Electrical safety

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.

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

Reference:
IOE 539: ELECTRICAL SAFETY NOTES
W. Monroe Keyserling, Ph.D. Professor
Industrial and Operations Engineering
The University of Michigan

Any continuous high voltage power supply that is capable of delivering more than 10mA 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 conditions. Paralysis have been reported at voltages below 24 V.

Typical reaction to alternating current at 60 Hz 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 to the current source.

Pulsed current

A total energy less than 16 Joule 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 do 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.