Shielded Control Cable & Communicating Controls

When I first started in the trade, we used to run into shielded control wires on the Carrier Comfort Zone 1 zoning systems and also on a Carrier VVT system I used to maintain at a bank. I knew it has something to do with electrical “noise” and that communicating systems often called for it, but I never looked any further into it.

Over the last decade, there has been a lot of different residential communicating systems that have come out. Some require shielded cable, some merely recommend it, and others don't mention it all.

The fact is that whenever controls work on a low voltage “signal” rather than a simple “on/off” control, they are more susceptible to induced charges from other nearby conductors, electronics, and even transients from electrical storms.

A shielded cable has a metallic jacket that surrounds the individual conductors and routes the induced charges to ground, keeping it away from the conductors inside.

As an example of this, I installed a Carrier Infinity system at my own house WITHOUT using the shielded cable. Almost every time there are lightning strikes nearby, the unit will throw a communications fault. Since I'm in Florida, that happens quite often.

If you do have the wisdom to run shielded cable, you need to remember to bond (ground) one side of the shield securely to a good equipment ground on one end and ONE END ONLY. If you ground both ends, you risk the shield becoming a path in the case of a ground fault, which could cause some bigger issues. If you ground both ends, you can also create a “ground loop” that can cause the very noise you set out to eliminate.

In some cases, you can perform a similar function by grounding leftover/unused conductors on one end if you failed to run a shielded cable. There is no guarantee it will solve the issue, depending on the severity, because the other conductors don't fully surround the conductors being utilized.

The lesson being, when working with communicating “signal” controls, run shielded cable whenever possible. I was looking around and found this spec sheet from Southwire on some shielded wire.

—Bryan

Related Tech Tips

Some Furnace Questions
I received an email from a podcast listener with some furnace-related questions. Based on the nature of the questions, I figured it would be better to ask an experienced furnace tech. Benoît Mongeau agreed to help by answering the questions.  My name is Matt and I am a newer tech (fully licensed this September, have […]
Read more
Gas Law Concepts for HVAC/R
If you went to school and learned the “gas laws” early on, you probably thought it seemed LAME. Then, later, things like mass flow rates, airflow conversions, and compression ratios seemed HARD. Well… It's because HVAC is hard if you don't understand the concepts behind the gas laws, even if you never learn to do […]
Read more
What Makes an Autotransformer Different?
The definition of a transformer is a device that changes the voltages in an alternating current circuit. You may have heard of an autotransformer or a buck-boost transformer, and these terms are usually used for the same type of device; they just highlight different aspects. A transformer does not need to be a buck-boost to […]
Read more

2 responses to “Shielded Control Cable & Communicating Controls”

  1. Great article,
    I cant tell you how many times on a 2 wire communicating system changing the wire to shielded stopped the intermittent communication failure.

  2. I often wondered about this. I was a simple minded ET before I moved into HVAC. I remember my first job as a service tech. My company installed many evolution units and we always had callback on a communication error. I remember asking my service manager about using shielded cable. His response, No you don’t need “All that”. Hmmm. “all that”. I guess they are still having lots of call backs. I have installed some systems using comfortbridge and have not had one call back because of a communication alarm. Keep in mind that I use shielded cable for all communication that is not wireless.

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from