...on People.
Bob's "Stupid Electrical Question" post got me to thinking about electrical shocks and I thought I'd share the following. someone might find this interesting. By the way, I didn't think Bob's question was stupid at all.
Electrical shock involves electrical stimulation of tissueand its effects range from a tingling sensation to the violent reactions of muscle tetanus to ventricular fibrillation. Thus electrical shock is measured in terms of current intensity at specific frequencies.
Macroshock is defined as a high value current level (mA) which passes through the body by skin contact with a voltage source. There must be two points of body contact. If the current passes through the heart, it may cause ventricular fibrillation or death.
Microshock is defined as low-value current (μA) which passes through the body. Generally, intact skin has high enough impedance to prevent microshock level currents from entering the body. However a beak in the skin; a cut or wound or perhaps even a metal sliver could lower the impedance enough to allow small currents to pass. Wet skin also has lower impedance. In this case even micorshock curents can be lethal.
Larger currents are required to cause death from Macroshock because the skin is a relatively good insulator. These are the effects of various current intensities in macroshock. This is at 60 Hz, 1 second contact, arm-to-arm.
1 ma Threshold of perception.
5 mA Accepted as maximum harmless current intensity.
10-20 mA "Let go" current before sustained muscular contraction.
50 mA Pain. Possible fainting, exhaustion, mechanical injury; heart and respiratory functions continue.
100-300 mA Ventricular fibrillation will start but respiratory center remains intact. Death may occur.
6A Sustained myocardial contraction followed by normal heart rhythm. Temporary respiratory paralysis. Burns if current density is high.
Remember this is assuming 1 second of exposure. If you grab a hot conductor and the current intensity is high enough, a sustained muscular contraction might make it impossible to let go of the conductor resulting in much longer exposure.
In contrast to the above, micorshock currents of 10 μA to 100 μA can cause fibrillation of death.
The frequency of the current is also important when considering shock. frequencies between 50 and 60 Hz are particularly potent where as lower or higher frequencies are not as threatening. 1 mA at 60 Hz establishes the threshold of perception for most people and 100mA at 60 Hz may cause respiratory difficulty, ventricular fibrillation and/or death. However if the frequency is raised to 1 KHz, these current levels do not cause such sensations or life threatening phenomenon. High fequencies in the megahertz region, for example, will not cause shock at all. Electrosurgical units operate in this frequency range and do not induce electrical shocks although they can cut, burn or cauterize.
The most danger from shock is ventricular fibrillation. Death can occur within minutes if not treated with CPR or defibrillation. (BTW, contrary to popular belief a defibrillator doesn't restart a stopped heart. It cause a momentary myocardial contraction which when released should 'reset' the heart. It requires a current of 6A or more passing through the chest.)
Leakage Current is defined as the low value electrical current (μA) that inherently flows (leaks) from the energized portions of an appliance or instrument to the metal chassis. All electrically operated equipment has some leakage current. This current is not the result of a fault but is a natural consequence of electrical wiring and components.
Leakage current has two major parts: Capacitive and resistive. Capacitive leakage current results from distributive capacitance between two wires or a wire and a metal chassis/component case. For example the "hot" copper wire forms one plate, the insulation forms the dielectric and the metal chassis (ground) forms the other plate of a capacitor. This capacitor is actually distributed over the entire length of the power cord and the longer the cord the greater the capacitance. A capacitance of 2500pF at 60 Hz on a 120 V power system gives approximately 1MΏ of capacitive reactance and 120 μA of leakage current. Examples of components that cause capacitive leakage current are power transformers, power wires and motors.
Resistive leakage current arises from the resistance of the insulation surround the power wires, transformer primaries and motor windings. Modern thermoplastic dielectrics on power lines and cords are of such high resistance that the resultant leakage current is negligible compared to capacitive leakage current. Older equipment however might exhibit higher resistive leakage current due to less resistive or compromised insulation.
The classical remedy for excessive leakage current is the third or safety ground wire. The hot wire in US systems is black and is the ungrounded wire. The neutral ground wire is white and is the return wire connected to earth ground in the main power/fuse panel. The safety ground wire (which normally carries no current) is green and is the ground current return only under leakage and fault conditions. Actually, two purposes of safety grounding are to drain off leakage current and blow the fuse or trip the circuit breaker in the hot line in case of catastrophic fault (such as hot wire shorts to grounded metal case or overload).
As an example of the effect of safety ground consider a piece of electrical equipment connected to a power system in which the leakage current through a 1 Ώ ground resistance is assumed to be 100 μA. If a person of 500 Ώ resistance touches the metal case, 0.2 μA of leakage current flows through the person and 99.8 μA flows through the safety ground. Clearly the safety ground is a much lower resistance connected in parallel with the person. Hence most of the leakage current flows through the safety ground. If the safety ground connection should become broken or defeated (3 to 2 wire adapter or 2 wire extension cord for example) all the leakage current flows through the person.
You can do the math for 1 A or 10 A of current in the event of a nonexistant safety ground and direct contact with the hot.
It is worth periodically checking the integrity of the safety ground especially in quipment that might get plugged and unplugged frequently. It might also be worthwhile to add an additional parallel safety ground that is independent of the power cord.
Just be careful.
Note: most of the italicized text above comes from my biomed textbooks. It was written with a hospital environment in mind however I think the information is valid for us as woodworkers or as users of electrical equipment in general. The same considerations to maintaining proper grounding of equipment apply. Periodically I check the safety ground integrity of my tablesaw, jointer, bandsaw, etc. These machines get plugged in and unplugged and tend to get moved around my shop so I like to know that the safety ground would be effective in the event of a problem.
Checking the safety ground is a simple matter with a DMM. Just measure the resistance between the ground pin on the plug and the metal chassis of the equipment. Ideally you should see no more than 0.5 Ώ of resistance.
Sorry this is so long.
Reply With Quote