advertisement
from the C&I department...
Difference Power Cables and Signal Cables
Application Questions and Problems topic
Posted by Grishma on 28 October, 2006 - 1:15 am
Please tell me the exact difference between Power Cables and Signal Cables used for Instrumentation Field Applications.

Regards
Grishma


Posted by Anonymous on 29 October, 2006 - 11:26 am
Dear Garishma,
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
3.co-axial 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.


Posted by Sunnichan on 29 October, 2006 - 11:32 am
Grishma,
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 confusedsunny@yahoo.co.in


Posted by Marc Sinclair on 29 October, 2006 - 9:29 pm
Hi,
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)

:)
Marc


Posted by Grishma on 31 October, 2006 - 12:39 am
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.

Regards
As always
Grishma


Posted by Marc Sinclair on 31 October, 2006 - 11:27 pm
Hi,

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)

Marc Sinclair


Posted by Trevor Ousey \(lists\) on 2 November, 2006 - 1:23 am
Hi Grishma,

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.

Regards, Trevor


Posted by aa on 27 December, 2008 - 5:38 am
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


Posted by Roy Matson on 29 December, 2008 - 6:59 am
Hi Grishma,
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.

Regards
Roy



Posted by vahid on 30 December, 2008 - 12:56 am
Roy,

May I ask you to give me some information regarding the use of "Drain Wire"? Can It be used as shield?

Thanks
Vahid


Posted by Roy Matson on 30 December, 2008 - 7:40 pm
Vahid,
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.

Regards
Roy


Posted by Roy Matson on 30 December, 2008 - 8:11 pm
Vahid,
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

Regards
Roy


Your use of this site is subject to the terms and conditions set forth under Legal Notices and the Privacy Policy. Please read those terms and conditions carefully. Subject to the rights expressly reserved to others under Legal Notices, the content of this site and the compilation thereof is © 1999-2014 Nerds in Control, LLC. All rights reserved.

Users of this site are benefiting from open source technologies, including PHP, MySQL and Apache. Be happy.


Fortune
The intelligence of any discussion diminishes with the square of the
number of participants.
-- Adam Walinsky
Advertise here
Advertisement
our advertisers
Help keep our servers running...
Patronize our advertisers!
Visit our Post Archive