When I was an undergrad, I did a lot of electrochemical etching. I put it on my resume for years, because interviewers were always interested in hearing about working with high voltage sources and terrifying chemicals.
That was when I started looking into high voltage design. I was amazed by the standards that are required for high voltage products. I was also relieved. While I can’t prevent grad students from McGyvering our high voltage products, I do have peace of mind knowing that there is board level protection in place.
Not every PCB design has the same rigorous rules on spacing that a high-voltage PCB design needs. In general, if the normal operating voltage of your product meets or exceeds 30 VAC or 60 VDC, then you should be very diligent about spacing rules in your high voltage PCB design. If you have a high-density board, especially with high voltages, you need to be even more concerned. PCB trace spacing requirements already help prevent signal integrity problems, but they are also important to prevent arc discharge. The high density makes spacing much trickier, and even more important for protection.
Spacing is more important in high voltage PCB design rules because a potential difference between conductive elements on a board can create an arc if the voltage between the conductors exceeds the breakdown voltage. Any arc that does occur poses a significantly higher risk to both the product and your users. To help mitigate that risk, there are standards for two primary spacing measurements in your PCB design: clearance and creepage distance.
When the voltage between two points overcomes the breakdown voltage, arcing can occur, damaging your product creating a safety hazard for users. Spacing on your board is a critical design parameter for preventing arcing.