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General Principles
Structured wiring allows for the flexible use of a buildings cabling network to run computers and telephones.  The system is totally flexible, in so much as the user can move apparatus from one office to another without resorting to any engineering work  This is achieved by point to point wiring and flooding the building with RJ45 sockets (intermediate patch enclosures can also be installed).  The whole system is built in accordance with the Category 5 wiring specification and only approved equipment may be used.  Cat 5 cable looks the same as telephone cable but is made to a higher specification.  All terminations on a structured cabling system are made without removing any twists from the cable and each socket is tested.  The test results are presented to the customer and this forms the guarantee.  As some guarantees last for 15 years or more - do not tamper with these wiring systems and under no circumstances repair a broken cable - it must be renewed completely.

Overview of Installation
The telephone system is wired to a central patch panel consisting of RJ45 sockets.  Extensions are cross connected from the telephone system patch panel to the building wiring patch panel using RJ45 plug ended patch cables.  These patch cables are straight connected and must not be confused with twisted cables (that look similar) that are used on ISDN equipment.  Patch cables may differ in colour and this is for identification purposes only, as they electrically the same.

At the end of the building cable is a RJ45 socket.  Telephones may have to be connected to the RJ45 socket and this may be achieved by the use of:-

  • An RJ45 to PST adapter.

  • By fitting an RJ45 plug ended cord to the telephone.

Adapters come in three types:-

  • PSTN Master, used for Exchange lines when connected to POT phones and Fax machines etc (has a ring circuit capacitor, test resistor and high voltage protector).
  • PBX Master, as above but without resistor and high voltage protector.
  • Secondary/slave adapter, used for supersets or apparatus that does not require a ring circuit capacitor.

All sockets and patch panels should be labelled and records should be kept of all connections.

RJ45 Wiring

Pinouts for RJ45 jacks (Serial and 10Base-T)

Connecting Telephone Apparatus
To ensure that telephones will work when connected to a structured cabling system the wiring must be correctly terminated at the first patch panel.  Extension wiring should be connected as follows:

Telephone system RJ45 socket on Patch panel
A 4
B 5
Earth 1

Only straight patch leads should be used to cross connect between patch panels (see diagram above).

BT adaptors
For a UK telephone to be connected to an RJ45 socket a converter adaptor must be used.  There are two sorts - one cord ended (as shown) and the other that plugs directly into the socket.

The wiring of the adaptor is shown below.  There are three sorts of adaptors: PSTN, PABX Master and Secondary.  Ensure that you purchase the correct version for your needs.  The PBX master is used when POT phones are connected and the Secondary type used for System Featurephones.

Be careful to ensure that if you use earth recall that the adaptors have a connection to pin 4 on the BT socket side of the adaptor.

Adaptor wiring

RJ45 Plug BT Plug
1 1 4 Recall - if wired
2 2 - Ring - Not connected
3 3 6
4 4 2 B leg
5 5 5 A leg
6 6 1
7 7 -
8 8 -

Common problems
Analogue telephone (POT) does not ring or only rings once:

  • Earth connected wrongly in Patch panel wiring.
  • Secondary adapter used (structured wiring system).

ISDN adapter does not work:

  • Ensure that correct cable is used between ISDN adapter and RJ45 socket (should be twisted type).

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Last revised: October 30, 2022