Shielded cable


Shielded cable

A shielded cable is an electrical cable of one or more insulated conductors enclosed by a common conductive layer. The shield may be composed of braided strands of copper (or other metal), a non-braided spiral winding of copper tape, or a layer of conducting polymer. Usually, this shield is covered with a jacket. The shield acts as a Faraday cage to reduce electrical noise from affecting the signals, and to reduce electromagnetic radiation that may interfere with other devices. The shield minimizes capacitively coupled noise from other electrical sources.

In single conductor signal cables the shield may act as the return path for the signal and is usually connected only at the signal source. In multiconductor cables the shield should be grounded only at the source end, and will not carry circuit current.

High voltage power cables with solid insulation are shielded to protect the cable insulation and also people and equipment.

Signal cables

By twisting two conductors of a balanced-line signal circuit into a twisted pair, some cancellation of inductively-coupled noise is obtained. However, a metallic shield layer over the twisted pair provides better suppression of noise. Coaxial cable is used at higher frequencies to provide controlled circuit impedance, but the outer tubular conductor is also effective at reducing coupling of noise into a circuit.

Applications

The use of shielded cables in security systems protects them from power frequency and radio frequency interference, reducing the number of false alarms being generated.

Microphone or "signal" cable used in setting up PA and recording studios is usually shielded twisted pair cable, terminated in XLR connectors. The twisted pair carries the signal in a balanced audio configuration.

The cable laid from the stage to the mixer is often multicore cable carrying several pairs of conductors.

Consumer grade microphones use screened wire with one central conductor in an unbalanced configuration.

Also see: High-end audio cables

Practical guide for shielded cables for industrial signal applications

There are 3 methods of introducing noise that can affect signal cables:
#Galvanically Coupled
#Capacitively Coupled
#Magnetically Coupled

Galvanically coupled noise is produced when a large current (fault, lightning) flows through the earth system and creates voltage differences between pieces of equipment that are not bonded together very well. Even 1 ohm can produce 1000 V if 1000 A fault or lightning current flows through it.

The importance of a good, low resistance, high current capacity earth cannot be over stressed.

Capacitively coupled noise is produced by changing voltages. The higher the rate of change the higher the coupled noise. Switching events, lightning, faults, can all cause this noise.

Capacitively coupled noise is reduced by using an ElectroStatic Screen around a twisted pair cable, and the screen is only earthed at one end. This gives the induced currents in the screen a path to earth. The twisted pair provides balanced coupling between the screen and the cores, and the induced current has very little effect on the signal.

Magnetically coupled noise is produced by magnetic fields from power cables laying near the signal cable. The magnetic field induces a current in anything conductive nearby.

Magnetically coupled noise is reduced by using an electromagnetic screen around a twisted pair cable, and the screen is earthed at both ends and any convenient point in between. This allows a circulating current to be induced in this screen which sets up its own magnetic field that opposes the original field. Low resistance magnetic material screens provide the best shielding. The twisted pair provides balanced coupling between the screen and the cores so there is very little effect on the signal.

The reality is that metal conduits, ductwork, and even ladder provide protection against magnetically coupled noise. It is better to have the source of the noise inside a magnetic material barrier, but the sheer number of cables and the heat generated prevent this. So they are on ladder which does help a bit. The use armoured cables and earth the armour at both ends helps stop the noise at the source.

This is why most signal cables are run in metal conduit or metal duct when they are in groups. There are no generated heat issues with 4–20 mA. The duct/conduit should be earthed properly at both ends and at any convenient point in between. The sections should be electrically bonded together to provide a continuous current path. Do not have the earth point only in PLC cabinet for that end. It should be earthed properly a couple of places before this, so that high induced currents are not being introduced to the PLC cabinet. For example, have a strong earth bond at the entrance to the switchroom, so that the only noise the section of duct inside the switchroom to the PLC cabinet is protecting against is any generated inside the switchroom. This can still be high, so the duct must avoid power cables.

Summary of most important measures to minimise the effects of noise and interference

1) Removal of sources of coupled noise
Avoid “high noise” areas
· Segregation of cables
§ Separate ladders / ducts – Have power cables on ladder and signal cables in ducting
§ Run signal cables in steel conduit
§ Magnetic material barrier between the power and signal cables if in same ladder/duct
§ Use armoured cables for power, and earth armour at both ends
§ Wrap power cables in “Trefoil” or similar to reduce magnetic field
2) Shielding
· Galvanic isolation – Have a good earthing standard
· Electrostatic shielding – Screen earthed at one end to a good earth (PLC end)
· Electromagnetic shielding –
§ Screen earthed at both ends
§ Use conductive, magnetic material for ladders, ducts
§ Electrically bond all sections of ladder/duct at joins
§ Earth the ladder/ duct at both ends and anywhere else convenient

All screens must be insulated from each other along the length of the cable. The electromagnetic and electrostatic screens especially. You can’t use the drain wire as an electrostatic screen, and the wrap as an electromagnetic screen.

Continue the integrity of the screens through Junction Boxes. Do not connect different screens together, or to earth. Do not use earth terminals.
· eg, for single pair overall screened cables feeding into a multipair overall screened cable all the single pair screens will be joined to the one multipair screen.
· eg, for single pair overall screened cables feeding into a multipair individual and overall screened cable each single pair screen will be joined to the corresponding individual pair screen, but not to anything else. The multipair overall screen is not connected to anything in the junction box.
· For both above examples the screens are all connected together at the PLC end.

3) Use twisted pair cable to reduce coupled noise effects.

4) Use proper earthing and bonding

Power cables

Medium and high-voltage power cables, in circuits over 2000 volts, will have a shield layer of copper or aluminum tape or conducting polymer. If an unshielded insulated cable is in contact with earth or a grounded object, the electrostatic field around the conductor will be concentrated, resulting in corona discharge, and eventual destruction of the insulation. As well, leakage current and capacitive current through the insulation presents a danger of electrical shock on contact. The grounded shield equalizes electrical stress around the conductor, diverts any leakage current to ground, and blocks capacitive current from flowing into objects (or persons) touching the exterior of the cable.

Shields on power cables are connected to earth ground. On high current circuits the shields must be connected only at one end, since induced current will flow in the shield. This current will produce losses and heating and will reduce the maximum current rating of the circuit. On very long high-voltage circuits, the shield may be broken into several sections since a long shield run may rise to dangerous voltages during a circuit fault.

External links

* [http://www.belden.com/pdfs/Techpprs/tpbroad.htm Belden Corp article for "Broadcast Engineering" magazine]


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