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A surge protector is an equipment that protects the machines against surges that comes from the source or inside the facility. Inside this device, there is a zinc oxide varistor, which allows the electric current to pass through it if only the voltage is greater than the rated. This passage is proportional to the voltage that reaches it. It guarantees that the output voltage does not increase.
Often, they are used into the main board, where each one takes a phase and, in the end, they are connected together to the ground.
When you want to protect a more sensitive equipment, like a computer, there are also commercially available surge protectors to install directly into the input of computer’s socket. Remember that this device may protect other equipments too, because it kind of absorbs the electric surge.
There are basically two sources of momentary surges: from the outside of the facility (caused by lightning, for example) or from the inside (by powering on and off the machines).
If the surges are coming from the outside, the most technically and economically measure to take is to install the surge protector into the main power box (at the entrance). Thus, the electric surges does not come in to your wiring, and you prevent Joule effects that those surges could cause.
In the other hand, if you have a welding machine, it is better to install the surge protector directly on it. Thus, you ensure that the surges generated by the machine does not reach the other machines (loss prevention through Joule effect is also applied here).
The usage of surge protectors leads to almost zero of energy saving, because the duration of surge current is so quick that it has no time to dissipate (a lot) through the wiring. But this saving depends on how often does the surges appear or are generated.