MDF AND IDF with Protective Devices |Main Distribution Frame and Intermediate Distribution Frame


MDF AND IDF with Protective Devices |Main Distribution Frame and Intermediate Distribution Frame


MDF AND IDF with Protective Devices

Read : Main Distribution Frame MDF|Basics of Telephone Exchange



 Main Distribution Frame (MDF) MDF stands for “Main Distribution Frame”. 

In standard practice, the MDF is created for the system side and far side, and cross-connect between the system side and far side tags is done by an open (red-white) jumper. 

 MDF is the first distribution point (DP) from the Telephone Exchange to subscribers. 

All the incoming cables from subscriber telephones and trunk lines from the other Telephone exchanges are terminated on one side of MDF known as the “Cable side”. 

The cables coming from the exchange equipment room are terminated on the other side known as the “System side”. 

These are terminated on KRONE TERMINAL BLOCKS. 

MDF uses IDC (Insulation Displacement Connector) terminal strips on which pairs of wires can be terminated. 

IDC terminal strips contain two parallel rows of terminals in the same molded plastic block.

 The terminals in one top row are internally connected by metallic strips to the corresponding terminals in the second bottom row.

 However, the connection is made through contacts by the metallic strips that may be broken by the insertion of a monitoring plug or a special disconnection plug known as a” Wedge Connector”.


IDC insertion tool:

It is used for making connections to the IDC terminals by punching the Cable pair. 

The two features of the tool are, 

a) It includes a scissor-action cutter that is designed to cut off surplus wire after the termination has been made. 

This occurs automatically as part of the punch-down action. 

b) It is provided with a folded-out metal hook for removing the wires from the terminals. It is shown in the figure

Purpose of MDF- Main Distribution Frame

 a) All incoming lines from subscribers and outgoing lines towards exchange are terminated at MDF 

b) It is a place suitable for testing the line side and equipment side. 

c) In case of cable pair failure, the line can easily be transferred on spare pairs available


 The incoming and outgoing lines are terminated at MDF on IDC disconnection module. 

These are available in capacities of 50 pair &100 pair. 

IDC disconnection module of Krone make is shown below.

IDF -Intermediate Distribution Frame

 Protective devices used in IDF

The Exchange equipment to be protected against A.C. power across, power induction, and lightning faults on Telecommunication lines. 

Otherwise, such hazards can potentially travel into the exchange modules and severely damage sensitive switching and transmission equipment. 

To minimize the effects of such occurrences, poly switch resettable devices can be used as over current protection as “Primary protection” at MDF in modular form. 

In MDF primary protection is provided. 

Primary protectors are Gas Discharge Tube (GDT) OR Carbon blocks or Integrated Protection Modules (IPM). 

The primary protectors limit the very high-energy transients. 

The secondary protectors are incorporated on the Telecom line card itself, which limit the voltage and current to acceptable levels. 

Here are other types of over voltage protectors such as “Metal Oxide Varistor (MOV) AND 

Thyristors Surge protection devices (TSPD) preferred choice for Telecom protection applications.

a) Gas Discharge Tube (GDT)

Gas Discharge Tube (GDT), operates on the principle of the arc discharge phenomenon. Electrically, GDT act as voltage-dependent switches. 


As soon as the voltage applied to the GDT exceeds the spark over voltage (70 volts to a few KV, depending on the type), an arc is formed in the hermetically sealed discharge region within nanoseconds. 

When the discharge has died down, the GDT extinguishes and the internal resistance immediately returns to values of several hundred mega ohms

Under normal operating conditions the high insulation resistance and the low self-capacitance contribute to the fact that a gas-filled surge arrestor has virtually no effect on the system to be protected. 


Typical applications are, 

 Telephone exchange substations 

 Cable distribution system 

 Telephone MDF 

 Subscriber’s terminals 

 Satellite reception system 

c) Metal Oxide Varistor (MOV)


Metal oxide Varistor are voltage dependent, non- linear devices which are manufactured using a semiconducting metal-oxide used for protection against transients. 


"A Varistor is an electronic component with an electrical resistance that varies with the applied voltage Also known as a voltage-dependent resistor (VDR), it has a nonlinear, non-ohmic current–voltage characteristic that is similar to that of a diode. 

In contrast to a diode however, it has the same characteristic for both directions of traversing current. 

Traditionally, Varistor were indeed constructed by connecting two rectifiers, such as the copper-oxide or germanium-oxide rectifier in anti-parallel configuration.

 At low voltage the Varistor has a high electrical resistance which decreases as the voltage is raised. 

Modern Varistor are primarily based on sintered ceramic metal-oxide materials which exhibit directional behaviour only on a microscopic scale.

 This type is commonly known as the metal-oxide Varistor (MOV)." 

 Applications of metal-oxide Varistor (MOV)

 Consumer electronics (video recorders, color TV, amplifiers, and car radios) 

 Domestic applications (heating controls, washing machines, and microwave ovens)

 Telecommunications (telephone handsets, telephone exchanges, fax, telex, and modems).  General industrial applications (machine control, air conditioning, medical analysis, transformers etc). 

 Lighting (electric ballasts) 

 Power supplies (washing machines, power switch etc). 

 Automotive electronics (protection of an ignition circuit output transistor, protection of the solenoid driver circuit). 

 Typical Protection design: Typical Protection design is shown in the figure shown below. 

It has ‘Primary protection’ in MDF by GD tubes and ‘Secondary protection in the Line Card by providing over current and over voltage devices like Resistor, PTC, and Fuse & Thyristors.

positive temperature coefficient resistor (PTCR)

A positive temperature coefficient resistor (PTCR) is a resistor that varies its resistance with its temperature.

 A resistor that changes its resistance characteristics as a function of its temperature is also called a thermistor. 

So the PTCR is technically a thermistor. Whenever a thermistor increases its resistance with an increase in temperature, it is said to have a positive temperature coefficient (PTC). 

If the thermistor decreases its resistance with an increase in its temperature, it is said to have a negative temperature coefficient (NTC)


A Thyristor is a solid-state semiconductor device with four layers of alternating P- and N-type materials. 

It acts exclusively as a bi-stable switch, conducting when the gate receives a current trigger and continuing to conduct until the voltage across the device is reversed biased, or until the voltage is removed. 

A Thyristor usually has three electrodes: an anode, a cathode, and a gate.   

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