contributed by Omy
Electricity is a lazy SOB. It will every time without exception take the easiest route home. Home being mother earth. Whether that path is through a ground wire, a pot of water, a drain pipe, or the human body. Pay careful attention to what your tying your sub up to.
Safety is paramount as you are not likely to get many second chances if any at all. Even a minor shock can cause you to rebound into a lethal circuit or trip over something. Since it’s an involutary reaction to the shock, you may experience bruises, bone fractures or even death from collisions or falls.
Very little current (the part that can kill you) is required to cause a problem. The current drawn by a 7.5 watt, 120V lamp passing across the chest, from hand to hand or foot, is enough to cause FATAL electrocution.
Table: The effects of electrical current on the body
| EFFECTS | CURRENT in mA (millamps, 1000th of an amp) | |||
|---|---|---|---|---|
| Direct Current (DC) | Alternating Current (AC) | |||
| Men | Women | Men | Women | |
| Slight sensation on hand | 1 | 0.6 | 0.4 | 0.3 |
| Perception ‘let go’ threshold | 6.2 | 3.5 | 1.1 | 0.7 |
| Shock – not painful and no loss of muscular control | 9 | 6 | 1.8 | 1.2 |
| Painful shock – muscular control lost by 50% | 62 | 41 | 9 | 6 |
| Painful shock – ‘let go’ threshold | 76 | 51 | 16 | 10.5 |
| Painful and severe shock – breathing difficult, muscular control lost | 90 | 60 | 23 | 15 |
The effects of current on the body as shown in the Table depends on:
- Circuit characteristics (current, voltage, resistance)
- Contact and internal resistance of the body (no its nothing to due with tolerance)
- The pathway through the body (does the path cross the heart)
- Duration of the contact
- Environmental conditions (are they grounded..standing in water..tied to a water pipe, etc)
To understand the currents possible affect on the human body, its important to understand the contact resistance of skin. The skins resistance can change as a function of the moisture present (more moisture, less resistance). This moisture present can also change during a scene due to chnages in ambient temperatures, humidity levels, level of anxiety, fright, etc.
| Examples | Dry Resistance (ohms) | Wet Resistance (ohms) |
|---|---|---|
| Finger touch | 40,000-1,000,000 | 4,000 – 15,000 |
| Hand holding wire | 15,000 – 50,000 | 3,000 – 6,000 |
| Finger-Thumb grasp | 10,000 – 30,000 | 2,000 – 5,000 |
| Hand around a 1.5″ pipe | 1,000 – 3,000 | 500 – 1,500 |
Body tissue, vital organs, blood vessels and nerve tissue in the human body contain water and electrolytes. These are highly conductive with limited resistance to current. As the resistance of the skin is broken down by the electrical current, resistance drops further and current levels increase.
Cuts, abrasions, blisters, etc. on the skin negate skin resistance, leaving only internal body resistance (which is really low… in the 200-1000 ohms range) to oppose flow. The amount of current passing through the body will be in accordance with Ohm’s Law (remember, Ohm’s Law is ABSOLUTE).
I (current in amps) = V (Volts) divided by R (Resistance in ohms)
Electrical currents passing through the body can cause muscles to lock up, resulting in an inability to release the hand’s grasp of the current source (which is why I have said you don’t always gets a second chance, if the current level is high enough it will hold you there… and if it does that, it’ll KILL you). This level is known as the ‘let go’ threshold current (for those electrically minded, this threshold does vary with frequency).
Playing with DC currents usually causes a single twitch and is considered less dangerous at lower voltage levels than AC circuits. On average, most females have a ‘let go’ limit of 6 – 10 milliamps. Most males have a ‘let go’ limit of 9 – 15 milliamps.
Sensitivity and potential injury will increase with time. A victum who cannot ‘let go’ of the current is much more likely to be electrocuted than someone whose reaction removes them from the circuit more quickly. In short, any prolonged exposure to 60 Hz current of 10 millamps or more may be fatal.
The most damaging path for electrical current is through the chest cavity and head. Fatal ventricular fibrillation of the heart (stopping the heart, in other words) can be initiated by a current flow as little as several milliamps. Death may occur immediately or take several hours. Prompt medical attention is required. I have heard of people who have recieved a serious shock in the morning and never went to the hospital… they felt fine… but they dropped dead at dinner time. Think of the electrical shock as ‘icebergs’. Most of the injury is remaining below the surface, unseen. Note that we are talking about a severe shock where typically there is damage (burns) on the skin. The current can damage internal tissues that over time will swell and can cut off needed circulation. Prompt medical attention can minimise the problems.
Play safe.