Basic Electricity
All HVAC equipment is electrically powered. As an HVAC technician, you must be able to understand electrical circuits and use electrical test equipment to find electrical problems when they occur.
Electrical power is produced in generating plants and distributed over high voltage power lines. A device called a transformer is used to step the voltage down to useable levels. The power brought into homes is 240 volts.
Once power arrives at the home, it is available as 240 volts for larger appliances and 120 volts for lights and smaller appliances.
Electrical current is the flow of electrons from a negative area to a positive area. When lightning occurs, electrons flow between the ground and clouds because there is a difference of electrical charge or electrical potential between them.
In a battery, a chemical reaction causes one pole to become negative in respect to the other. This develops a potential between the two poles and allows current to flow.
To harness the electrical energy in the current, it must be connected to something that offers resistance to the current. A light bulb is an example of a resistance, also known as a load.
When current flows through a light bulb, the electrical energy is converted to light and heat. The energy consumed by the light bulb, called power, is expressed in watts.
Current is expressed as amperes or amps. Voltage is expressed as volts and resistance is expressed in ohms.
Batteries produce direct current known as DC. The electricity delivered to a residence and the kind most commonly used in HVAC is alternating current known as AC.
Some HVAC products may contain DC circuits. In that case, alternating current can be converted to direct current by devices called rectifiers.
The voltage, current, resistance and power values in any circuit are closely related. Knowing this relationship is important for understanding how electrical circuits operate.
The relationship can be expressed in a simple mathematical formula known as Ohm’s Law. Using Ohm’s Law, voltage, current or resistance can be calculated if any two of the values are known.
For example, a 120-volt circuit contains a 60-ohm resistor. Current can be calculated by dividing the voltage by the resistance for a current of 2 amps. (120/60 = 2)
If the power rating of a load and not its resistance is known, Ohm’s Law cannot calculate the current. Instead, the power formula must be used. The power formula states that power is equal to voltage times the current.
Variations of the formula can be used when any two values are known. For example, if the power rating and applied voltage is known, the current in the circuit can be found by dividing the power rating by the voltage.
The basic electrical circuit contains a power or voltage source, a load such as a light bulb, a switching device and conductors such as wire to carry the current.
Wire conductors are covered with an insulator to prevent electrical shock.
In a series circuit, there is only one path for current flow. The total resistance is equal to the sum of all the resistances in the circuit. In a series circuit, current is the same through all the loads.
In a parallel circuit, each load is connected directly to the voltage source in much the same way as individual rungs are connected on a ladder.
Total current in a parallel circuit is equal to the sum of the individual currents.
The total resistance in a parallel circuit is always lower than the lowest resistance of an individual circuit.
Combination series-parallel circuits can be found in some electronic circuits.
Many devices used in HVAC equipment such as motors, relays, transformers and solenoids depend on the principle of magnetism.
When current flows through a wire wrapped around an iron core, a powerful electromagnet is produced.
Unlike permanent magnets, electromagnets can be turned on and off, which makes them very useful in electrical circuits.
Circuit diagrams, also known as wiring diagrams or wiring schematics, use symbols to represent electrical components. Most electrical components fall into two categories, load devices and control devices.
Load devices consume power by converting it to heat, light or mechanical energy.
Electric motors convert electricity to mechanical energy. They are used in nearly all HVAC systems to drive compressors and to power fans.
Most motors in HVAC equipment operate on AC power.
Electric heaters, sometimes called resistance heaters are used in many HVAC applications.
A common use of electric resistance heaters is in heat pump systems where supplemental heat is sometimes required.
Lights in HVAC systems are used to indicate equipment status.
Switches control the flow of current to loads or other control devices.
The action of switches is described by the number of poles (number of circuits to the switch) and the number of throws (number of circuits fed by the switch). For example, a single pole, single throw switch controls a single circuit.
Heavy-duty switches called disconnect switches are used to turn the main power to HVAC units on and off.
Always confirm with a voltmeter that power is off before working on HVAC equipment, even if the disconnect switch is turned off.
Temperature-controlled switches are known as thermostats. Thermostats can open or close a circuit at a preset temperature. The thermostat in a home heating system closes and turns on the furnace when temperature drops below the thermostat set point.
Pressure switches, also called pressurestats, open or close in response to pressure changes.
Pressures that are too high or too low can damage the compressor in an HVAC system. The most common use for pressure switches is for compressor protection.
A short circuit occurs when current bypasses the load. This extremely low resistance connection causes current to rise rapidly.
Fuses protect against short circuits. They contain a metal strip that melts if the rated capacity of the fuse (in amps) is exceeded.
Special delayed-opening or slow-blow fuses are used in HVAC equipment. They will not open with the brief current surges that occur when motors start. They will only open if the current surge remains.
Circuit breakers serve the same purpose as fuses. Their main advantage is that they can be reset if they trip.
