Electrical safety
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Work environment
Keep your workspace well lit and tidy. Most serious accidents happen because of a chain of unlikely events. A messy workspace increases the chance of freak accidents that can start a chain of events that results in serious damage.
Tools
The tools should be isolated and clean. Inspect them regularly to make sure there are no cracks in the insulation. Never depend solely on the insulation of your tools, the insulation is an extra layer of protection in case of failure elsewhere.
Effects of electrical current on the human body
Continuous current
The formulas for 0.5% and 99.5% chance of fibrillation caused by electrocution
- I0.5% = 165 / SQRT (t)
- I99.5% = 495 / SQRT (t)
Where Ip is the current in milliamps at which p percent of the population experiences fibrillation. t is the time of exposure in seconds where t is less than 5 seconds.
Reference: IOE 539: ELECTRICAL SAFETY NOTES W. Monroe Keyserling, Ph.D. Professor Industrial and Operations Engineering The University of Michigan
Note that fibrillation is not 100% sure to kill you, but it is likely to do so.
Hand-to-foot resistance with wet and undamaged skin
- Skin resistance (hand) = 500 Ω
- Internal resistance (hand-foot) = 400 Ω
- Skin resistance (foot) = 500 Ω
- TOTAL BODY RESISTANCE (sum) = 1400 Ω
Typical reaction to current passing through a healthy human body
- 1 mA - you can feel it
- 5 mA - pain
- 6 mA - female let go
- 10 mA - paralysis/male let go
---- No longer Safe ----
- 15 mA - hazardous
- 30 mA - breathing stops
- 75 mA - irregular heartbeat
- 100 mA - dangerous to life
- 4 A - the heart stops
- 5 A - death/skin burns
The body reacts differently to alternating current than to direct current. Alternating current might cause paralysis or spasms, direct current might cause clenching that may lock you firmly to the current source.
Any continuous high voltage power supply that is capable of delivering more than 10 mA should be treated with respect. If you get stuck you might die if no one finds you. Traditionally voltages below 48 V is thought to be safe under normal working conditions. Paralysis have been reported at voltages below 24 V. Note that 48 V can under certain conditions give a current of (48/1400) = 34 mA through the body which is enough to stop breathing and will result in death if you are not removed from the current source in time.
Pulsed current
A total energy less than 16 joules is thought to be safe in the meaning of not causing fibrillation in a healthy human. It is known that smaller energies have caused death and serious injury because of secondary effects or illness. Violent spasms and shock are some of the dangers.
Capacitors are not to be trusted when it comes to rated capacity and voltage. A capacitor that is thought to be safe may have a larger capacity than stated and may be overcharged by a significant amount.
Another hazard of capacitors is dielectric absorption. This effect sometimes allows capacitors to recharge themselves without any external power. Any hazardous capacitor should be stored with a short circuit across the terminals in order to prevent this and should always be considered potentially charged otherwise.
High frequency current
Nerve receptors does not respond to high frequencies (response typically drops beyond 1kHz), this has created a myth that the skin effect makes the current flow in the upper layers of skin and is harmless. In reality the human body is not a uniform conductor and the current may take any number of paths through the body and may cause serious damage without any discomfort.
