UNDERSTANDING ELECTRICAL SCHEMATICS

1 INTRODUCTIONIn this section, you will take alook at a packagedgas/electric system, which is a relatively complexheating and cooling unit. The wiring diagram for thisequipment is more difficult than those you studied inPart 1 but again, the schematic as a whole can besimplified by breaking it down into its basic you examine individual control circuits and theassociated components that they operate, the overalldiagram becomes easier to understand, as do thevarious machine functions. The SCHEMATICS in thissection may include some symbols with which youare not familiar. For your convenience, many of theschematic symbols currently used and recognized bythe HVAC/R industry are collected in Figure 16 at theend of this SYMBOLSG enerally speaking, a wiring schematic shows thecondition of a piece of equipment when there is nopower being applied to the unit.

2 Therefore, if a switchis depicted as being normally open(N/O) or normallyclosed(N/C), remember that the position of theswitch is shown as it appears when there is no powerapplied to that circuit. If there is any deviation fromthis practice, there will be an explanatory note on you may know, a switch is characterized by thenumber of contacts (or poles)and the number ofpositions (or throws)it has. Think of the number ofpoles as the number of circuits that the switch cancontrol at one time, and the number of throws as thenumber of paths a single circuit can take. Figure 1A,for example, shows both a normally open and a nor-mally closed single-pole, single-throw(SPST) type of switch either opens or closes one 1B shows a single-pole, double-throw(SPDT)switch. Again, only one circuit can be controlled atany given time, b ut in this case the switch has two dif-ferent connected positions, which means that it candirect current to either of two ServiceEngineers Society1666 Rand RoadDes Plaines, Illinois 60016 UNDERSTANDING ELECTRICAL SCHEMATICSPart 2by Howard L.

3 Pemper, CMS 2003 by the Refrigeration Service Engineers Society, Des Plaines, ILSupplement to the Refrigeration Service Engineers 4A nA. Single-pole, single-throw (SPST)Normally closed (N/C)Normally open (N/O)B. Single-pole, double-throw (SPDT)C. Double-pole, double-throw (DPDT)Figure 1. Switch symbolsA switch that can control more than one circuit at atime is shown schematically as having more than oneset of contacts. Look back at Figure 1C on the previ-ous page. It shows an example of a double-pole, dou-ble-throw (DPDT) switch, which can control twocircuits at the same time. The dashed line representsthe mechanical connection, and tells you that thecontacts move together, but are not connected 2 above shows a few of the many othervariations that are possible in depicting and temperature controls are switches, too,and they also may be configured with various combi-nations of poles and throws.

4 The position of theswitch arm in the schematic symbol indicates theoperation of the control. In Figure 3, for example, thetemperature switch (RS-2) is shown with the armabovethe contacts. This signifies that the switchopens on a rise in temperature and closes on a dropin temperature. The pressure switch (AFS-2) isshown with the arm belowthe contacts. This signifiesthat the switch opens on a drop in pressure andcloses on a rise in example of an SPDT limit switch (LS) is shown inFigure 4. When there is an increase in temperature,the contacts C to N/C move to the N/O the temperature decreases, the contacts C to N/O move back to the N/C electrically operated control switches. Theschematic symbols used to represent relays are thesame as those for manually operated switches,except that relay symbols often include a solenoidcoil.

5 There are several possible ways of depicting thesolenoid coil. Figure 5 shows two different schematicrepresentations of a DPDT relay. Note that multiple-pole relays, like multiple-pole switches, are connectedmechanically but not a type of heavy-duty relay that handleshigher voltages and higher currents than a control2 Double-pole, single-throw (DPST)Double-pole, double-throw (DPDT)Three-pole, single-throw (3 PST)Figure 2. Multiple-pole switches1312RS-2 AFS-2 Figure 3. Temperature and pressure controlsCN/ON/CLSF igure 4. SPDT limit switchrelay. Contactors appear nearly identical to relays onschematic diagrams. Some manufacturers employcontactors that use a single set of contacts. A busbar is placed over the connection where the otherset would be, as shown in Figure 6. Figure 16 at theend of this chapter includes many other symbols forswitches and BASIC DIAGRAMLet s take alook at a generic schematic of a pack-aged heating/cooling unit.

6 In order to illustrate thevarious options that may be possible with a unit of thiskind, the schematic has been put together by takingparts from several different manufacturers. Becauseof its complexity, the schematic is broken into threeparts. Figure 7 (spread across pages 4 and 5) showsthe high-voltage components, and Figure 8 ( pages 6and 7) shows the low-voltage or control circuits. Therenormally is no point-to-point or line diagram with thistype of schematic, but a component layout is oftenprovided. This is shown in Figure 9 ( pages 8 and 9).EQUIPMENTIn order to service any piece of equipment, you firstmust know what you are working with. Even beforeyou begin a visual inspection of the equipment itself,a quick look at the schematic will give you a generalidea of the type of equipment and its components. InFigure 7, for example, it s easy to spot the two com-pressors therefore, you can assume that this is atwo-stage cooling system.

7 Likewise, in Figure 8 youcan see two ignition systems again, you can con-clude that this is a two-stage heating a quick glance at the schematic, you have deter-mined what the unit is. As you become more experi-enced, you also may have a good idea of the kinds ofproblems that you may encounter with a particulartype of you saw earlier, relays can and do have single relay may have afunction in two,three, or sometimes four different circuits. Its contactsmay be located in various parts of the schematic, andyoumust know how to find them if you are to knowhow the unit works. In our generic diagram, severalrelays provide lockouts for the cooling and heatingsections, which means that the two sections cannotcome on at the same control relay CR-1 at the bottom of Figure 8(line 183).Now look at the detail shown in Figure 10A(found on page 10).