Special HACR circuit breakers are used in HVAC equipment. They have a delayed trip and perform the same function as a slow-blow fuse.
Solenoid valves are valves that contain an electromagnet. When the electromagnet is energized, the valve opens.
Solenoid valves are used to control the flow of liquids or gasses. For example, the flow of water to a humidifier can be controlled by a solenoid valve.
Relays, contactors and starters are magnetically-controlled switches commonly used in HVAC systems.
Relays can have one or more sets of normally open (N.O.) or normally closed (N.C.) contacts, depending on their application. When the relay is energized, the relay contacts change position.
Relays are used in control circuits. A closed switch energizes the relay coil, which, in turn, opens or closes the relay contacts. These contacts, in turn, control a load such as an electric motor.
A contactor is nothing more than a heavy-duty relay and is typically used to control a compressor. Starters are used to start large electric motors and usually contain current overload protection.
Transformers are used to raise or lower voltage using the principle of induction.
Transformers contain a primary winding energized by a voltage and one or more secondary windings where higher or lower voltage is produced, depending on the number of windings.
If the secondary voltage is higher than the primary voltage, indicating more windings in the secondary, the transformer is a step-up transformer. If the secondary voltage is lower, indicating fewer windings in the secondary, it is a step-down transformer.
Overload devices stop current flow if current or temperature limits in an electric motor are exceeded.
Thermal overloads sense the temperature of motor windings and open if temperatures become excessive. Magnetic overloads operate like relays and open if motor current becomes too high.
The possibility of electrical shock is always present when working on HVAC equipment. You must always take the necessary precautions to protect yourself and others from electrical shock.
The amount of electrical current passing through the human body depends on the voltage present and the resistance of the body.
While higher voltages are sometimes encountered, the 115 volts used to operate portable power tools is sufficient to cause severe injury or death.
If your skin is damp, it lowers your resistance which can increase the current flow through your body. Surprisingly, an electrical current of less than one amp can be fatal.
Less than 1mA No sensation.
1mA to 20mA Sensation of shock, possibly painful.
May lose some muscular control
between 10mA and 20mA.
20mA to 50mA Painful shock, severe muscular contractions, breathing difficulties.
50mA to 200mA Up to 100mA, same symptoms as above, only more severe.
Between 100mA and 200mA ventricular fibrillation may occur. This typically results in almost immediate death unless special medical equipment and treatment are available.
Over 200mA Severe burns and muscular contractions.The chest muscles contract and stop the heart for the duration of the shock, followed by death unless special medical equipment and treatment are available.
Circuit diagrams are important for understanding how equipment operates. They come in a variety of formats.
Wiring diagrams show how the wiring is physically connected. They contain helpful information such as individual wire color and physical connections.
A simplified schematic diagram is designed to make troubleshooting easier. The ladder diagram is an excellent example of a simplified schematic diagram.
The layout of the ladder diagram makes troubleshooting with methods such as “hopscotching” easier to follow.
Electronic controls, sometimes called solid-state controls, are rapidly replacing electro-mechanical controls in HVAC products due to their reliability and the enhanced features they offer.
Various electronic components, including microprocessors, are mounted on printed circuit boards. Most printed circuit electronic controls are not field-repairable. If a control fails, it is replaced.
Microprocessor controls offer enhanced capabilities such as remote monitoring of complex systems and self-diagnostic features. Such features are not available with more conventional electro-mechanical controls.
When troubleshooting electrical circuits, it is often necessary to measure voltage, current and resistance, using ammeters to measure current and multimeters to measure voltage and resistance.
Analog meters, which require the technician to interpret a scale, are being replaced by direct-reading digital meters.
Current can be measured by placing a multimeter in line (in series), with the load. This requires that the circuit be disconnected for the meter to be attached and the measurement made.
A clamp-on ammeter enables a technician to safely measure current by simply clamping the jaws of the meter around a single wire through which current is flowing.
Very low current can be measured with a clamp-on ammeter by wrapping additional turns of wire around the ammeter jaws. This provides a multiplying effect, enabling the low current to be read on the meter movement.
A multimeter is a single instrument that can measure voltage, resistance, or current. Analog and digital versions are available.
To measure voltage, the multimeter is connected in parallel with or across the energized component.
When troubleshooting an energized circuit with a voltmeter, attach one lead of the voltmeter to the common side of the circuit with an insulated alligator clip and probe the circuit with the other lead to minimize the shock hazard.
The ohmmeter function of a multimeter is used to check circuit continuity or to measure the resistance of various components. The component or circuit to be measured must be isolated or disconnected from the rest of the circuit.
When using the ohmmeter function, system power must be off. Otherwise damage to the multimeter will result.
If the ohmmeter battery is dead, resistance can still be determined with the multimeter. With the circuit energized, measure voltage across the component in question. Obtain the current reading through that leg of the circuit. Then use Ohm’s Law to calculate the resistance.