Some Electrical Requirements for Instrumentation 2

[nightfox1925]

I do hope this isn't an out of topic to discuss but it is related to Electrical requirements to instrumentation.

We have both 4-20mA analog and discrete 24Vdc digital signal paired, individually sheilded cables, both rated at 300V. We have some restrictions in routing them on new conduits and there is one existing conduit that we may use which happens to carry a digital 120I do hope this isn't an out of topic to discuss but it is related to Electrical requirements to instrumentation.

We have both 4-20mA analog and discrete 24Vdc digital signal paired, individually sheilded cables, both rated at 300V. We have some restrictions in routing them on new conduits and there is one existing conduit that we may use which happens to carry a digital 120VAC cable. Can we run the 24Vdc digital instrumentation cables along with the 120VAC digital instrumentation cables? or are they supposed to be routed in separate conduits?

Another is can we route both 4-20mA analog and 24Vdc digital instrumentation cables in a common conduit since both of them insulated at 300V and both are considered as a NSL 1 (noise susceptability level 1 per IEEE 518)?

Thanks for the assistance in advance.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[itsmoked]

Did you go circular on that question?? Kinda lost me.

However your questions are common and really hard to answer as one's experence with the particular control system is really most important in the answer.

In some cases it would be no problem at all. In other cases it could be non-functional.

Strictly speaking if all the wire jacketing is rated at the same voltage or higher you are technically free to cram them into the same conduit.

If the DI you refer to doesn't pass much current then it may cause no difficultly running the 4-20mA with it. If the distance is short it may be no problem. If the distance is hundreds of feet.. Maybe a problem.

If it is a new installation you really want the low voltage stuff by it's self. Otherwise do what you are stuck with realizing you may be forced to do remedial stuff to cure problems later.

Sometimes a digital signal may cause a brief glitch on the analog signals. In lots of cases that short glitch doesn't effect anything negatively. In other cases it could cause control excursions and be a problem. I can't say from here in your case.

You can typically run all the 24V or less stuff 0-5V, 0-10V, 4-20mA, 24VDC-DI, DO, moderate distances and not have any problems.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[nightfox1925]

Thanks Keith. Do you have any reference link I can use to understand my case here? Why is it that longer distances will pose an interference? is this due to sheath voltage accumulation? What if all the sheath drains are grounded at a single point, will this be a problem?

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[danw2]

ISA's 'Recommended Practice' ISA-RP60.8-1978 Electrical Guide for Control Centers has 2 short paragraphs, one on control wiring, one on signal wiring, that make statements similar to and usually found buried in manufacturers' installation guides for instrumentation, regarding the routing of control wiring (120Vac cable) and low voltage signal wiring.

The copyright restriction prominently displayed on its cover forbids distribution of any part of it.

http://www.isa.org/Template.cfm?Sec...=/Ecommerce/ProductDisplay.cfm&ProductID=2528

Dan

 

[itsmoked]

I don't know of a document.

Capacitive coupling is just that. Just like a capacitor the more the plate surface area the larger the capacitance. The larger the capacitance the lower the impedance. The lower the immpedance the more coupling and the lower the frequencies that can effectively couple. The longer the distance the cables co-mingle the greater the capacitance. Shielding is supposed to help this and does. But shielding isn't perfect usually.

Inductance is the same story. Twisting the wire pairs helps here as the shielding does absolutely nothing in the magnetic realm. But the twisting is not perfect either. So as the lengths increase inductive coupling increases.

Again, the things controlled and the sensors can make a huge difference to this whole installation. Do you care about 1/10th degree readings or are 1 degree readings adequate?

Questions like this are what make this subject so hard to deal with. This is why an EE generally has to say, "For me to answer your wiring questions with any certainty I can only say don't run analog signals with digital signals." But the reality is, yes, you can mix them, successfully, in a lot of cases.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[danw2]

I'm in Keith's camp on this. There are recommended practices, but sometimes mixing low voltage and high voltage does work "well enough". Other times it doesn't.

The text below is very similar to the ISA recommendations, but it comes from pages 20 & 21 (32,33 pdf) of a Honeywell UDC 3200 single loop controller user manual, 51-52-25-119, at

http://hpsweb.honeywell.com/NR/rdonlyres/F5959894-390E-47E7-ACF7-05972085E869/28577/515225119.pdf

-------------- start ------------------
Control/Alarm Circuit Wiring
The insulation of wires connected to the Control/Alarm terminals shall be rated for the highest voltage involved. Extra Low Voltage (ELV) wiring (input, current output, and
low voltage Control/Alarm circuits) shall be separated from HAZARDOUS LIVE (>30 Vac, 42.4 Vpeak, or 60 Vdc) wiring per Permissible Wiring Bundling, Table 2-5.

Electrical Noise Precautions

Electrical noise is composed of unabated electrical signals which produce undesirable effects in measurements and control circuits.

Digital equipment is especially sensitive to the effects of electrical noise. Your controller has built-in circuits to reduce the effect of electrical noise from various sources. If there is a need to further reduce these effects:
• Separate External Wiring—Separate connecting wires into bundles (See Permissible Wiring Bundling - Table 2-5) and route the individual bundles through separate conduit metal trays.

Use Suppression Devices—For additional noise protection, you may want to add suppression devices at the external source. Appropriate suppression devices are
commercially available.

ATTENTION
For additional noise information, refer to document number 51-52-05-01, How to Apply Digital Instrumentation in Severe Electrical Noise Environments.

Permissible Wiring Bundling
Table 2-5 Permissible Wiring Bundling
Bundle No. Wire Functions
1 • Line power wiring
• Earth ground wiring
• Line voltage control relay output wiring
• Line voltage alarm wiring
2 Analog signal wire, such as:
• Input signal wire (thermocouple, 4 to 20 mA, etc.)
• 4-20 mA output signal wiring
Digital input signals
3 • Low voltage alarm relay output wiring
• Low voltage wiring to solid state type control circuits
• Low voltage wiring to open collector type control circuits
--------------------- end ------------------

Most manufacturers print similar recommendations when low voltage signals are involved.

Dan