Voltages above 30VAC or 60VDC are usually considered hazardous. From an operating safety point of view, this is where spacing rules become very significant.
PCB design trends are always heading towards size reduction and increased component density in the search for miniaturization. Although new technologies and components make this possible, there are times, especially when high voltage circuits are present, when board designers struggle to keep up. This is happening because, high voltage boards used to be separate components in a multiple board system. High voltage circuits require special design rules which include increased electrical clearances that are meant to keep the operator safe. With the current miniaturization trends, the high voltage system has to be placed on the same board and the designer has to find ways to implement these rules.
PCB Clearance is defined as being the shortest path, or distance, measured in air, between two electrical contacts. Sometimes, if the equipment is subjected to intense mechanical influences, like vibration, larger clearance values are required.
Creepage is also defined as the shortest distance between two conductive elements, but measured along the surface of the board or, in general, the insulating material.
Both PCB clearance and creepage requirements depend on a number of factors among which are voltage values and even altitude. In most PCB design software tools all spacing rules are referred to as clearance rules. When working with high voltage boards there is a big difference between the two. Creepage and PCB clearance requirements should be clearly understood as they can be easily violated. For example, creepage requirements are always greater than the corresponding clearance requirements, or at least equal. Board designers often find themselves concentrating on either creepage or PCB clearance rules between two electrical nodes on the board while unintentionally forgetting about the other requirements. Electrical spacing rules have to be under constant supervision during all stages of the board development to make sure they are satisfied and in accordance with the operating safety standards.