Copyright Goodheart-Willcox Co., Inc. Chapter 17 Electrical Troubleshooting Fundamentals 391 6 at the top of the diagram. The rest of the diagram lists compo- nents and conditions required for proper operation. After list- ing the items, each is verified to either be operating correctly or incorrectly. When an item is checked and found to be oper- ating properly, the item IS NOT the problem, and it is crossed off in the IS-IS NOT chart. An item remains on the chart until you determine that the item IS NOT part of the problem. The IS-IS NOT chart starts with the most obvious items. Figure 17-22 shows the start of an IS-IS NOT chart for electri- cal testing of a heat pump system that is not providing cooling. After beginning an IS-IS NOT chart, the next step is to begin checking the items listed on the chart. In this heat pump example, checking for the correct voltage to the thermostat is the easiest first check. The thermostat dis- play is blank, so the technician notes that the thermostat IS NOT powered up. Therefore, this item is left unmarked on IS-IS NOT chart: it may be the problem! The technician then removes the thermostat cover and uses a DMM to check if there is power to the thermostat. There IS NOT 24 V at the thermostat, so this item remains unmarked on the IS-IS NOT chart. Based on the IS-IS NOT diagram, it is time to check that there is power to the dis- tribution box and that no circuit breakers are tripped. The technician measures voltage at the disconnect box and the contactor for the outdoor unit. The voltage mea- surement IS as expected, so the technician marks off the item on the IS-IS NOT chart. She also confirms that there is proper voltage to the indoor unit. These items are now marked off as IS NOT on the diagram. See Figure 17-23. All of the items on the IS-IS NOT chart have been classified, but the root cause of the problem has not yet been identified. When this occurs, add more items related to the unmarked items to the IS-IS NOT chart. Figure 17-24 shows the next checks performed by the technician. This process would con- tinue until the root cause of the problem is identified. Problem—No Cooling Power to Thermostat Power to Indoor Unit Power to Outdoor Unit Power to 24 V Circuit Breaker Closed Breaker Closed Goodheart-Willcox Publisher Figure 17-22. An IS-IS NOT chart can be a useful tool for electrical troubleshooting. This simple example shows the beginning of an IS-IS NOT chart for troubleshooting a heat pump system. Problem—No Cooling Power to Thermostat Power to Indoor Unit Power to Outdoor Unit Power to 24 V Circuit Breaker Closed Breaker Closed Goodheart-Willcox Publisher Figure 17-23. As you verify that an item IS NOT the cause of the problem, cross it off the IS-IS NOT chart. Problem—No Cooling Thermostat Display Working Power to Indoor Unit Power to Outdoor Unit Power to Thermostat Power to Transformer Power from Transformer Power to Circuit Board Breaker Closed Breaker Closed Power from Circuit Board Goodheart-Willcox Publisher Figure 17-24. Continue adding items to the IS-IS NOT that are related to any items that have not yet been marked as IS NOT.