The contribution of the shape and size of polymers towards ignition for four kinds of polymers, PP (polypropylene), HIPS (rubber reinforced polystyrene), ABS (acrilonitril-butadiene-styrene copolymer) and PC (bisphenorA type polycarbonate) was studied. The critical heat flux (CHF), which means that the specimen ignites after infinite time, depended on the shape and kind of polymer at the plane position heating. But it was not affected by these conditions at the side position heating. The CHF and other phenomena of ignition were analyzed and the two regions in one of which the shape and the kind of polymer affects them and in another region they was not affected. It will be useful to design electrical parts that reduce fire risks. (c) 2007 Wiley Periodicals, Inc.