From The Physics Teacher, Dec 1969, p. 505-6. We are indebted to the Honeywell Inc. Corporate Research Center, Hopkins, Minnesota for their kind permission to reproduce sections from their comprehensive safety manual.


The critical factor in electrical shock is the current (Amperes) that passes through the person who becomes part of an electrical circuit. As little as 0.050 A may cause a fatal shock. The resistance of the contact by the body may vary according to circumstances from 100 to 500,000 ohms. The resistance of a person touching two electrodes with dry fingers is about 100,000 ohms, and with both hands immersed in a salt solution the resistance is about 700 ohms.

Because of the many variables in contacts with live conductors, electrical equipment must always be treated with respect. A number of methods may be used for guarding personnel against accidentally contacting live electrical elements. In general, exposed electrical equipment operating at 50 V or more should be guarded by enclosure or location. This not only protects against fatal shock, but keeps shocking currents in the neighborhood of the threshold of perception. The body responds to even small electric currents, the effect of ac being more serious at power frequencies. With 60 cycles ac the threshold of perception has an average values of 1 mA. The "let-go" current is 6-9 mA. Above this value the current causes such a violent contraction of the muscles that the victim cannot release his grip. At 25 mA, breathing becomes difficult or impossible, causing asphyxiation resulting in respiratory paralysis. In such cases the patient can be saved by immediate and continued artificial respiration. Obviously, the electrical supply should be cut off or the patient safely removed from the electrical supply before such first aid is administered.

At 100 mA, the heart is affected by ventricular fibrillation, which is nearly always fatal. Currents of 1 A and more cause severe burns.

To eliminate hazards due to high voltage, terminals should be guarded by a rail or screen and posted with danger signs. Guards should be sufficiently strong and rigid to prevent their being displaced if a person should strike against them.


                                                       Current, in mA
Effect                                              50Hz    60Hz      DC

Threshold of perception through skin.                  1       1       5
Threshold of muscular decontrol let-go current.       15     6-9      70 
Threshold of danger to life from failure of
 heart, respiration                                   20      25      80
Threshold of fibrillation                            100     100     100


Permanent high voltage sources should be will marked with warning signs. Temporary high voltage setups should be marked with bright red tape.


Where it is practical, all inside wiring should be enclosed in conduit or equally effective protection, and firmly attached to sturdy structural elements. The conduit should be grounded effectively.

Switches and disconnectors in circuits supplying hazardous operating equipment that must be shut down for maintenance should be arranged so that they can be locked in the open position to safeguard against their being closed when work is in progress on a unit that they control.

Switches, fuses, circuit breakers, and other control devices should be identified so that their open and closed positions are quickly recognized and the circuits that they control are easily established. Switches should be carefully considered so that they will be installed in places where there is the least possibility of them being operated accidentally. When it is not practical, the blades of knife switches should not be live when the switch is open. In cases where an open switch may ball into the closed position by gravity, the switch should be provided with a restraining device to prevent the switch closing accidentally. If enclosed switches can be used they are preferred.

When a switch and/or fuse is installed in equipment, the switch must be in the hot lead and between the line and the fuse. It is not necessary to break the neutral line with the switch.

In the case of a variable autotransformer, such as a Variac or Powerstat, it is important to observe polarity, otherwise there could be 115 V to ground from the autotransformer output, even though the dial is at zero.


Protection against stray electric currents due to shorts or other faults in electrical systems can be provided by arranging for a previously determined safe path for stray currents to ground.

If a low resistance path to ground is provided, protective devices will be operated quickly and the hazards of shock will be reduced because the voltage to ground of the metal parts of the system will be limited.

Regardless of the quality of insulation in electrical systems there is a possibility that this insulation will break down and cause a short circuit. In some cases, fuses and circuit breakers will automatically open the circuit if a short occurs. This action, however, may not necessarily occur.

Grounding should be provided on all exposed metallic non-current-carrying parts of electrical supply equipment (i.e., frames of generators, transformer cases, switch boxes, conduit, cable sheaths, etc.) carrying 110 V or more.

The grounding circuit must have low enough resistance to operate any fuse or circuit breaker if a short occurs.


Operations where flammable vapors, gases or dust, or explosive substances are present, or locations where such materials are apt to occur in flammable concentrations require that electrical equipment (i.e., switches, motors, wiring, etc.) be safely installed so as to reduce the possibility of arcs, sparks, or overheating causing the ignition of the materials. Plugs that can be de-energized before removal from receptacles should be used. The specifications of the National Electrical Code for Hazardous Locations should be considered the minimum requirement for such situations.

Conductor wires, when exposed to corrosive or damp conditions should be of a type that is resistant to the exposure. An example is the use of portable lamps in damp places. They should be equipped with a socket of noncombustible, nonabsorbent insulating material, a handle and approved cord protected by a nonabsorbent insulator and a basket-type guard to protect the bulb. All extension cords should be the three pole type to provide for adequate grounding. It is preferable to use only electrical equipment approved by the Underwriter's Laboratories, 207 East Ohio Street, Chicago, Illinois for the type of location or operation concerned.


Storage batteries, when being charged, evolve hydrogen as a product of the chemical change that takes place in the electrolyte. In some cases the gas may accumulate in the battery to such a degree that when the caps to the battery's cells are removed, the electrolyte (which is composed of sulfuric acid) will spray out of the opening. It is also possible for an explosive mixture of hydrogen and air to accumulate in the battery especially if the water level is low. Care is required, therefore, in maintenance operations on storage batteries.

Rooms or enclosures where storage batteries are charged should be ventilated to remove explosive gas concentrations. Unless it has been determined that there is sufficient ventilation to remove dangerous accumulations of gases, smoking, the use of open flames, or of tools that are not made of non-sparking metal (nonferrous), and power tools should be avoided in the room. The electric system (wiring, switches, etc.) should meet the specifications of the National Electric Code for this type of hazardous location.