Why use Twisted Shielded Pair Cable
This question is best answered in the full context of wire selection. A basic background in electricity will yield a basic answer, like “to reduce noise”. That’s true.
Let’s go deeper, talk about it in terms of the physics, and find out why it reduces noise.
The answer lies in the basic principles of electrical circuit theory, where there are three key properties to consider: resistance, capacitance, and inductance.
How do we ensure that a wire will not overheat, when we apply voltage and current to it? By selecting a wire gauge that is sufficiently large, otherwise it would have excessive resistance. Don’t use 22 AWG telephone wire to power a room air conditioner. Don’t waste money buying a 12 AWG cable just to hookup a doorbell circuit.
Alright, that one was easy. On to the tough ones. There are two kinds of electromagnetic induction: capacitive coupling (electrostatic), and magnetic coupling (electrodynamic).
How do we ensure that the signal transmitted over a wire, will not be degraded by excessive noise, when we place the wire into an environment, that has ambient electrical fields which tend to capacitively couple with the active signal-carrying wires. By covering the wires with a continuous metallic shield or foil, which is grounded at one or both ends. The ambient electrical fields capacitively couple with the shield instead of the active signal-carrying wires. This helps prevent your wires from playing the role of a capacitor anode or cathode.
How do we ensure that the signal transmitted over a wire, will not be degraded by excessive noise, when we place the wire into an environment, that has ambient electromagnetic fields which tend to inductively couple with the active signal-carrying wires. By twisting the wires continuously, from end to end.
- Transformer effect. The ambient electromagnetic fields magnetically couple with thousands of very small twisted coils, instead of the entire untwisted “single-loop” signal-carrying wires (a transformer with a 1:10000000 ratio induces infinitely less noise than one with a 1:1 ratio; lowering your effective loop area, ELA). This helps prevent the wires from playing the role of a transformer primary or secondary coil.
- Oppositely-wound. One wire is a clockwise spiral, one wire is a counter-clockwise spiral, they cancel each other out.
- Crosstalk. Twisting the pairs counters crosstalk effect between pairs in multi-pair cable, the wire nearest to the noise-source is exchanged in every half-twist.
If you like this kind of stuff, you should study electrical engineering.
Circuit Breakers; Ground Fault Circuit Interrupters; Arc Fault Circuit Interrupters
There’s a power tool, whose housing is metallic. The tool has an internal frayed wire, and it’s touching the housing. Because the tool has a three-wire grounded cord, there is a low-resistent current path from the frayed wire to the housing, causing a short circuit.
The circuit breaker will trip. Circuit breakers protect against large overcurrents.
There’s a power tool, whose housing is plastic. The tool has an internal frayed wire, and it’s touching the housing. Because the housing is plastic, there is no current path from the frayed wire to the housing, so the breaker does not trip. The tool gets wet, and you pick it up to operate it. There is now a small low-resistent current path from the frayed wire to the wet housing through your body, causing a small-current ground fault. It’s not a large enough current draw to trip the circuit breaker. GFCI’s are threshholded differential-current meters. When the absolute value of Black Current minus White Current exceeds 0.5 milliamps, it trips. The Ground Fault Circuit Interrupt (GFCI) will trip. GFCI’s protect against small current ground faults.
There’s a power tool. The tool has a frayed power cord, so matchstick-head sized defect in the cable jacket has left a hard-to-see exposed speck of copper wire, which is touching the carpet. It’s not enough current draw to trip the circuit breaker, or the GFCI. The Arc Fault Circuit Interrupt (AFCI) will trip, when it detects a non-periodic series of “bad” current arcs, which otherwise would eventually burn the carpet. AFCI’s are designed to ignore periodic arc faults, since they commonly occur in any motor that has brushes and an armature. Brushes use “good” arcing. Tens of thousands of people die annually from all sort of avoidable electrical hazards. Electrical Regulatory Bodies, expecially the NEC board, are always researching what to impose, to help minimize the risks to the public.