© 2003 Liberty Mutual Group – All Rights Reserved 1

Construction Safety Talks

Electrical Grounding

No. 73

How This Talk Applies To My

Crew:

Some Things To Know Before You Give Your Talk.

How many times, as a supervisor, have you heard these words spoken about electrical power tools?

"It's only 110 - it can't hurt me."

There are many accident reports that show that a 110 volt shock can be fatal. It's not the voltage that will get you; it's the current (measured in amps). This chart shows your body’s reaction to various levels of current. A fatal electrocution can occur around 100 milliamps - one tenth of an amp!

less than .001 amp no sensation.

.001 to .002 amps threshold of perception - very slight tingle

.002 to .010 amps muscular contraction (mild to strong.)

.010 to .025 amps painful shock - inability to let go due to strong muscular contractions.

over .025 amps violent muscular contraction.

.050 to .100 amps ventricular fibrillation (can be fatal) - irregular heart beat and pumping

over .100 amps paralysis of breathing (fatal)

The skeptics in the crew are right. A number of conditions must be in place for this to occur.

• the body’s resistance is low because the worker is hot and perspiring.

• the path of the current crosses through or near the heart.

• contact with the energized source and ground must last several seconds or longer. The body may move upon initial shock, breaking the path.

An electrical grounding system helps to prevent electrocution. Check if your job is using an assured grounding system or GFCI.

© 2003 Liberty Mutual Group – All Rights Reserved 2

Explain to your crew the importance of using one of these two systems.

Ground Fault Circuit Interrupters (GFCI)

This device measures current of the hot (black wire) and the neutral (white wire) conductors - they should be the same. If they are not, it means that current is leaking through another path to ground. The GFCI will trip when it senses a difference as little as 5mA (milliamperes).

Nuisance tripping of a GFCI can be corrected with some simple investigation. If a long extension cord is used, the diameter of the conductors may be too small or there may be internal problems between the conductors. A 14 gage 25 foot cord can handle your 13 amp circular saw, but a 50 foot cord such be 12 gage. Look at all connections, are any wet or in water? Test the tool, it may have a fault in it. Test the GFCI, they can also break. But don't bypass it! Never move the GFCI to the tool end of the cord! This leaves the extension cord unprotected.

Assured Grounding System

This requires frequent checking and marking of all tools and cords. Normally done on a job by the electrical sub-contractor. If used on your job, you should explain the operation in detail.

For assured grounding to be used, third wire grounding (complete with grounding prong) devices such as outlets and extension cords, are necessary. This provides a low-resistance path to ground. Should they come in contact with a fault in the cord or tool, the current will take the path of least resistance. In reality it will split into two parallel circuits going to the ground, most through the third wire but some through your body.

Try These For Show And Tell!

Point out the type of grounding you have at your site and explain how it works to the crew. (OSHA allows both an assured grounding system or the use of GFCIs.)

Questions You Can Use To Get Them Talking:

• Do all tools/cords have a third prong (except double insulated)?

• How can I tell if my power tool is double insulated? (by the specification plate on the tool)

© 2003 Liberty Mutual Group – All Rights Reserved 3

Attended By:

The illustrations, instructions and principles contained in the material are general in scope and, to the best of our knowledge, current at the time of publication. No attempt has been made to interpret any referenced codes, standards or regulations. Please refer to the appropriate code, standard or regulation making authority for interpretation or clarification.