A look inside a typical Strowger exchange

A look inside a typical Strowger exchange

Strowger exchanges are made from racks 10ft 6in high, although some smaller exchanges used in villages were built from 8ft 6in racks. The racks are arranged in rows, and in all but the smallest exchanges, travelling ladders ran on overhead tracks to give easy access to equipment at the top.

All early Strowger exchange equipment was painted Battleship Grey. In the late 1950s the Post Office tried alternative colours to reflect light better and brighten the atmosphere inside exchanges. The new standard, Light Straw, was progressively used for new equipment from the 1960s onwards — also used for Crossbar, electronic (TXE), and even some early System X digital exchange racks. System X would later revert to grey.

All lines enter the exchange through the Main Distribution Frame (MDF). The MDF is the same height as the equipment racks, with connection blocks on each side. Cables enter from underground via a cable chamber, feeding up through the floor to connection blocks on the "line side" of the MDF, where fuses in each wire protect against power surges.

The other side of the MDF is called the "exchange side." Here, connection blocks terminate cables leading to the Intermediate Distribution Frame (IDF) — the heart of a Strowger exchange, where incoming and outgoing sides connect together. The exchange-side blocks are arranged in phone number order, with jumper wires run between the two sides of the MDF so any outgoing line to a particular street can be assigned any phone number. Lightning protection is fitted here too, along with test jacks that let a line be tested "both ways" to find whether a fault is inside the exchange or outside.