Cable Specs

[vreijb]

Anyone can give input about specifying instrument cables for power plants. The cables will be for:
-Analog input 4-20mA or RTD
-Digital Input/output
-Input/Output cables from Swicthgear to PLC for control and monitoring
I am looking specifically whether to provide shield on Digital input/output cables and whether to have them twisted pairs or simply control cables
Your input will be appreciated.

 

[JLSeagull]

Consider individually shielded twisted pairs as the conservative approach for the 4-20 mA, RTD (triad) and low level signals like thermocouples or transducer probes. I recommend this approach for industrial plants and anywhere that noise can be disruptive. Some people accept home run cables with twisted pairs and an overall shield in processes that lack significant sources of noise. As the money is in the installation labor not the difference in cable cost, I would stay with individually shielded twisted pairs.

We also use individually twisted shielded pairs for 24 Vdc digital contacts, etc. This is just for cable standardization. Assure that the cables are tray rated if any cable tray is used. I am a fan of 600 Volt insulation but this is left over from some old NEC issues.

Ethernet uses unshielded twisted pairs. Copper Ethernet is also limited to 100 meters.

If looking for a cable standard, consider specifying NEMA WC 57 (ANSI/ICEA S-73-532) Standard for Control, Thermocouple Extension and Instrumentation Cables.

Some people think that Instrument Cables are "power limited" cables. UL 13 applies to power limited cables. However, these signals are not NEC 825 power limited cables unless you use Intrinsic Safety Barriers. Intrinsic Safety would be very unusual in a power plant. Perhaps your fire alarm signals are power limited signals but not the 4-20 mA or alarm signals.

 

[ScottyUK]

Hi jJLSeagull,

I.S. is actually pretty common in European plants, especially the younger generation of CCGTs because the installed cost is usually much lower than the flameproof Ex'D' alternative. The old stations built years ago 'when we built things things properly' have a lot of Ex'D' equipment. I.S. is the only option under the ATEX Directive for Zone 0 environments where a continuous explosion hazard exists.


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If we learn from our mistakes I'm getting a great education!

 

[JLSeagull]

Typically, when the domestic US plants use IS it is a belt and suspenders thing that also includes explosion-proof fittings. I think that the ISB replacement cost reduced the IS interest state side. Perhaps had replaceble fuses arrived earlier IS would be more common in the US. We also tend to over-classify areas adjacent to hazardous areas just in case a plant modification would add combustible material.

My point related to this topic is that novice engineers and designers want to apply NEC article 825 for DCS and SIS wiring. The overcurrent protection in such systems rarely qualifies as power limited circuits except when IS barriers are applied. Even those tiny 1/4 Amp micro-fuses can usually be replaced with a fuse closer to 10-Amp.

 

[dpc]

For power plants in the US, I'd think about specifying Tray Cable (TC) so that you can use the cable in cable tray.

600 V insulation is a good thing - although you hear a lot of whining from some of the vendors.

Definitely individual twisted shielded pairs for signal cable.

 

[vreijb]

I have worked most of the time in persian gulf and in oil and gas projects. We always considered:
single pair cables with shield for analog and digital signals
multipair cables with individual and overall shield for analog signals
multipair cable with overall shield for digital signals

I am new in the north american market and my colleagues are arguing that they only consider instrument cables for 4-20mA and RTD signals. For the rest alarm and signalling digital inputs, cables are considered to be control cables with multcore and without shields.
is this common way of doing it in power plants

 

[JLSeagull]

Some plants use 120 Vac signals for alarm and shutdown. Power plants may use 125 Vdc due to the use of station batteries. For 120 Vac and 125 Vdc it is common to use #14 AWG color coded control cable bundles instead of shielded instrument cable. This was true for the hydrocarbon processing business before 24 Vdc was the norm for alarm, shutdown and status signals.

 

[protons]

You can look at hear, this is what we use for 4-20 mA.

http://www.tecnicable.com/productos/cable.asp

EOZ1 (ROZ1) - Z10Z1

 

[JLSeagull]

¿Tiene una direcion que lodar mas rapido?