A spark-protected alternator has an external cavity on an end plate of the alternator housing with commutating (slip) rings on an alternator shaft extension and mating metal brushes positioned within the external cavity. The external cavity is effectively sealed from ambient atmosphere surrounding the alternator, to provide protection from sparks generated between the brushes and commutating rings wherein a minimum area of the housing end plate is utilized. Integral extensions of the end plate form side walls of the external cavity that partially radially surround the shaft extension and commutating rings. A brush holder in which the brushes are attached has projections which mate with slots in the side walls to radially close the external cavity, and an end cap (including a gasket) closes an open end of the external cavity, while the end plate effectively closes the other end of the external cavity. The brush holder forms part of the walls that close the external cavity, and this minimizes the size of the external cavity required to seal the commutating rings and brushes from the ambient atmosphere.