Electrical clearances are an important requirement for all boards. Too tight a clearance between tracks and pads may lead to hair-line shorts and other etching problems during the manufacturing process. These can be very hard to find once your board is assembled. At least 15 thou is a good clearance limit for basic through-hole design, with 10 thou or 8 thou being used for denser surface mount layout.
As a rule of thumb, an absolute minimum of 8mm equals to 315 thou spacing should be allowed between 240V tracks and isolated signal tracks. The clearance varies based on whether the tracks are on internal layers and the external surface. They also vary with the operational height of the board above sea level, due to the thinning of the atmosphere at high altitudes. Conformal coating (a non-conductive spray often applied over the tracks to resist moisture, corrosion, etc) also improves these figures for a given clearance.