Ohm’s Law and Kirchoff’s Voltage Law

Having a working knowledge of Ohms law and Kirchoff’s Voltage law is important to understand some of the dangers associated with electrical wiring. Lets look at a dangerous practice where many individuals have died that can be explained with the practical application of Kirchoff’s and Ohm’s laws.

Kirchoff’s Voltage Law. The sum of the voltages of a closed (series) circuit is equal to the voltage source. This means that the voltage drop of all resistors (E1, E2…) in a series circuit will equal the voltage source (ES).

ES = E1 + E2 + E3 + E4 ….

Ohm’s Law. The relationship between current, resistance and voltage.

Amperes, I = E/R

Resistance, R = E/I

Voltage, E = I x R

Many in the electrical trade have heard the comment that “that neutral can bite you worst than the hot.” However, this is not true, but the grounded (neutral) conductor can be as potentially dangerous as the ungrounded (hot) wires. Lets look at an example where a person gets in series with the neutral of a 120V circuit.

The resistance of each conductor is 0.20 Ohms (100 ft of 12 AWG), the load has a resistance of 200 Ohms, and the individual has an internal resistance of 1000 Ohms. To determine the contact voltage (potential) between the worker and the two white neutral wires, we need to:

Step 1. Determine the total resistance of the series circuit.

RT = R1 + R2 + R3 + R4

R1 = Hot Wire = 0.20 Ohms each

R2 = Load = 200 Ohms

R3 = Person = 1000 Ohms

R4 = Neutral Wire = 0.20 Ohm

RT = 0.20+ 200 + 1000 + 0.20

RT = 1200.40

Step 2. Determine the current of the series circuit.

I = E/RT

I = 120V/1200.40

I = 0.0999667A

Step 3. Determine the voltage drop of each resistor.

E = I x R

E Line = 0.0999667A x 0.20, E Line = 0.02V

E Load = 0.0999667A x 200.00, E Load = 20.00V

E Person = 0.0999667A x 1000, E Person = 99.96V

E Neutral = 0.0999667A x 0.20, E Neutral = 0.02V

Step 4 Determine the sum of the voltage drops of the circuit.

ET = 0.02V + 20V + 99.96V + 0.02V, = 120V

As we can see, the contact voltage between the two neutral wires is almost 100V, therefore, the electric shock hazard (100V) from the neutrals is not greater than the shock hazard from a hot and neutral (120V). Nevertheless, the danger of electrocution exits any time the contact voltage is in excess of 30V.

Many deaths are caused by the neutral conductor, simply because people do not treat this wire with the same level of respect and caution as the hot wires. In addition, the surprised factor of receiving a shock from the neutral can cause someone to fall off a ladder.

WARNING: A Ground Fault Circuit Interrupter will not protect against a neutral-to-neutral or a line-to-neutral connection.

Ohm's Law Graphic

Copyright © 2002 Mike Holt Enterprises,Inc.
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