8 As you can see, control relayCR-1 has five sets of contacts that are operated byone coil. The first two sets of contacts, CR-1a andCR-1b, are found on lines 42 and 48, respectively(see Figure 10B). The third set, CR-1c, is found online 77 (see Figure 10C). The fourth set, CR-1d, isfound on line 149 in the low-voltage section of theschematic (see Figure 10E). The last set, CR-1e, isfound on line 90 (see Figure 10D). Remember thatwhen a number in the right-hand margin of Figure 10 Ais underlined, it designates a set of normally closed(N/C) 5. DPDT relaysCoilL1T1L2T2 Figure 6. Two-pole contactor with bus bar4T2L2T3L3T1L1 BLKYELBLUBLKYELBLUCOMP-1T2L2T3L3T1L1 BLKYELBLUBLKYELBLUCOMP-2CC-1CC-2TB-1208/ 230-3 IFMOFM-1 CAPT1L1T2L2T3L3 IFMCT1L1 OFMC-1T1L1T2L2T2L2 OFMC-2 YELBLUBLUYELBLKYELBLKYELBLUBRNBRNBRNBLUB LUYELBLKYELYELBLKBLKBLKBLKBLKYELYELBLKYE LBLU115 V208V230 VTRSF-1 TRSF-1 OFMC-1 OFMC-2CB-1110 AGNDOFMC-1C1C2FU-18AC2 OFMC-2C131C2C2 GRYATS13CR-1aSCROFM-2 CAPSCR1234567891011121314151617181920212 2232425262728293031323334353637383940414 24344454622, 2429, 31 Figure 7.

9 High-voltage circuits5 BRN31 BRNGRYGRYCC-1CC-1C1C2C2CC-1C2 BRNTDR-1 TDR-146 ASR-1236 BRNOFC-2 LPS-1 CCTDR-1214613 LMHPS-1CR-1bASR-1 OFC-1 BLU/REDOFC-1 BRNCC-2CC-2C1C2C2CC-2C2 BRNTDR-2 TDR-2C1C2 ASR-223C2 BRNOFC-2 BRNGRYCC HTR-1 LPS-2 TDR-2211313 LMHPS-2CR-2aASR-2CR-1cOFC-2 BLUOFC-2220V120 VHeater220V120 VHeaterCC HTR-1 BRNGRYBLKWHTCC HTR-2CR-2bBRNIFMC-2 IFRHIFRC13151313CR-713CR-1eBRNIDFMRCC HTR-2 IFMCC1C2 IDFMC1C2C1C2C2 IFMC2 USLV-1C2 IDFMC221 CCPSUSLV12123147484950515253545556575859 6061626364656667686970717273747576777879 8081828384858687888990919293941, 3, 49, 10, 1215, 17, 184848, 536363, 68 Figure 7. High-voltage circuits (continued)6111332 BLKBLUWHTYEL115 VTRSF-2 TRSF-2GV-124 VCB-23 AIGN PCBVIOVIOVIOT1T2T2 TRSF-2C2 IGN-1 IGN-2CN/ON/CLSGV-1 GVR-1 CSaRS-1 AFS-1 IGN-1 IGN-1 IGNFS1123 VIOVIOT1T2T2GV-2 GVR-2 CSbRS-2 AFS-2 IGN-2 IGN-2 IGNFSGVR-2 GVR-1 IGN PCBIGN-1 IGN-2CR-7 VIOCR-7C1C2T2959697989910010110210310410 5106107108109110111112113114115116117118 1191201211221231241251261271281291301311 32133134135136137138139140141142143 Figure 8.

10 Low-voltage circuits74546 IFRHC2HR-2bCR-2c101279HR-1c46HR-1b13HR-2 a13312HR-1aCR-1dLOGICTDR-3 IDFMRVIOIDFMRC1C2C2CR-2C2HR-2C2CR-2 VIOHR-1C1C2C2 IFRCVIOIFRCC1C2C2HR-1 VIOHR-2C1C2C2CR-1C2 USLV-1 VIOCR-1C1C2C2CR-2 VIOCR-2C1C2C2 IDFMRLOGICVIOVIOIFRHC1C23CY1Y2W1W2GR1441 4514614714814915015115215315415515615715 8159160161162163164165166167168169170171 1721731741751761771781791801811821831841 85186187188928285158, 146161, 152, 14963, 80, 14642, 48, 77, 149, 90 Figure 8. Low-voltage circuits (continued)8 GVR-2 GVR-1 IGN PCBCY1Y2W1W2 GRC2C1151296314118521310741CR-1C2C163524 1 IFRCC2C1635241CR-2C2C1635241 IFRHC2C1635241HR-1C2C1635241 IDFMRC2C1635241HR-2C2C1635241CR-7T2T1GV- 1T2T1GV-2N/CCN/OLS13RS-113RS-212 AFS-112 AFS-212 CCPSIGN-2 IGN-1 TRSF-1 TRSF-2 Figure 9. Component layout9L2L1L3T2T1T3C1C2CC-2L2L1L3T2T1T3C 1C2CC-1C1C2L1T1L3T3L2T2 IFMCC1C2L1T1L2T2 OFMC-2C1C2L1T1L2T2 OFMC-11234 ASR-11234 ASR-2220 V120 VLMOFC-1220 V120 VLMOFC-2 TDR-1 TDR-2312 TDR-3 CAP-1 CAP-2231897564 COMP-1&2208/230/3-phaseFigure 9.