Safe Use of Electric Powered Hand Tools

Safe Use of Electric Powered Hand Tools

Most people at some time or another have received a low voltage electrical shock and have been fortunate enough to escape with only a slight tingle. Shock hazards are most acute in wet or damp locations or adjacent to large areas of metal.  They develop in a cord when a live conductor in some way becomes exposed or comes in contact with some other metal, causing it also to become capable of giving a shock.

The shock occurs when a person’s body makes a connection between the conductors of a circuit, or between a live conductor and the ground.  The current tends to flow to the ground, and will pass through a person’s body as a conductor.  The severity of the shock depends upon the voltage and current of the circuit and how good a contact is made.  The following table shows the relationship between current and its effect on the human body.

Current Values (Milliamperes)

0-1 Threshold of feeling
1-8 Mild to strong surprise
6-15 Let-go
15-25 Muscular freeze
75-100 Ventricular fibrillation
200-1000 Nerve block, unconsciousness

It must be remembered that there is wide variation in voltage required to send a current though the human body since the resistance of the body varies from 1,000 ohms when wet to 500,000 ohms when dry (Ohm is a measure of electrical resistance of the circuit in which a difference of 1 volt produces a current of 1 ampere).

The risk of electric shock from using an electric hand tool is very great. It can be fatal. Sometimes the windings in the motor become defective and cause a short.  When this happens, heat is generated and the insulation on the coil may be burned through.  If the bare wires inside come into contact with a metal casing, it too becomes energized.  If there is no ground wire to carry the current from the casing back to the outlet, the current will flow into the person holding the tool.  The person may suffer a fatal shock, especially if his/her hands are moist and the ground is damp.

For this reason, all power tools that are not insulated must have a ground wire.  To make sure the grounding is effective, it must be continuous from the tool to the outlet.  There must not be any broken or missing prongs on the plugs, either on the tool cord or extension cords.  The best way of ensuring a good ground connection is through the use of a multi-prong plug to fit into a suitable receptacle, with the grounding terminal well grounded.

At construction sites or during any outdoor work, there should be a Ground Fault Circuit Interrupter (GFCI).  This sensitive device will cut off current even when there is a small short.  A tiny amount of current is all it takes to stop the heart from beating.  Extension cords with GFCI are available.

Double insulated saws, drills and power tools normally have a plastic case, but it is best to read the information on the case to be sure.  They are the only ones that need just a two-prong plug and receptacle.