by Nick Gromicko, CMI®
Inspectors should understand electrical concepts in order to perform competent electrical inspections. This article seeks to clarify some elementary electrical terms and concepts that are sometimes confused.
Basic Electrical Measurement Units
- Voltage, measured in volts (V), is the measure of potential energy per unit of charge. Using a “water-in-pipes” analogy, voltage in the electrical system is similar to water pressure in a plumbing system. High "pressure" or voltage in an electrical conductor means that the conductor is capable of delivering a lot of electricity to the user.
Most household current is “pushed” at 120 or 240 volts, although these values are nominal, and considerable variation occurs. Most (but not all) modern electrical equipment can handle small voltage variations and differences without any problem. For example a 240V appliance can usually handle 216V fine. Sensitive electronic equipment may require the installation of a voltage stabilizer.
- Resistance, measured in Ohms (Ω), is the measure of the restriction of flow of electrical current through a material. All materials, except superconductors, have a resistance above zero. Metals have lots of free electrons; therefore, they have a low resistance, so they are used in wiring. In an electrical circuit, it is important to use cables that have a low enough resistance to adequately transfer the necessary current for the application. Thick wires are required for high-power applications because these wires have low resistance.
Consider the incandescent light bulb. Thomas Edison and other early researchers discovered that if resistance in a wire is great enough, the wire heats up and glows and produces usable light. They used this knowledge to create the incandescent bulb, in which a current is applied to a highly resistant, ultra-thin filament, causing it to glow. Standard copper wires, unlike light bulb filament, have little electrical resistance. Yet, even copper wire can glow and start a fire when it is too resistant for the current running through it.
- Amps are a measure of the number of electrons flowing in the same direction along a conductor. Also known as the current, this value is proportional to the applied voltage and the resistance of the material. For example, if a light bulb is connected to a battery, the current flowing through it would be calculated using I = V / R, where I is the current, V is the voltage of the battery, and R is the resistance of the light bulb. This relationship is known as Ohms Law. You can see from this example that in order to double the current flowing in the bulb, you would need to double the voltage applied to it.
- Power is a measure of the overall amount of work being done in a system in relation to time (or energy used per second). In an electrical system, power can be calculated by using the formula P = VI. From this, you can see how the voltage and current in a system relate to the overall amount of power used. The unit of a Watt (W) is equivalent to joules per second; therefore, one Watt is equal to one joule per second.
Alternating Current and Direct Current
- Alternating current (AC) is almost universally used for a home's electrical power. The amount of voltage applied to an AC circuit is constantly changing from zero to a maximum and back to zero in one direction, and then from zero to maximum and back to zero in the other direction. Because voltage is the pressure that causes current to flow, the current will also change from zero to maximum. Each complete change from zero to maximum to zero is called one hertz (Hz). Hertz is often abbreviated as "cps" (cycles-per-second) or Hz, which you will see marked on some electrical devices.
- Direct current is most commonly found in homes in the form of electrical energy stored in batteries. In a DC circuit, the amount of voltage and the direction of application are constant. The amount of voltage is determined by the type and size of the battery. The direction of current flow is also constant and, as in AC circuits, the amount of current flow is determined by the resistance. Batteries convert chemical energy to electrical energy. The chemical energy can be in wet form, as in a car battery, or in dry form, as in batteries used for flashlights, toys and portable music devices. Some batteries are designed to be recharged from an AC source. The voltage from all batteries, unless recharged, will gradually decrease. AC power can be converted to DC power for some uses in the home. The conversion is performed by a device called a rectifier or current converter.
Which one is dangerous: voltage or current?
A common adage goes, “It's not voltage that kills, it's current!" This is essentially correct. However, if voltage presented no danger, no one would ever print and display signs saying: "DANGER -- HIGH VOLTAGE!" It is electric current that burns tissue, freezes muscles, and fibrillates hearts. However, electric current doesn't just happen on its own -- there must be voltage available to motivate electrons to flow through a victim.
High voltage is not inherently dangerous. Track your feet across carpet on a dry winter day and you will charge your body to several thousand volts. If you then touch metal, the resulting static discharge will have a voltage many times greater than a typical home’s electrical system, yet you will be perfectly safe because the current is not sustained.
A person's body presents resistance to current. The following two variables partly determine whether an electric shock will cause bodily harm:
- individual body chemistry. Some people are highly sensitive to current, experiencing involuntary muscle contraction with shocks from static electricity. Others can draw large sparks from discharging static electricity and hardly feel it, much less experience a muscle spasm.
- where contact is made with the skin, such as from hand-to-hand, hand-to-foot, foot-to-foot, hand-to-elbow, etc. An electric shock that travels from one hand to the other will pass through the heart and potentially lead to cardiac arrest. The same current, if it travels through just one hand, will not be as dangerous. Also, contact with a wire by a sweaty hand or open wound will offer much less resistance to current than contact made by clean, dry skin. Sweat and blood are rich in salts and minerals, which make them excellent conductors.
In summary, electrical terms such as volts, amps, ohms and watts describe distinct electrical phenomenon, although they are dependent on one another.