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Breaking Down Complicated Schematics

Oct. 11, 2012
The symbols in an electrical schematic — such as controls, switches, relays, and circuits — provide signposts to the form and function of an HVACR system

Welcome to Part 2 of a four-part series of excerpts from the Refrigeration Service Engineers Society publication, “Electricity for HVACR Technicians.” (You can find Part 1 here). This publication won a Contracting Mechanical Systems WEEK Product Showcase Award in 2009, in the category of Education. NOTE: this series is not intended to serve as a replacement for concentrated, formal classroom and/or field training by a qualified electrical professional. This article will provide a review of reading complex electrical schematics. As you examine individual control circuits and the associated components that they operate, the overall diagram becomes easier to understand, as do the various machine functions.

Switch Symbols
Generally speaking, a wiring schematic shows the condition of a piece of equipment when there is no power being applied to the unit. Therefore, if a switch is depicted as being normally open (NO) or normally closed (NC), remember that the position of the switch is shown as it appears when there is no power applied to that circuit. If there is any deviation from this practice, there will be an explanatory note on the schematic.

A switch is characterized by the number of contacts (or poles) and the number of positions (or throws) it has. Think of the number of poles as the number of circuits that the switch can control at one time, and the number of throws as the number of paths a single circuit can take.

Figure 2-1A shows a normally open and a normally closed single-pole, single-throw (SPST) switch. This type of switch either opens or closes one circuit. Figure 2-1B shows a single pole, double-throw (SPDT) switch. Again, only one circuit can be controlled at any given time, but in this case the switch has two different “connected” positions, which means that it can direct current to either of two paths.

A switch that can control more than one circuit at a time is shown schematically as having more than one set of contacts. Look at Figure 2-1C. It shows an example of a double-pole, double-throw (DPDT) switch, which can control two circuits at the same time. The dashed line represents the mechanical connection, and tells you that the contacts move together, but are not connected electrically. Figure 2-1C shows an example of a double-pole, double-throw (DPDT) switch, which can control two circuits at the same time. The dashed line represents the mechanical connection, and tells you that the contacts move together, but are not connected electrically. Figure 2-2 shows a few of the many other variations that are possible in depicting multiple-pole switches.

Pressure and temperature controls are switches too, and they also may be configured with various combinations of poles and throws. The position of the switch “arm” in the schematic symbol indicates the operation of the control. In Figure 2-3, for example, the temperature switch (RS-2) is shown with the arm above the contacts. This signifies that the switch opens on a rise in temperature, and closes on a drop in temperature. Note that the pressure switch (AFS-2) is shown with the arm below the contacts. This signifies that the switch opens on a drop in pressure and closes on a rise in pressure.

An example of an SPDT limit switch (LS) is shown in Figure 2-4. When there is an increase in temperature, the contacts “C” to “NC” move to the “NO” position. When the temperature decreases, the contacts “C” to “NO” move back to the “NC” position.

Relays are electrically operated control switches. The schematic symbols used to represent relays are the same as those for manually operated switches, except that relay symbols often include a solenoid coil. There are several possible ways of depicting the solenoid coil. Figure 2-5 shows two different schematic representations of a DPDT relay. Note that multiple-pole relays, like multiple-pole switches, are connected mechanically, but not electrically.

A contactor is a type of heavy-duty relay that handles higher voltages and higher currents than a control relay. Contactors appear nearly identical to relays on schematic diagrams. Some manufacturers employ contractors that use a single set of contacts. A “bus bar” is placed over the connection where the other set would be, as shown in Figure 2-6.

Series Circuits
In a series circuit, components are arranged one after another, so that the same current flows through all of the components in one continuous path. Figure 2-7 shows an example of a number of switches and a relay coil connected in series. The direction of current flow is indicated by the arrows. Notice that the current must pass through all of the switches before it can energize the coil. If any of the switches is open, the coil cannot be energized.

Parallel Circuits
In a parallel circuit, there are two or more separate paths or branches for current flow. A parallel circuit arrangement allows any one of a number of controls or switches to energize a load. Look at Figure 2-8A, for example.

When either of relay contacts IFRH or IFRC is closed, current will pass through the circuit and energize coil IFMC. However, note that both IFRH or IFRC must be open in order for the coil to be de-energized.

A single switch can energize several loads at the same time in a parallel circuit. All of the loads will get the same amount of current when the switch closes. In Figure 2-8B, for example, when switch CR-1a is closed, current will pass through the circuit and energize coil OFMC-1. And as long as thermostat ATS is closed, coil OFMC-2 will be energized.

Series Parallel Circuits
A series-parallel circuit is a combination of series (or single-path) and parallel (or multiple-path) circuits. Such a circuit allows some operations to proceed while stopping others. Series-parallel circuits are primarily used in control and safety applications. The detail shown in Figure 2-9 illustrates a series-parallel circuit. When switch CR-1b closes, current flows through both branches of the circuit. As long as all of the switches ASR-1, OFC-1, HPS-1, and LPS-1 are closed, current passes through the upper branch and energizes coil CC-1. If any of the switches is open, however, the current will be allowed to pass only through the lower branch to coil ASR-1. With all of the switches in the upper branch closed and coil CC-1 energized, current simultaneously passes through the TDR-1 contacts to the TDR-1 relay. This is a “delay-on-make” time-delay relay, which means that after a specified period of time, the TDR-1 contacts will open. This type of control is commonly used for low-pressure bypass during low ambient conditions.

When coil CC-1 is energized, note that current also passes through the 120-V resistor, through the resistor marked “Heater,” and through differential pressure control OFC-1. This is an oil failure control. When oil pressure is sufficient, differential pressure control OFC-1 will open, thus taking the heater out of the circuit and preventing the OFC-1 contacts from opening.

Connections & Wiring
In order to conserve wire and space, some manufacturers terminate more than one coil connection at the same point. This practice is depicted schematically in Figure 2-10A, where the terminals from the IDFMR and the IFRH are taken off the same terminal connection. Note that the schematic shows two connections or wires at that point (the arrows point to the connection points). Because line diagrams are not used in complex schematics, the component diagram will show the terminal connections on the relays and other devices (see Figure 2-10B).

In some cases, a control or relay may not be shown as a replaceable item, or even as a component that can be tested. In Figure 2-11 for example, the timing contacts for the igniters are on the printed circuit board, but they cannot be physically replaced. The contacts themselves are usually shown enclosed in a “box” drawn with dashed or dotted lines. This same approach is used for the gas valve relay shown in Figure 2-11, and in some instances for fan relays as well.

To learn more about Electricity for HVACR Technicians and other educational offerings from RSES, visit Click the “eLearning” link for online versions of this course.