Btu Buddy 118: Tim Gets a Shock
Now, Bob’s company has promoted him to help train a new employee, right out of a school specializing in HVAC, just like Bob was. Bob is now Tim’s Btu Buddy. Tim is anxious to travel with Bob. Tim realizes that he is right out of school, with the theory and lab work that he accomplished in school, but still needs help. He knows that he worked with many of the components of the systems in the school, under ideal conditions with good light and air conditioning. Now it is into the field, sometimes under the house with poor lighting, or out on the rooftop in the sun, where the real action is. He is naturally and normally reluctant, but he has Bob to help guide him.
Bob and Tim arrived at their fifth service call of the day. They were both wet from servicing a heat pump outdoor unit in the snow. This was a no heat call for a gas furnace. The weather was cold and there was no heat at all in the house. The owners had put water to boil on the stove to stay as warm as they could.
The homeowner met them at the door and explained that the heat shut off during the night. The indoor fan was not running.
Bob and Tim went to the basement workshop where the furnace was for a look around. Tim was standing with his hand on a metal workbench when he reached up and touched the furnace ductwork. He jumped back and said, “I just got shocked. What in the world is going on?”
Bob said, “Don’t touch that ductwork again. It must be electrically hot. Get your meter and place one lead on the workbench and the other lead on the duct and see what you get.”
Tim got his meter and checked and, sure enough, the meter read 115V (Figure 1).
Tim said, “Now what?”
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Bob responded, “The fan is not running, so maybe the fan is the problem. Turn off the breaker and let’s isolate the fan motor and see what we can find. Before you touch the furnace again, after you turn off the power, check the furnace for power to ground again. There may be a bare wire laying on the furnace ductwork somewhere else. Let’s not take a chance on another shock.”
Tim turned to turn off the breaker and said, “This is not a breaker system. It is a fused system.”
Bob then said, “Unscrew the fuse marked furnace.”
Tim unscrewed the fuse and checked for power again, and said, “The circuit is dead now.”
They removed the wires from the furnace motor and using their ohmmeter, checked continuity to ground and found the motor had a circuit to ground inside it.
They replaced the motor and before they started the system up, they looked to see why the ductwork was not grounded. They found that the system was old and only had two wires in some old fiber-covered cable running from the electrical box to the furnace. They replaced this wire to the fuse box with a three-wire cable and wired the green wire to the neutral bus bar, which is grounded. (NOTE: In some code areas this wiring function must be performed by a licensed electrician. Some code areas allow the HVAC technician to complete this service procedure.)
They then started the system up and checked it out, changed the filters, and oiled the motors. It was performing like it should when Tim asked for an explanation of grounding the system.
Bob explained, “From a safety standpoint the object of grounding the system is to protect the heart muscle from electric current flow. It only takes a very small amount of current flow through the heart muscle, actually about 15/1,000 of 1 amp or 15 milliamps, to cause the heart to fail.”
Tim asked, “Why wasn’t I hurt from the shock I received earlier? I felt it all over. It was scary.”
Bob said, “Your whole body shared the electrical flow along with the heart muscle. Never take a chance. It pays to use your voltmeter. I had not noticed the age of this installation when we walked in or I would have warned you about touching the furnace or ductwork before checking it to ground. Any home that was wired earlier than about 1970 is subject to have a two-wire electrical system. The later systems have a green third wire that runs from the frame to the grounded neutral bus bar which is grounded to earth ground. This is called an appliance ground. That was the reason for replacing the two-wire system with a three-wire system. I remember one night when I crawled under a house to work on an electric furnace. I was lying on the wet ground and started to touch the furnace; then I thought, I had better check this to ground with a voltmeter. It was electrically hot. If I had touched that furnace, it may have been the last thing I ever did. A well learned important lesson.”
Tim said, “Yes, that would make a believer out of you.”
Bob said, “Years after the green ground wire was introduced, the ground fault interrupter (GFI) was introduced (Figure 2). These are placed in locations where an unsuspecting person may get an electrical shock. The first time I ever saw one was one time where I had my vacuum pump under a house to evacuate a water source heat pump system. I started my pump and it ran for a few seconds then shut off. I was aggravated. I came out from under the house and told the electrician that the electrical outlet next to the heat pump was defective. He checked it out and disconnected the vacuum pump and reset the GFI. Then he plugged the vacuum pump in and it tripped again. He told me the vacuum pump motor must be grounded. I took it to the truck and checked it with the ohmmeter and sure enough it had a very slight ground.”
Tim asked, “What did you do to finish the job?”
Bob said, “I put a cap on the pump inlet so I could run the pump and plugged it into a circuit that didn’t have a GFI and let it run for about 15 minutes and checked for ground again and the ground was gone. I carried it back under the house and started it again on the system with the GFI and it ran fine.”
Tim asked, “What was the cause of the ground and why did it go away?”
Bob answered, “The vacuum pump was in the truck side compartment and had been there for days in cold, damp weather and had sweated until it was damp. Running it warmed it up and dried it out and the minor ground went away.”
Tim said, “It sounds like GFI outlets are really effective.”
Bob said, “Yes, they have saved many people from electrical shock. Electrical safety devices have all been well thought out and should be heeded. Every device that has been added is because of some major failure where damage has been done. They are vital because electricity is invisible. It does not make a noise to tell you it is present. You will not know it until you either detect it or it bites you. Sometimes the bite is deadly. It is good practice to be able to say ‘I have not had an electrical shock.’ It is not smart to brag about the electrical shocks that you have had.
“Another thing worth noting is that when an electrical circuit runs through a GFI it may continue on to other fixtures. If the GFI trips, it will stop the electrical flow to the other outlets downstream.”
Tim then said, “This business just gets bigger and more interesting all the time. Thanks for the lesson.”
Publication date: 1/21/2013
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