Edison Screw - Edison Screw-cap Fittings

Edison Screw-cap Fittings

Three-way lamps have a d suffix to indicate double contacts, usually E26d or E27d, or rarely E39d. The second contact is used for the lower-wattage filament of the two inside the lamp. This extra contact is a ring located around the main contact. Unlike bayonet sockets, three-way and regular lamps are interchangeable, although the low filament or low setting doesn’t work if mismatched.

The medium Edison screw has seven threads per inch, or about 3.6mm per thread. In the U.S., the Energy Independence and Security Act of 2007 requirement for greater energy efficiency only applies to the medium Edison screw, all other being considered "specialty" lamps.

Diazed fuses (IEC_60269#D-system_.28DIAZED.29) DII uses the same E27 thread as lamps, but have a longer body and cannot be screwed into a lamp holder (socket). A lamp base is too short to contact the bottom terminal of a fuse holder. However it's possible (but not useful) to screw a DII fuse holder without a fuse in an E27 lamp holder.

Screw bases suffer from two disadvantages. First, the metal screw itself forms one of the contacts for the circuit. If the lighting system is not correctly wired, or a lamp is plugged into a non-polarized outlet, the metal screw can become energized, presenting an electric shock hazard to anyone attempting to change the lamp. Second, if the lamp unscrews in the socket for any reason, it can lose contact with the center contact and stop working until it is screwed in tighter. This is not always a disadvantage, however, as it provides a method for turning a lamp off without a switch.