1.2 What the current actually does to you

1 Four things decide the outcome

OSHA 3075 lists them, and the order does not matter because all four apply at once. The severity of an electric shock depends on the amount of current flowing through the body; the current's path through the body; the length of time the body remains in the circuit; and the current's frequency.

Notice what is not on that list. Not voltage on its own — voltage matters only because it drives current through whatever resistance is in the way. Not how tough the person is. Not how experienced. The body is a resistor, and Ohm's law is indifferent to seniority.

2 The milliampere table

OSHA publishes a table of the general relationship between the amount of current received and the reaction it causes, for current flowing from hand to foot for one second. OSHA credits it to W. B. Kouwenhoven, "Human Safety and Electric Shock", Instrument Society of America, 1968, and reproduces it in OSHA 3075. It is worth reading slowly, because the numbers are far smaller than people expect.

A 15-ampere branch circuit is 15,000 milliamperes. The bottom row of that table — the one marked "death probable" — is 10,000. The breaker protecting the circuit is not protecting you; it is protecting the conductors from overheating, and it will happily deliver a lethal current through a person without ever tripping.

3 The let-go range, and why freezing is worse than being thrown

Somewhere between about 6 and 30 milliamperes, depending on the person, the current stops being something you feel and becomes something that operates you. OSHA 3075 explains the mechanism: the electrical stimulation causes the muscles to contract, and this "freezing" effect makes the person unable to pull free of the circuit.

Then it gets worse in a way most people have never heard: freezing "is extremely dangerous because it increases the length of exposure to electricity and because the current causes blisters, which reduce the body's resistance and increases the current." Duration goes up, resistance goes down, current goes up — a loop that runs while the person cannot let go of the tool.

That is the reason for the sentence in the previous chapter. OSHA: "Longer exposures at even relatively low voltages can be just as dangerous as short exposures at higher voltages. Low voltage does not imply low hazard."

4 Path: why hand-to-hand is worse than hand-to-foot

The table above is measured hand to foot. Current takes the route between the two contact points, and what sits on that route decides the damage. A path that crosses the chest puts current through the heart and the muscles that drive breathing; a path from one hand to the other does exactly that. This is the physical reason behind an old field habit — keeping one hand out of the work, in a pocket, away from any grounded surface — which is not a regulation but is not superstition either.

It is also why standing in water matters so much, and why 29 CFR 1910.334(a)(4) requires portable equipment used in highly conductive locations, "such as those inundated with water or other conductive liquids", to be approved for those locations, and 1910.334(a)(5)(i) forbids plugging in energized equipment with wet hands.

5 Involuntary reaction: the injury that is not electrical

Even a current too small to be dangerous in itself can hurt you badly, because of what your body does in response. OSHA 3075 lists the consequences of involuntary muscle reactions: "a wide range of other injuries from collisions or falls, including bruises, bone fractures, and even death."

1. 1
   A worker on a ladder takes a shock too small to stop his heart.
2. 2
   The involuntary reaction throws him back off the ladder.
3. 3
   The recorded injury is a fall, and often the recorded cause is a fall.
4. 4
   The electrical hazard that started it is never corrected, because nothing on the report points at it.

6 How to tell if a shock was serious — you cannot

This is the part of the chapter that saves lives after the fact. OSHA 3075: "A severe shock can cause considerably more damage than meets the eye. A victim may suffer internal hemorrhages and destruction of tissues, nerves, and muscles that aren't readily visible. Renal damage also can occur. If you or a coworker receives a shock, seek emergency medical help immediately."

Current enters and exits through small wounds and does its damage in between. A worker who took a shock, feels shaky, and says he is fine may have injuries that will not present for hours. On a jobsite, "he got a little tingle off that saw" is not a story to laugh at; it is an incident, it is a reason to take the equipment out of service, and it is a reason for the person to be seen.

7 Frequency, and why AC is the dangerous one

The fourth factor in OSHA's list — the current's frequency — is the one that gets left out of jobsite conversations, and it matters because almost everything on a site runs on it. Alternating current at ordinary power frequency reverses direction many times a second, and that repeated stimulation is what drives the sustained muscular contraction described earlier in this chapter. It is the physical reason the let-go range exists at such low currents.

This is also why battery-powered equipment changes the conversation without ending it. A cordless tool removes the cord, the ground path and the let-go mechanism from the task, which is a genuine reduction in electrical risk. It does not remove the hazard from the room: the panel is still there, the temporary lighting is still there, and the charger plugged into a wet receptacle is still a cord-and-plug exposure under 1910.334.

Two practical consequences follow from the whole table. First, a shock that lasted a long time is worse than a shock that felt severe, so the question after an incident is "how long were you on it", not just "how hard was it". Second, a person who was thrown clear was, in a narrow sense, lucky: OSHA's own table notes that in the let-go range a person can be thrown away from the circuit if the extensor muscles are stimulated — the same event that causes falls also ends the exposure.