A lamp base has a projection that carries lamp contacts and has an outer surface formed with retaining formations arrayed with irregular spacing. A socket forms a recess into which the projection can be inserted with play and apertures provided with contacts engageable with the lamp contacts. Retaining formations in the socket fit with the lamp-base retaining formations in only one defined angular alignment of the lamp base and the socket. The projection has a central plug projecting toward the socket and forming a lamp key coded for a predetermined lamp wattage. The socket has a hole corresponding to the plug and itself forming a socket key coded for the maximally admissible lamp wattage for the socket. The lamp base only fits into the socket when the lamp wattage indicated by the lamp key does not exceed the maximally admissible lamp wattage defined by the socket key.