Anyone who has ever gotten a jolt from an electric shock probably has a healthy respect for electricity. Electricity likes to flow through a path – we do not want to be any part of that path. Here are the possible paths we would need to avoid: If one of our hands touches a place where there is high-voltage, while 1.) another part of our body is in contact with the ground (whether we are sitting or standing), or 2.) our other hand is touching the metal frame of the TV (which is technically at ground potential). See Psychology Wikia - Electric Shock.
All domestically purchased TVs are guaranteed to be safe because their designs have been tested and approved by Underwriters Laboratories (UL), an independent safety testing agency, for products in the United States, or the Canadian Standards Association (CSA) for products in Canada. As long as you don’t tamper with the TV, or remove the back cover, you will be safe.
But as soon as you remove the back cover, all bets are off. If you plug-in the TV with the back cover removed, you are exposing yourself to potentially hazardous, high-voltage places inside of the TV, and you are therefore putting yourself at risk of electric shock. Professional technicians are constantly aware of this risk when they service televisions. If the repair requires this kind of high-risk exposure, and you decide to take on the challenge of doing the repair, then it is imperative that you are aware of, and acknowledge this risk.
The procedures in this web site that may expose you to the aforementioned risk will display the following warning:
While performing these procedures, you may need to plug-in the TV. If you do, there will be places in the chassis where high voltages exist, and you could hurt yourself (to say the least) if you touch the wrong thing. Yet there are other circuit boards that would be harmless to touch. Just to be on the safe side, the warning will remind you not to touch any circuit board in the TV while it is on.
At the most, the procedures will only ask you to use the probes of a multimeter to touch certain parts of a circuit board to measure voltages at test points. Or you may be asked to use an insulated tool to adjust controls on circuit boards. Most of the time the procedures will ask you to have the TV unplugged if you need to touch anything. For example, if you would need to replace a circuit board, you would remove the back cover, replace the board, and then plug-in and turn-on the TV.
So, I exhort you to heed the warning wherever you see it, exercise good common sense, and don’t be careless.