Please tell me the exact difference between Power Cables and Signal Cables used for Instrumentation Field Applications.
Signal cables has to carry 4-20 mA DC current only.
Analog signal transmission typically consists of two-wire signal leads or three-wire signal leads. In systems that require high precision and accuracy, the third signal lead, or shield, is necessary.
Normally it's used in RTD signal transmissions.
In the three-wire configuration, the shield is grounded at the signal source to reduce common-mode noise.
There are 4 types of signal cables are used to carry analog signals.
1.Plain pair cable
2.twisted pair cable
4.shielded pair cable
These cables are normally single pair cable which has the cross sectional area of 0.5 mm2 to 1.5 mm2.
But in power cables the cross sectional area is too larger.
Signal cable will use for low & high level analog signals such like filed instruments, RTDs T/c's. etc.. Power cable will use for powering the instrument or machinary. if you are using a 4 wire micro motion transmitter. signal cable will cary 4 to 20 mA signal and power cable will carry 24DC/110/230vac. both cables canot be run in a same route. there will be some segregation between them. please refer NEC. for more details write to me at firstname.lastname@example.org
Signal cables carry signals and power cables carry er.. power. Signals are just data modulated power, whereas power is just energy, but to really throw your teacher, mention Microphone 'Phantom Power', power over Ethernet and BPL (broadband over power lines)
Thanks for all the responses posted.
Well, I do understand that signal cables carry all types of instrumentation signals and power cables would carry power. But, physically, what differentiates one from another?? I mean, can they be used interchangeably? Should the power cables be shielded like the signal cables? For normal 24VDC operations, can the diameter of power cables be same as the signal cables? Power cables are usually multi-strand copper cables whereas signal cables can be copper or tinned-copper wires, right? can they be used interchangeabley?
All I was trying to know was the subtle differences that might forbid the use of one cable in the "other" application.
Cable should be selected to reliably and safely conduct (and insulate) the voltage and current required. The cable also may need to be of a specific construction to minimise the effect of external radiation (coaxial, twisted pair etc) or to minimise the radiation of energy (screened, shielded, coaxial) or even to maximise the amount of radiated energy (aerials!). They also need to be selected to conform to various regulations and schemes (insulator thickness, colour etc.) High voltages and frequencies call for other other types of cable construction, (look up 'skin effect') it's a science in itself. :) Having said all that, within an industry there is usually a conventional set of cable types used. I still find myself using 0.75mm YY to carry 4mA sensor signals, and 1mm tri-rated cable for panel wiring, massively over specified for the carried power, but cheap, convenient and compliant. (and available in a wide range of EN. UL and other wacky colour scheme colours)
I guess the terms Power and Signal are broad references to cable types without being specific. There would be no reason that a "signal" cable could be used in a 'power" cable application, if the voltage rating of the insulation complied and the current rating was adequate. Also thermal properties of the isulation need to be taken into account.
As for shielded power cables, now a days we use shielded cables for motors supplied from VS Drives, to comply with EMC standards. Basically the shield stops 'noise' getting out, while in "signal" applications it is to stop 'noise' getting in. Shielding in shielded cables varies depending on intended use, and as such using any old shielded cables in sensitive applications can be problematic.
In short power cables are those which are used in power transmission & distribution where as control cables are signal carrying cable. generally signal cables are shielded to minimize data loss as they are operated at low voltage whereas power cables are operated at high voltage. so u cannot replace high voltage cable with signal cable as their voltage capacity is low.
alpesh guj india
>All I was trying to know was the subtle differences that
>might forbid the use of one cable in the "other" application.
Signal cables will be smaller gauge and consist of multiple individually shielded twisted pairs.
Control cables will be non shielded multi conductor cables for on-off circuits to control solenoids, motor control or any higher current load. Voltages will be 24VDC, 120VAC, 125VDC and higher. Typical standard for control cable will be #14 AWG teck (armoured) cable.
Power cables will be the large gauge cables used to feed motors, transformers, MCC's, etc.
These cables are not interchangeable. Your 4-20 mA, RTD or thermocouple signals need a shielded set of wires.
Signal cables are not a heavy enough gauge to prevent too much voltage drop or actual cable overheating and damage when used in a control circuit. Control circuits could easily be 1 to 5 amps.
Signal cables normally are twisted with a foil shield, power cables normally have a ground wire but no shield.
As Trevor said provided the signal cable is rated for the voltage you could also use it for power provided with the appropriate over-current protection (most of the signal cable I see is rated for 120 Volts), however if someone received a shock because they confused a power cable (120AC) from a 4-20 mA you might have a hard job convincing them it was a good idea.
It is common practice to use signal cable to power 4 wire 24 VDC instruments, power and signal can run in separate pairs of a multi-pair cable, this saves a lot on cable and fittings.
May I ask you to give me some information regarding the use of "Drain Wire"? Can It be used as shield?
I think you are refering to the bare wire that accompanies each pair, yes that makes contact with the foil shield. Ground that at the control room end either at the I/O rack or marshalling panel. The field end must not be tied to ground or you will get a current flow which causes noise on the signal pair. It's normal to cut and tape the shield at the transmitter end.
If you are referring to the bare wire that accompanies each pair. Yes, that makes contact with the foil shield. Ground the wire at one end only (usually at the control panel, DCS I/O or marshalling panel.
The shield should be connected all the way to the transmitter in the field where it is cut and taped to prevent an accidental ground. You need to check that carefully once installed because electricians have a habit of grounding both ends.
When you check the field device end take a roll of tape. Snip off and tape any that are connected to ground. Some devices have an internal shield, it's OK to leave that connected provided there is no ground path. A good test is to lift the ground at the control panel end and check it to ground with a multimeter, it should read several MegOhms
I had a query. Can we give 230vac power and 24vdc potential free supply to instrument from the same cable? 24dc for digital signal and not the analog signal.