The lift that slowed the office
A new office works well all week. Then the lift motor starts. Every time the lift moves, a row of computers drops its link for a second.
The data cable runs beside the lift power cable. The power cable makes a field that bleeds into the data. This lesson explains what happens and how to stop it.
Key words
| Word | What it means |
|---|---|
| Electrical signal | data sent as changes in voltage along copper |
| Light signal | data sent as pulses of light along fibre |
| Interference | unwanted signal that disturbs the real one |
| EMI | electromagnetic interference: a disturbance from nearby electrical kit |
| Crosstalk | signal leaking from one pair of wires into another |
| Static | a small build-up of electric charge that can disturb a signal |
| Shielding | a metal layer round wires that blocks interference |
| Segregation | keeping data cable and power cable apart |
| Tampering | secretly interfering with a cable or its signal |
| Laser | a light source that makes a narrow, strong beam |
| LED | a light source that makes a wider, weaker beam |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can compare electrical and light signals, say how tampering shows up, and explain how to keep data cable free from interference.
Starter: noise
Have a go
Starter: noise
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Electrical and light signals
| Signal | Carried on | Made by | Weak point |
|---|---|---|---|
| Electrical | Copper cable | Voltage changes | Picks up interference |
| Light, laser | Single mode fibre, long runs | A laser | Loss from bends and dirty joins |
| Light, LED | Multi-mode fibre, short runs | An LED | Loss from bends and dirty joins |
The cells students fill in are left blank in this preview.
Security: tampering and signal loss
Someone may try to listen to a cable. Copper can leak signal that a clever tap can read. Fibre gives out no electrical field, so it is harder to tap. But a fibre can be bent to let light leak, and that tap shows up as signal loss.
Protect cables with locked cabinets, conduit and tamper seals. Treat a sudden loss as a possible warning.
Where interference comes from
EMI comes from motors, lifts, fluorescent lights, big power cables and radio. Static comes from charge building up. Crosstalk is signal leaking from a pair into the next pair.
Fibre does not pick up EMI at all. That is why it is used in noisy places and between buildings.
Segregation from electrical cables
Keep data cable away from power cable. Use separate trunking, or one trunking with a metal divider. Where they must meet, cross at right angles, not side by side.
The standard BS EN 50174 gives the minimum gaps. The gap depends on the cable type and on the power load, so check the current standard.