B - W = Blue with thin white bands
W - B = White with thin blue bands
O - W = Orange with thin white bands
Colours shown in brackets are for use when
existing GREY or Cream cable is
(O) - Orange (B) - Blue (G) - Green
pin 4 has no function in a domestic installation it is usually
for the sake of neatness.
The quality of wiring carried out by many
older BT wiremen is often approaching art in its perfection.
often has Orange, White, Green and Black wires.
Usually (but not always) Orange and White are the
active pair and go to connections 2 and 5 in the master
socket. In some master boxes (such as the type
with a removable front section) they go to two connectors
marked B and A.
way round they are connected usually doesn't matter
but some modems
older USA sourced ones) and some answering machines
are fussy about polarity, so it's wise if possible
to check the voltage on the line and connect -48V
to the Aleg (5) and 0V to the Bleg (2)
the master socket.
If you have underground wiring with a small grey
connection box by the door the internal cabling will usually be the same type
and colour as the extension cabling.
with White Bands
used but usually connected for neatness
colour code shown above is the one which would normally be used by BT. HOWEVER
it isn't always adhered to, especially if internal wiring in a new house has been
installed or modified by previous occupants.
should never rely solely upon the colour code - always check both ends of the
Terminals 2 and 5 swapped (2 at one socket
connected to 5 on another and vice versa)
poor speech quality, possibly poor bell. -
3 and 2 or 3 and 5 transposed
but no speech (or very poor speech) and can't dial out. -
Wire between terminals 2 or 5 broken.
You have an installation where
the main socket works and the remote in the garage roof doesn't. You need
to test the continuity of the circuit. So you can use a very long lead with
and a test meter - or cheat.
Firstly, disconnect the BT line completely.
the remote end bridge any two terminals (make a note of which two).
Measure continuity between these two wires at the master socket end - should be
no more than a few ohms.
- Repeat for the second
pair of wires.
- If either show a fault swap the
combinations - so if you tried 2 and 4 and that was OK, and then 3 and 5 and that
failed, you know 2 and 4 are both good so trying 2 and 5 and 2 (or 4) and 3 will
show you the faulty wire.
- so you have tested the cables and there is 150 yds of cable buried in the newly
decorated wall and only 2 wires have continuity - what do I do??
This is about the only occasion where you cheat and use a second master socket.
The second master gives you back your
ring signal so connect the two working wires to terminals 2 and 5 on the new master
and hope for the best!
REN measures the load
a device places on the line when ringing. A normal BT line will support
a REN of at least 4, in other words at least a total of 4 phones/fax/modems
should work on any line so long as their REN figures added together don't exceed
The REN is normally found on a label
at the base of the machine
(near the green approval symbol).
practice you can quite often exceed this number because devices with a REN of
1 may actually have a real REN of only a fraction of 1.This is an anomaly of the
test procedure used. Moreover many lines can drive a REN of more than 4.
Note that some elderly fax and answering machines can have very high REN's
(and they really are high!). If some or all of your phones fail to ring
or some ring very anaemically then its possible you have exceeded the REN.
Try unplugging devices until they work.
You can get REN Boosters which
will increase the ringing capacity of a line if desired, although if you get to
this stage you should probably be thinking of installing a small PABX.
- Peter Parry - WPP