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.

Click here for the full range of Protective Devices and Fuses

Click for an article on protection - 1961

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
This obviously a small exchange as the MDF is of a small size and not even half populated.
There is an engineer working on a test desk.
The test desk is testing facility which can be connected to any line.
The blocks on the rear are where the external cables terminate and the front blocks are internal terminations to the IDF.
They are cross connected by a twisted two wire called a "Jumper" wire.
Note the travelling ladder - all MDF's were fitted with these.


Schematic showing three floors of a hypothetical telephone exchange.
Basement shows the incoming cables from the street rising up to the MDF.
The Ground floor (centre part) shows the MDF.
The first floor shows the IDF to the left and exchange equipment to the right.


This is the line side of the MDF
These are fuse mountings (old style) - not all are populated with fuses
They are numbered and this is called the Bar Pair


MDF in a London Exchange
A large exchange judging by the length of the MDF
This is the exchange side


The same MDF as above but showing the exchange side in close up
These are lightning protector mountings



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Last revised: January 22, 2024