Electric shocks, are they dangerous?

[Quandary]

When I was a sixth former I did a few tests on my classmates. They all survived, but...

Being a bit more serious ... there was a huge variation in the resistance of the subjects. A factor of 12 between highest and lowest across a relatively small sample. So some people are very much more vulnerable to electric shocks than others. I did find that those of us who messed around with electrics and electronics had a far higher resistance than the others, but that could've been just a coincidence (just pre-empting JD questioning the statistical significance of my measurements )

I remember you, I was in primary school and you were about 15, you had bolt on hand and ankle cuffs made from metal Meccano strips and what I now recognize as a magneto. You used to offer to excuse some of my mates torture if they captured and held us down while you turned the handle. You were a right B******.

 

[lpdsn]

... and what I now recognize as a magneto.

Too obvious.

What you need is a big AF choke. Leave them wondering how you gave them a shock from a 1.5V battery.

 

[LittleSister]

you had bolt on hand and ankle cuffs made from metal Meccano strips and what I now recognize as a magneto. You used to offer to excuse some of my mates torture if they captured and held us down while you turned the handle. You were a right B******.

There are people who will pay good money to get that sort of treatment!

 

[DownWest]

I had a similar problem with an old OB motor. OK at tick over, but a hefty belt as soon as you opened the trottle. After scratching my head, I figured a bad joint in the HT circuit. Finally guessed at the plate that carried the coils and contact breakers and rotated to vary the timing. Put a flexible wire between the plate and the block, which cured it.

 

[penberth3]

There are many variations to the dangers of electrocution. AC 50 hz is most dangerous because the alternating voltage tends to make muscles of the hand or arm clench so you can not let go the source. DC or a single pulse as in ignition is not so dangerous because it generally makes you jerk away from contact...

I would avoid touching anything electrical, but I'm sure you've got that the wrong way round. DC current, which is obviously in one direction, causes muscles to grip and is the most dangerous.

 

[elton]

I would avoid touching anything electrical, but I'm sure you've got that the wrong way round. DC current, which is obviously in one direction, causes muscles to grip and is the most dangerous.

AC cause the muscles to freeze, DC cause them to either to tighten or release, depending on the direction of current.

 

[macd]

AC cause the muscles to freeze, DC cause them to either to tighten or release, depending on the direction of current.

Regardless of muscle effects, DC is inherently less lethal due to its lesser effect on the heart (but see below). This is one reason that nice Mr Edison was happy to promote the use of the electric chair. His mains electricity was 110V DC. His rivals used AC, the same as the chair. Unfortuntely for him, frying people didn't have the PR voltage he hoped and AC prevailed.

Incidentally, I don't think muscles give a stuff about direction of DC current, any more than did Sr Galvani's frogs.

This research paper gives an interesting low-down on this and the 'let-go phenomenon': https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2763825/
(Note the bits I've placed in bold, some of which were news to me. The 'let go' findings quoted refer to voltages of less than 600.)

A factor that makes a large difference in the injury sustained in low-voltage shocks is the inability to let go. The amount of current in the arm that will cause the hand to involuntarily grip strongly is referred to as the let-go current. If a person's fingers are wrapped around a large cable or energized vacuum cleaner handle, for example, most adults will be able to let go with a current of less than 6 mA. At 22 mA, more than 99% of adults will not be able to let go. The pain associated with the let-go current is so severe that young, motivated volunteers could tolerate it for only a few seconds. With current flow in the forearm, the muscles of flexion and extension are both stimulated. However, the muscles of flexion are stronger, making the person unable to voluntarily let go. Nearly all cases of inability to let go involve alternating current. Alternating current repetitively stimulates nerves and muscles, resulting in a tetanic (sustained) contraction that lasts as long as the contact is continued. If this leads to the subject tightening his or her grip on a conductor, the result is continued electric current flow through the person and lowered contact resistance.

With alternating current, there is a feeling of electric shock as long as contact is made. In contrast, with direct current, there is only a feeling of shock when the circuit is made or broken. While the contact is maintained, there is no sensation of shock.** Below 300 mA DC rms, there is no let-go phenomenon because the hand is not involuntarily clamped. There is a feeling of warmth while the current travels through the arm. Making or breaking the circuit leads to painful unpleasant shocks. Above 300 mA, letting go may be impossible. The threshold for ventricular fibrillation for direct current shocks longer than 2 seconds is 150 mA as compared with 50 mA for 60-Hz shocks; for shocks shorter than 0.2 seconds, the threshold is the same as that for 60-HZ shocks, that is, approximately 500 mA.

** Elsewhere the article states that damaging interruption of heart rhythm is typically limited to a "vulnerable" phase of the cardiac cycle. Since the shock effect of DC is brief and transient, it will often not coincide with this phase. With AC (i.e. continuous) shocks lasting more than one cardiac cycle, it inevitably will.

 

[PuffTheMagicDragon]

Would it not be more assuring if you simply changed the leads and then kept them clean?

 

[Spirit (of Glenans)]

[/QUOTE]It is possible that the shock is coming from a lower voltage part of the ignition system which comes out as a wire which is grounded to stop the motor[/QUOTE]

This points to a problem with the wiring of the on/off switch.

 

[Spirit (of Glenans)]

Not boaty, but many years ago I was standing on an aluminium stepladder in a chemist shop window, working on a display. There was a small neon sign, advertising a brand of cosmetics, in my way. I attempted to move it slightly, whereupon the wire, (carrying probably 10Kv), fell off the terminals. striking at the stepladder least three times on the way down. Didn't half make me jump

 

[Daydream believer]

My father would hold some unsuspecting persons arm or hand & remove the HT lead on a vehicle that was ticking over & put his thumb on the contact. The current used to run through him & make the unsuspecting byestander jump.

 

[earlybird]

I once chartered a boat with a Stuart-Turner twin. Raising the anchor regularly delivered quite sharp electric shocks from the chain. Someone eventually happened to notice a fat spark jumping from the engine HT lead, which was damaged near the magneto (IIRC) connection. When fixed, the shocks stopped and the engine ran much better.

 

[lpdsn]

Would it not be more assuring if you simply changed the leads and then kept them clean?

Sensible advice, but I don't think you're really entering into the spirit of the thread.