What are the most important safety precautions when working with power supplies?

They are required under the NEC (National Electrical Code). At the first sign of trouble with your business' power supply, the switch instantly disconnects power, preserving your business from a variety of dangers including surges, short-circuits, and potentially harmful overloads.

The large capacitors in a PSU can store power for hours or even days, even if the wall switch or the PSU switch are turned off and the power cable is pulled out. In short - be very careful around the legs of the large capacitors, as they can really pack a punch! Be sure to short the legs of capacitors using eg. a screwdriver while making sure you're isolated from that circuit. That will discharge the capacitors and make it safe to proceed.

Muy importante el uso de un sistema de puesta a tierra para proteger todo aparato eléctrico y/o electrónico. Esto ayuda incluso a proteger a los usuarios.

For those who work in electric workshops and other such factories, make sure to keep a pair of insulated rubber gloves with you all the time. Make sure that you are wearing one all the time when dealing with the electrical circuit or branch circuits. Never deal with anything at your workplace with bare hands

Use proper grounding techniques: Always use proper grounding techniques when working with high voltage power supplies. This will help to prevent electrical shocks and fires. Don't touch high voltage components: Never touch the high voltage components of a power supply, such as the high voltage transformer or high voltage capacitors. These components can store a dangerous amount of electrical energy and can cause serious injury or death. Don't work alone: Never work on a high voltage power supply alone. Always have a qualified person nearby in case of an emergency.

It is very important to check whether the equipment operates on direct current or alternating current. Connecting a device that operates on an AC power supply to a DC power supply, or vice versa, may damage the device or power supply. Even if a DC power supply is required, it is not recommended to use it near heat-sensitive equipment because the series power supply is a linear power supply and generates heat. On the other hand, switching power supplies can suppress heat generation, but they generate a lot of noise, so you should avoid connecting devices that may malfunction due to noise.