MAIN DISTRIBUTION FRAME (MDF)
The Main Distribution Frame (MDF) is where all the external main cables (incoming and outgoing cables from the street) are terminated. Below the floor is the cable chamber and the external cables are fed to the MDF via tubes or through a false wooden floor.
On the line side of the frame there used to be provision for fuses and on the exchange side there was provision for protectors.
Fuses would blow with high currents and were in place to stop power contacts from electrocuting engineers working in the exchange or damaging exchange equipment. Protectors are a device used to retard a lighting hit. The lightning would arc to earth via the MDF metal work and to ground via a lattice of copper earth plates buried under the exchange.
Fuses and protectors are no longer used and normally you would find metal links in place of fuses.
Cables from the MDF that are used for exchange telephone subscribers go to the Intermediate Distribution Frame (IDF), which is located elsewhere in the building. The IDF distributes cables to the exchange equipment.
Byron Telephone Exchange MDF
Schematic showing three floors of a hypothetical telephone exchange.
This is the line side of the MDF
MDF in a London Exchange
The same MDF as above but showing the exchange side in
Last revised: January 22, 2024