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44 The PCB Design Magazine • July 2014 side and the dc secondary circuits within those products. Starting with section 2.10.3, midway through the document, the standard defines what clearances are required so that over-volt- ages, including transients and peak voltages that may be generated within the equipment, cannot cause faults. A series of ta- bles, 2J through 2M, explains how to calculate clearances with respect to the details of your application. Next, sec- tion 2.10.4 explains how to determine the spacing re- quired for creepage. Clearance vs. Creepage What's the difference between clearance and creepage? Clearance is the shortest distance between two conductors measured through air. Creepage is the shortest distance be- tween two conductors, measured along the surface of the insulation separating them. Consider the ways an electrical fault between conductors can occur if the clearance or the creep- age spacing is too small. If the clearance is too small, a transient over-voltage event can result in arcing between the conductors, especially if there is dust in the air or humidity. This is a virtually instantaneous fault that does not recur until another such over- voltage event. Faults result- ing from insufficient spacing for creepage can take much longer to occur. Over time, usu- ally stemming from dust buildup and moisture, a path (in other words, a track) for current to leak from one conductor to another can devel- op and cause a gradual breakdown of the sur- face of the insulating material between them. The mechanism for this is not a voltage spike— though that could contribute—but the contin- ual presence of high voltage combined with an insulating material whose comparative tracking index (CTI) is too low. CTI is a measure of a material as an electrical insulator and is expressed as the voltage at which the surface of a material breaks down under a standardized test. The manufacturers of PCB laminates publish CTI figures for their products. For convenience, the break - down voltages are lumped into six categories: The highest rating is 0 for with- standing values of 600V and greater and the low- est is 5, for less than 100V. For example, conventional FR-4 has a CTI of from 175– 249 volts, which places it in rating category 3. The paramount objec- tive of all electrical safety standards is of course to prevent any danger of shock. If no one will ever be near a circuit while it is powered, the clearance and creepage allowances among its conductors that are subject to hazardous voltage can be the mini- mum that ensure the cir- cuit will function and not deteriorate. IEC-60950-1 specifies five increasing levels of mandatory insu- lation related to the physi- cal location of a circuit relative to human contact, the voltage, and the operat- ing environment. Naturally, the best tactic to avoid the dan- ger of shock or circuit degradation is to space traces as far apart as possible but very often that distance will not meet creepage requirements. What can be done in those cases? If turning to a material with a better CTI is not feasible, routing a slot in the space between design for manufacturing Consider the ways an electrical fault between conductors can occur if the clearance or the creepage spacing is too small. If the clearance is too small, a transient over-voltage event can result in arcing between the conductors, especially if there is dust in the air or humidity. This is a virtually instantaneous fault that does not recur until another such over-voltage event. Faults resulting from insufficient spacing for creepage can take much longer to occur. " " CONSIDERINg CREEPAgE AND CLEARANCE continues