call mode which are connected together provide a path for earth loops between A leg to earth Voice signal lost networks. For example between two different computer systems. This same isolation issue applies at the patch panel where A to B short Voice signal lost screened cable is used. Screened patch panels should conduct the Open circuit A Capacitance imbalance screen through from inlet to outlet cable without cross coupling between circuits. Cross connection between screens can lead to Open circuit B Capacitance imbalance earth loop problems if more than one computer system is being Unshielded twisted pair cable has a relationship between the handled by that patch panel. The best solution is to provide a mutual capacitance between pairs and each line capacitance to selectable facility SO that screen coupling between sockets can be earth. The capacitance to earth is generally 1.5 times the mutual made if appropriate for that installation. capacitance. This applies provided the cable has an earthed shield 33/8 Structured cabling or is laid in an earthed environment, tray, soil or damp duct etc. This 4. Interface between external and internal cable. property can be used to analyse some otherwise difficult faults. 5. Test point. Voice communications between the two ends of the pair is import- 6. Isolation point. ant for fast and efficient testing. This can be achieved by radio or some test equipment makes use of the pair under test. Many designs of MDF's exist ranging from the basic angle iron There are more sophisticated pieces of test equipment, for frames dating back to 1893, to the more modern modular frames example Time Domain Reflectometer (TDR). This works on an seen today. With modern electronic exchanges the frame has, in echo sounder or radar principle of timing the echo from the other many cases, become the largest single unit within the PTT switching end of the cable or from any breaks, joints or changes in capacitance equipment. Because of this, considerable work is taking place with along the length o