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A look inside a typical Strowger exchange

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

All early Strowger exchange equipment was painted in Battleship Grey. In the late 1950s the Post Office tried out alternative colours which would reflect the light better and make a brighter atmosphere inside exchanges. The colour chosen as the new standard was Light Straw which was progressively used for new equipment from the 60s onwards. Light straw was 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 and has connection blocks on each side. Cables enter the exchange from underground via a cable chamber and then feed up through the floor to the connection blocks on the 'line side' of the MDF. Here fuses are fitted in each wire of the line to protect against power surges

Cable chamber Main Distribution Frame

Cables entering from an underground cable chamber


Line side of Main Distribution Frame showing fuse mount termination blocks. (80 or 100 lines per block)

The other side of the MDF is called the 'exchange side'. Here connection blocks terminate cables leading to the Intermediate Distribution Frame (IDF). The IDF is the heart of a Strowger exchange where the incoming and outgoing sides of the exchange are connected together. The blocks on the MDF exchange side are arranged in phone number order and jumper wires are run between the two sides of the MDF so that any outgoing line to a particular street can be assigned any phone number. Lightning protection is also fitted on the exchange side blocks and there are test jacks which allow a line to be tested 'both ways ' to determine if a fault is in the exchange or outside.

A Strowger exchange

A Strowger exchange

The engineer is standing on a travelling ladder. The racks contain 2 motion selectors which do the actual connecting of the calls as we shall see in a minute. The blue insulated bus bars carry 50v DC power to the rack suites and down to fuse boxes (the black boxes) on the left side of each rack.

This is a light straw coloured exchange!

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