Eight wires, one wrong
A new desk will not connect. The plug fits. The cable looks fine. A tester reads the eight wires and shows that wire 6 does not reach the far end.
Most faults in a cabled link are physical: something is cut, crossed, bent, dirty or the wrong kind. Learn the common ones and you can name a fault from a test result.
Key words
| Word | What it means |
|---|---|
| Open | a wire that does not reach the far end |
| Short | two wires touching that should not touch |
| Reversed pair | the two wires of one pair swapped at one end |
| Split pair | wires joined to the right pins but twisted with the wrong partner |
| Wire map | a test that shows which wire reaches which pin |
| Pin-out | the order of the wires on the pins of a plug |
| Duplex mismatch | one end sends both ways at once and the other end does not |
| Macrobend | a large bend that lets light escape from fibre |
| Microbend | a tiny bend from pressure that leaks light |
| Containment | trunking, tray or conduit that holds cable |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can name the common physical faults and match a wire map result to its fault.
Starter: wiring
Have a go
Starter: wiring
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Physical faults and their signs
| Fault | What is wrong | Typical sign |
|---|---|---|
| Wrong cable type | Cable cannot carry the signal, such as a telephone cable for data | No link, or very slow |
| Wrong pin-out | Wires on the wrong pins: a wire map error | Fails wire map, no link |
| Open | A wire is cut or not punched down | A pin shows no connection |
| Short | Two wires touch | A pin reaches two far pins |
| Bad port | A dirty, damaged or contaminated connector or socket | Link comes and goes |
| Damaged cable | A cut, crush or tight bend, including macrobends and microbends | Poor test result, loss of signal |
| Bent pins | Pins in a socket are pushed over | One wire does not connect |
| Duplex or speed mismatch | The two ends disagree on speed or on sending both ways | Link is up but very slow |
| Wrong containment | Cable run too tight, near power, or against the rules | Noise, or a breach of standards |
The cells students fill in are left blank in this preview.
Reading a wire map
A wire map lists each near pin and the far pin it reaches. A good cable reads 1 to 1, 2 to 2, and so on.
An open shows a pin with nothing. A short shows a pin reaching two. A reversed pair shows two pins of a pair swapped. Crossed wires shows pins reaching the wrong far pins.
Worked example: four cables tested
| Cable | Near pin to far pin | Fault |
|---|---|---|
| A | 1 to 1, 2 to 2, 3 to 3, 4 to 4, 5 to 5, 6 to 6, 7 to 7, 8 to 8 | None. Pass |
| B | 6 has no far pin | Open |
| C | 7 and 8 both reach far pins 7 and 8 | Short |
| D | 1 to 2 and 2 to 1 | Reversed pair |
The cells students fill in are left blank in this preview.