I would say in my experience most electrical problems with pos systems are due to the owners negligence.
Here are some free ideas that you can do to protect your computers and POS equipment.
1) Do not overload your electricity plug like these people did.
Shift some of the items to another plug preferably on a different circuit.
2) On the same circuit try not to put the computer with printers, air conditioner units, refrigerators, etc. As a rule of thumb, if the electric item has moving parts, it can create an electric surge. Especially keep computers away from heavy-duty machinery, for example, commercial refrigerators and elevators.
2) Do not switch everything on at once. When an electrical device gets switched on what it does is create a power surge. This surge can go into other electrical machines. I have seen a defective air conditioner fry a computer.
3) Make sure the cables are not damaged; this is typically a problem with old cables.
4) Turn the computers off at night if not needed, besides saving power and giving you better security as hackers cannot get in if the computers are off, if an electric surge happens when the computer is switched off, it probably will not affect the computer.
5) Be careful of liquids near your computer or its equipment, I have seen what a milk shake did to a pos system when a customer accidentally spilt some of it on the point-of-sale desk. Clearly, some items are not meant to be scanned. In the event of such a spill turn everything off immediately in the area.
6) Do not move, open or rearrange your electric items while they are switched on. The other point is that the hard drive in the computer can get damaged when it's moved while its working.
7) Finally do something if you see something is wrong, for example, you smell of burning plastic, see sparks, black scorch marks, have fuses blowing regularly, etc.
8) If you have a power board with a surge protector, use it.
9) If you have a UPS, make sure the computer is on the circuit.
If you have any more ideas, please let me know.
