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 used.
(O) - Orange (B) - Blue (G) - Green
pin 4 has no function in a domestic installation it
connected 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 A and B.
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 B leg (5) and 0V to the A leg (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
with White Bands
with Orange Bands
used but usually connected for neatness
with Blue Bands
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 cable.
2 and 5 swapped (2 at one socket connected to 5 on
another and vice versa)
Very poor speech quality, possibly
poor bell. -
Terminal 3 and 2 or 3 and 5 transposed
but no speech (or very poor speech) and can't dial
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.
- In 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
- 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
OK - 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
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 4.
The REN is normally
found on a label at the base of the machine
(near the green approval symbol).
In 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