content

The Fundamental Laws of Electricity and Safety

Feb. 1, 2002

Perhaps you're familiar with the myth that getting between the neutrals can hurt you worse than touching the hot. In fact, grounded (neutral) conductors can be as dangerous as ungrounded (hot) wires. The danger of electrocution exists any time the voltage is in excess of 30V. Consider what happens when a person becomes part of the neutral of a 120V circuit. Ohm's and Kirchoff's Law will be the basis

Mike Holt

In our example, the resistance of each circuit conductor is 0.20 ohms (100 ft of 12 AWG), the load has a resistance of 200 ohms, and our accident victim has a skin contact resistance of 1,000 ohms. To determine the voltage (potential) with our accident victim as part of the neutral current path, we need only perform four short calculations:

Step 1. Determine the total resistance of the series circuit using Ohm's Law.

RT=R1+R2+R3+R4
R1=Hot Wire=0.20 ohms
R2=Load=200 ohms
R3=Person=1,000 ohms
R4=Neutral Wire=0.20 ohms
RT=0.2 ohms+200 ohms+1,000 ohms+0.2 ohms
RT=1,200.40 ohms

Step 2. Determine the current of the series circuit.

I=E÷RT
I=120V÷1,200.40 ohms
I=0.0999667A

Step 3. Determine the voltage drop across each resistor.

E=I×R
E Line=0.0999667A×0.20 ohms=0.02V
E Load=0.0999667A×200.00 ohms=20.00V
E Person=0.0999667A×1,000 ohms=99.96V
E Neutral=0.0999667A×0.20 ohms=0.02V

Step 4. Using Kirchoff's Law, verify the sum of the voltage drops of the circuit is equal to the voltage source.

ET=E1+E2+E3+E4
ET=0.02V+20V+99.96V+0.02V=120V

By applying two fundamental laws in four math steps, we have proven:

The voltage potential between the accident victim and the neutral wire is almost 100V.
The electric shock hazard (100V) from the neutral is not greater than the shock hazard from a hot and neutral (120V), but it is just as dangerous.

You must realize that it's not necessary to get between two broken neutral conductors to receive a shock. You can receive the same shock if you contact the end of the neutral at the load and any grounded surface. And remember, even a ground fault circuit interrupter (GFCI) will not protect you against electric shock from a neutral-to-neutral (series) or a line-to-neutral (parallel) connection.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.