Now the part you all have been anxiously waiting for — actually getting to use meters to diagnose problems.
BUT… before we can do this, we need to always discuss safety. We will sometimes be dealing with live circuits so we need to make sure we do this safely.
Safety is ALWAYS the primary concern for everyone. On the job injuries can be significantly reduced when proper guidelines are followed. Always make yourself aware of all company, local, state and/or OSHA regulations. It is said that 90% of accidents are caused by human error.
Job Site Safety: Keeping the job site clean of trash, extra tools and equipment will significantly reduce the chance for injures.
Lock-Out Tag-Out:Basically put your lock with a tag with your name on it to lock out the disconnect of the equipment you are working on. That way, no one can turn the unit on accidentally while you are working on it.
Personal Safety: Personal safety includes remaining aware of your surroundings at all times, using properly maintained tools and meters.
Personal Protection Devices (PPD) Such As: Hard Hat, Safety Glasses, Gloves, Safety Shoes, Ear Plugs/Muffs, Respirator and Safety Harness. Ensure That Your PPD’s provide the intended protection. Like your tools, they should be inspected regularly, used properly as directed by the manufacturer’s instructions, and never altered or modified in any way.
When working around rotating and moving components, such as motors, pulleys and belts, these can pose a serious risk. Loose fitting clothing, such as neckties, baggy pants, and over-sized gloves should be avoided. Long hair should be tied back or kept under a hat.
Jewelry, such as necklaces, metal watch bands, and rings should also be avoided. If any of these items encounter an energized circuit, serious injury or even death is a likely result.
Work smarter not harder. Every day at almost every job we work around equipment with the power on. Be sure to turn off the power to the unit being serviced and check to be sure disconnects did turn off the power, do not assume it is off.
Extreme caution and common sense are necessary to prevent electrical injury. Electrocution occurs when as small as 6 to 200 mA of current flows through the heart disrupting the its normal operation and causing death. Shock is an injury that occurs to the skin or internal organs as a result from exposure to an electrical current. About 1,000 people die annually of electric shock in the United States. The outcome of an electric shock depends on the intensity, the route, the victim’s state of health, and the speed and adequacy of medical treatment.
Needless to say, SAFETY SHOULD ALWAYS BE FIRST,especially when working on electrical circuits.
In order to make this easier for every one, I have created a very basic Power Point set of slides explaining the basic usage of meters and what the reading you see mean. This is attached at the end of this posting. If you take the time to review it, and have read the previous posts in this series, the proper use of meters should become very apparent.
So what meters do you need to do most electrical checks of a system? You need to have meters the can check voltage, amperage, and ohms. These are the 3 main electrical checks you perform on almost all equipment. Added features like a μF scale for capacitors is always nice, and these can be found on multimeters that have multiple scales on them.
Of course the discussion of analog or digital meters always comes up. I’m old-fashioned and grew up with a Simpson 260 and to this day I will never get rid of my analog meter. The great thing about analog meters is that you can actually see the needle bounce when you have a switch that is making and breaking quickly. Most digital meters do not react quick enough. Most analog meter have selectable scales, so you can really zero in on what you are looking for.
Don’t get me wrong, I also have digital meters. You can get very accurate reading providing you understand how it works. Case in point, I had a contractor who was reading the resistance on a sensor in a heat pump and he said it was 13.2 ohms. If this was truly the case, he had a bad sensor. But then I asked him what scale he was on? He said, “I don’t know, the meter is auto-ranging. All it shows is 13.2”. So I asked if there was a K also on the screen? Turns out, there was a K so the sensor was reading 13,200 ohms which was a good sensor. It’s little errors in the usage of meters like this that can lead to false diagnostics. Always be aware of what your meter is actually telling you. What scale it is reading? Are you on A/C volts or D/C volts? Are you reading A/C amps or D/C amps (like micro amps)?
In order to use meters properly. you need to know what you are trying to read and what scale you are using.
One other common error I get is in reading resistance or ohms. How many times have all of us forgotten that “0” means NO resistance (which is usually a short) and ∞ or OL or 1 is an open circuit. Switches should either be open or closed in most cases. Light bulbs should read some form of resistance.
For specific diagnostics, like compressors, there is a previous posting just on how to diagnose compressors. I would encourage you to go to that posting as see how to accomplish that portion of an air conditioning system.
As I stated in the first posting in this series, I hope no one is offended by these postings and that it refreshes your memories of how to properly work on and check electrical issues. If you would like to see anything more specific, feel free to use the respond section of the post and send me a message asking what you specifically need info on. I will always try to respond to requests for information to help make jobs easier.
Here is the Power Point presentation – hope it helps!