AC v DC Shocks

Which is more dangerous, a shock from AC, or DC?

You sometimes come across the view expressed that a shock from a DC source is more dangerous than a shock from an AC source because DC will “hold” you while AC will “throw” you.

Just to be totally confusing you sometimes see exactly the reverse claimed, AC holds while DC throws.

So what is the truth?

The reality is that both AC and DC are, in practical terms, EQUALLY dangerous.

These urban myths started about a hundred years ago with the birth of electrical distributions systems and the contest between Edison's DC system and Tesla/Westinghouse's AC system.

Before either were in common usage a furious marketing battle was fought for some years by proponents of both systems. Edison in particular claimed that Tesla's AC was deadlier than his DC system and was not above some dirty tricks to support his position.

The event that produced this myth was the letting of a contract for an electric chair for which both Westinghouse and Edison bid. But Edison was cunning and deliberately over-quoted so the contract went to Westinghouse, thus allowing Edison to then claim that AC was chosen because it is more deadly than DC - and a myth was born.

In fact the risk that a shock may turn deadly is highly situational and has almost nothing to do with the type of current, AC or DC.

The old saying “It's the volts that jolts, but the mill's that kills” contains the truth that the higher the voltage, and the better the electrical connection, the greater the risk.

Since muscles are electrically activated by nerve currents any externally applied current will cause muscle contraction. The direction of the current has nothing to do with it. So a DC current will cause a continuous contraction, while AC will produce a series of contractions with each cycle.

Because mains frequencies, 50 or 60Hz, are faster than muscle response times, and the voltage generally much greater than the fractions of a volt in the nerves, the result is a gross contraction of the muscles.

If such a gross contraction causes you to grip the conductor you are in serious trouble. But if you happen to touch the conductor in such a way that the gross muscle contraction withdraws your hand, you are in luck.

This gives rise to the rule that if you really must touch-test for voltage you should use the BACK of your hand. That way the grip reflex will be away from the conductor.

Mind you, the contraction may still form your hand into a fist, violently contract your arm muscles, and result in you equally violently punching yourself in the face, and I once scored a bloody nose just like this (but better bled than dead).

More often the violent motion will result in you striking something else, such as the edge of a cubicle or chassis, causing injury, loss of skin, and even a broken bone or two.

Even if you don't get much of a contraction an unexpected electric shock, or even a pin-prick where voltage might be expected, can cause violent movement due to surprise, and I have lost skin to this over-reaction more than once.

This gross muscle contraction is far in excess of any possible normal exertion and a serious shock will result in muscle pain equal to very severe exertion and perhaps even damage to muscle tissue. This is why shock victims feel like they have been kicked by a horse, sometimes for days afterward.

The primary danger with any electric shock is the current that flows through the chest cavity and heart. Since the heart is basically a muscle it is also subject to disruption due to external current, but the heart is a special muscle which follows a complex self-triggered cycle of energisation called the “PQRS wave”.

The biggest danger is that the external current will disrupt this orderly PQRS contraction sequence and the various component muscles will contract in a disorderly fashion resulting in a fluttering called fibrillation, and ceases pumping. This disorderly fibrillation can continue until death.

A current as low as only 5 to 10 milliamps through the chest cavity can induce fibrillation, and even less if the victim already has a heart condition. This is less than the current drawn by the average LED or torch bulb, so don't expect any fuse to save you.

We've all seen the TV shows where the doctor yells “STAND CLEAR!” and applies the paddles of the de-fibrillator to the chest of the patient who then arches their back as the jolt is delivered.

So if an electric shock can cause deadly fibrillation, how come applying more electricity can DE-fibrillate the heart?

The answer is in the degree. Light to moderate currents through the chest disrupt the heart rhythm. But if a very heavy current is applied the whole heart goes into an all-over major contraction like a clenched fist. When this current is stopped the heart relaxes and will often re-commence it's orderly PQRS rhythm.

This 'Emergency Room' drama gives us some clues as to exactly what not to do. The defib' paddles have a large contact area, and contact is enhanced by using a conductive gel. The paddles are placed on the outside of the chest either side of the heart to maximise the current flowing through it.

The defib' itself consists of a high voltage DC supply of around 300-500 volts, a capacitor bank, and a method of setting the time (0.5-1sec) and amount of charge delivered (joules, or watt/seconds).

Make no mistake, defib's can be DEADLY if you don't already have a fibrillation, or if poor contact limits the current, such as between the hands (and thus across the chest).

So this gives us a couple of other potentially life-saving tips; when probing a live chassis put your free hand in your back pocket, not resting on the grounded chassis; but do rest your bare forearm of the hand you are probing with against the chassis. That way any current flow will be diverted before it gets to your chest and heart. It won't hurt any less, but it may save your life.

When you put all the above together it explains why shocks from voltage sources of 1-2000 volts tend to be LESS deadly than 110 or 240. So much current flows through the heart that the shock itself is like the defibrillator action - the heart clamps, then starts beating again when released.

With very high voltages (say above 6.6kV distribution) other factors come into play, in particular burning. The outer layer of the skin is a surprisingly poor conductor, but underneath we are basically a large bag of salt-solution and highly conductive. So if you receive a shock from something sharp that penetrates the skin the shock will be much more severe.

Similarly if a high current is allowed to flow for more than about a second the skin carbonises and the contact resistance drops dramatically allowing a large current to flow resulting in gross heating of the body.

In some cases of lightning strike or contact with very high distribution voltage (22kV upward) the tissue is burnt internally by these high currents, so even if you survive the cardiac shock you may still suffer terrible burns.

And here we encounter the only real difference beween DC and AC. AC passes through zero volts 100 (or 120) times per second, every half cycle, while DC is naturally continuous.

This means that if you strike an arc from DC there is no moment when the current falls to zero where the arc may cool enough to snuff out, so very high voltage DC will tend to follow you as you fall away, maintaining connection for longer.

This is generally only an issue with high energy supplies at voltages in excess of 1000 volts, such as railway traction, but it is a significant factor in the operation of ALL switches and contact breakers.

This is the reason why switches have an AC current rating roughly five times their DC current rating - it is much harder to break a DC current than an AC one because the arc that is always formed when a current is broken (even if microscopic) is snuffed at the next zero-crossing, while with DC a long plasma arc may result.

A spark (such as a car ignition system or lightning) is where air breaks down and conducts, but an arc is where vaporised metal forms a conductive gas cloud and may continue for some time, perhaps seconds, until something else gives up.

Such prolonged arcs are not uncommon around high power valve output stages. The temperature within the arc may be in excess of 6000°C (hotter than Oxy) and lead to explosive expansion and a spray of white-hot metal which is a specific risk to the eyes.

So in summary;

• always check even “dead” wiring with your trusty neon screwdriver before touching,
• then make first contact with the back of the hand.
• When probing live gear be careful not to provide a path across your chest by placing your free hand in your pocket,
• and lean your active arm on the grounded chassis to bypass any shock current to ground before it gets to your chest.
• Don't work on high voltage unless you are WIDE AWAKE and SOBER.

“Beware the lighting that lurks in charged capacitors” and always treat the power mains with the greatest respect for its ability to deliver a huge amount of energy into a fault even before any protection trips.

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