power supply ( it doesn't matter if it's a switching or normal power supply, as long as it's isolated ), make sure nobody goes sticking his hand inside the chassis, etc. then it should be fine.

 

[giftiger_wunsch]

If the device has an insulated case, the transformer is well insulated, and the output is only 28VAC, your precautions are perfectly adequate.

Edit: If you're still concerned, you could consider using an RCCB?

 

[Hero999]

Alot of what your're reading on here is fluff. It really doesn't matter if AC or DC is safer. The bottom line is to take all safety percautions

Generally I agree.

What I'm talking about is risk assessment: if I were to work on something connected to a 50VAC supply, I would take more precautions than I would if it were connected to a 50VDC supply.

 

[bountyhunter]

Just think of it as an isolated source, not connected to the mains in any way.

Even an "isolated" secondary is connected to the primary via inter winding capacitance which can buzz you pretty good. If the transformer has a grounded shield between windings it will reduce this effect.

 

[kchriste]

No matter what you are working with, it pays to be carefull. Think of what would happen if your wedding ring, or some other jewelery on your body, shorted the supply from a 12V car battery.

 

[giftiger_wunsch]

No matter what you are working with, it pays to be carefull. Think of what would happen if your wedding ring, or some other jewelery on your body, shorted the supply from a 12V car battery.

That would indeed be rather dangerous, but generally the contacts on a car battery are too far away to be shorted so easily.

 

[Hero999]

The problem is connections on other components such as relays and PCBs aren't that far away; they should be fused though.

 

[marcbarker]

I dunno about all this health and safety mania, it all seems too risk-averse to me. I think a few occasional mild electric shocks are good for your survival, and creates a healthy sense of respect. It's a bit like riding a bike as a kid you learn not to jam your front brake on hard, rather than remove the front brake off the bike to remove the risk.

The trouble is that if everyone does their best to eliminate all risk and increase 'health & safety' in every aspect of life, we lose any judgment of risk.

Then one day when the 'completely de-skilled and over-protected' End User does "everything they are supposed to do to the letter", in a world of health and safety gone crazy,grabs hold of a live fridge door handle, we are then all in disbelief!

 

[giftiger_wunsch]

You don't need to intentionally expose yourself to risk in order to learn how to avoid it. That defeats the purpose.

Using your example of the front brakes on a bike, most people are warned what will happen and thus never do it, rather than being flung head-first from it before realising it's a bad idea

 

[BaCaRdi]

Thanks for the explanation

I used to live in West Orange, NJ shame on me.. I should know this..hehe

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).

 

[be80be]

Can kill you

A low-voltage (110 or 230 V), 50 or 60-Hz AC current through the chest for a fraction of a second may induce ventricular fibrillation at currents as low as 60 mA. With DC, 300 to 500 mA is required. If the current has a direct pathway to the heart (e.g., via a cardiac catheter or other kind of electrode), a much lower current of less than 1 mA, (AC or DC) can cause fibrillation. If not immediately treated by defibrillation, fibrillations are usually lethal because all the heart muscle cells move independently. Above 200 mA, muscle contractions are so strong that the heart muscles cannot move at all.


you can read it all here Electric shock - Wikipedia, the free encyclopedia

Funny how I said that at the get go just shows how well people read

 

[giftiger_wunsch]

Funny how I said that at the get go just shows how well people read

The fact that you made your opinion known early on means we're forbidden from discussing the issue further then?

 

[be80be]

The fact that you made your opinion known early on means we're forbidden from discussing the issue further then?

Where did I say stop reading
I said it's funny I posted this at the start and then it was like no one read it
then two pages later some one posted the same as me and it's Like wow
I didn't no that thanks for pointing it out

 

[giftiger_wunsch]

Perhaps it helped that the later explanation was actually provided by the poster, rather than simply quoting wikipedia. Just a thought.

 

[marcbarker]

You don't need to intentionally expose yourself to risk in order to learn how to avoid it. That defeats the purpose.

Using your example of the front brakes on a bike, most people are warned what will happen and thus never do it, rather than being flung head-first from it before realising it's a bad idea

It's about knowledge of risk, not the absolute prohibition of anything and everything deemed to be 'risky'. People who do dangerous sports understand the risks they expose themselves to, and they themselves are responsible for their own survival. On the other hand, people who never take risks are boring old people like bank managers and accountants!

In a security system, the system administrators deliberately allow hackers to penetrate defenCes so that they can be strengthened. An auto-immune system allows 'germs' to attack the host, so that the adaptive immune system learns. I say the same goes for awareness of electric shocks, just like you have a rational fear of risky driving manauvres because of previous 'near misses' which teaches you a lesson.