An understanding of electrical safety is essential in order to prevent possible fires, electrocution, etc. Often, when growing up, we are told that we must be careful when using electricity. However, many questions may arise that require answers. For example, in the labs that are conducted in digital electronics, we find that we can touch "live" wires that have a 5 volt potential, and yet we do not feel anything. We routinely handle small batteries without any ill effects. One is left to wonder how much is "safe" and when does it become dangerous? In this discussion, various examples using numbers and Ohm’s Law should help to clarify some of these questions. By measuring quantities and using some simple algebra and arithmetic (i.e. science), we can obtain a good understanding of potential hazards, etc., as well as the importance of remembering some fundamental concepts.

First, let’s review Ohm’s Law. It states that the voltage in a circuit equals the current multiplied by the resistance. This is often written as:

V = IR (Ohm’s Law)

where V represents voltage measured in volts, I represents current measured in amps, and R is the resistance measured in ohms. This law applies to any circuit, including one that may include you!

Electical accidents occur when a person makes contact with a live wire (or device) and completes a conductive path to ground. Hence, it is imperative to keep this in mind in order to prevent accidents. How serious an accident will be is dependent on the variables (i.e., V, I and R) of Ohm’s Law when a circuit is completed.

The amount of current travelling through the body is determined by electrical resistance, which can vary widely. The resistance of (very) dry skin can be approximately 100,000 ohms (100K) per square centimetre. The resistance of wet skin, on the other hand, may be as little as 500 ohms (0.5 K) or less. One can imagine that a person standing in a bathtub full of water, or having a shower, who then reaches out to touch a radio, light switch, etc., can become an excellent conductor for an electrical circuit! (i.e. wet skin offers little resistance to electron flow)

What can happen when current flows through the body? A current as small as 10 to 20 mA (milliAmps or thousandths of an Amp) can paralyze muscles and prevent a person from letting go of a live conductor (e.g. wire, appliance, etc.). Approximately 18 mA (only 18 thousandths of an amp) paralyzes chest muscles and breathing stops (death will occur in minutes unless the current is stopped). About 100 mA will cause the heart muscle to enter ventricular fibrillation (uncoordinated heart contractions that prevent blood from being pumped to the brain, body, etc.) which will also result in death. Keeping these values in mind, let’s apply Ohm’s Law to some examples.

In most homes, schools, offices, etc., the voltage supplied to the wall sockets, lights, etc. is about 120 volts. If we look at the example given above, with wet skin having a resistance of 500 ohms, what current will flow through the circuit? Using algebra to rearrange Ohm’s Law, we find:

I = V / R

Substituting values, we have: I = 120 / 500 = 0.24 A or 240 mA (more than 20 times a lethal amount!)

(i.e. it is a very bad idea to be near anything electrical when wet)

Even hands that are moist from slight perspiration, (a normal situation), the resistance may be only about 1,500 ohms. Again substituting into Ohm’s Law: I = 120 / 1500 = 0.08 A or 80 mA (still 8 times a lethal amount!)

Now let’s consider handling batteries, or live 5 volt wires. Assuming moist hands (i.e. 1,500 ohms), Ohm’s Law tell us that: I = V / R = 5 / 1500 = 0.003 A or 3 mA (much less than half the lethal amount). In fact, most adults can barely detect a current of 1 mA.

In conclusion, we can see that increasing voltage and/or decreasing resistance will lead to an increase in current, which could be lethal. Hence, the safest situation is a very low voltage with a very high resistance, which is the situation we have in the electronics lab (i.e., very low voltage of 5 volts, and very high resistance of 1500 ohms or more, therefore currents are very small). Handling 5 volt circuits is quite safe, but always keep in mind that when using electricity at home, school or at work, thinking about safety is crucial. Learning how electricity works and remembering how to safely handle this powerful tool is an essential skill to master.

Written by E. Montgomery, December 21, 2000