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Protecting Your Sensor’s Signal With The Right Cable

notre dame gargoyleUntil micro-scale wireless technology is perfected, the sensors we use for monitor and control systems will require cables for power and communication. And that means the performance of those systems is dependent on the cables we use.

So why can’t we use the same cables for everything? Why does cable type, or size, or length matter?

Those are valid questions, with important answers. Let’s explore them, shall we?

EMI & Cable Type

So, why do some sensors come with twisted pair cables and some come with what seems like plain old regular cable by comparison? What’s the difference?

Short answer: it comes down to signal safety and integrity. Certain signal types are more susceptible to interference from stray electromagnetic fields than others. For those signal types, the extra protection afforded by special cables is essential for maintaining a stable signal.

Ok, that was short. How about the long answer?

Alright, since you insist:

Let’s start with a quick review of electromagnetic fields. (See what you started! Electromagnetic fields! Ahhhhhhhhhh!)

Current traveling through a conductor creates a magnetic field that is concentric to the conductor. To visualize this, wrap the fingers of your right hand into a fist, with your thumb sticking up. Now imagine that your fingers could wrap all the way around your thumb, through your wrist, to your knuckles. Your thumb, then, is like the conductor, with the current travelling out through the tip, and your fingers show the electromagnetic field, generated by the current, wrapped around the conductor.

Now it stands to reason (and all sorts of fancy-pants math) that if a current through a conductor can generate an electromagnetic field, then an electromagnetic field can create a current in a conductor. But when it comes to sensors and control systems, we don’t want random stray electromagnetic fields creating currents in our cables; we only want the currents that our sensors and controls systems put there.

And that’s where signal protection via special cabling comes in.

There are several factors that determine the extent to which a conductor will be influenced by an electromagnetic field, but one is used multiple ways to mitigate that influence: parallel paths. It’s much easier (because All The Fancy-pants Math) for parallel conductors to induce current in each other than it is for nonparallel conductors. So, to protect a conductor from the unwelcome advances of stray electromagnetic fields, we do as much to prevent parallel paths as possible.

First, we twist two conductors together: Ta Da! Twisted Pair cables! The advantage, and reasoning, here should be obvious: it’s awfully hard for a conductor to be parallel to anything when it is constantly twisting.

Second, we put a thin mesh of conducting material around the twisting conductors: Shielded, Twisted Pair cables. Where twisting cables seeks to keep a conductor from any possible parallel path, the shield actively seeks to create a possible parallel path, intercepting any electromagnetic interference before it can get to the protected conductor.

Ok. That makes sense, and the E&M stuff wasn’t even too scary. But what kinds of signals need to be protected? Any? All? Just a few?

Fair question, and the answer is straightforward, even if it sounds evasive: low-current analog signals, and high frequency digital communications. See? Straightforward and evasive, all at the same time. So let’s make it slightly less evasive.

First, low-current analog signals generally come in two flavors: 4-20 mA and mV/V. In general, low-current signals are more susceptible to interference from environmental electromagnetic fields, since smaller currents create smaller fields, which are easier to disrupt. So twisted pair or shielded twisted pair wiring is required to protect these signals.

Second, high frequency digital communications, such as Ethernet, rely on twisted pair cabling. All Cat 3, 5, 6, and 6a telecommunications wiring is twisted pair. Other communication and control standards, such as Modbus, Hart, and Fieldbus, specify that shielded twisted pair cabling must be used for all or most of a network.

Can other signal types, such as 0-5 VDC, benefit from twisted pair cabling? To an extent, yes. Using twisted pair cabling certainly won’t hurt the signals. However, twisted pair is often more expensive than unpaired cable of the same size. So using twisted pair cables for everything doesn’t always make sense. But any signal will benefit from the protection of shielded cable.

Power Supply, Voltage Drop, and Cable length

Usually, voltage drop is only a concern for power distribution cabling. If a run of cable is too long for a given amperage and size of cable, the cable size (cross-sectional area, in essence) is increased, reducing the resistance and the resultant voltage drop. But in low-power signal and control scenarios, when voltage drop is an issue, rather than changing the diameter of the cable, the length of the run is limited.

For an example, let’s look at the mv/V signal again. Imagine that we have a sensor some 300 feet from our control equipment, on a circuit with 22 AWG cable and an arbitrary 10 mA current. The resultant voltage drop would be 0.0495 V. Not a lot, right? Well, for power delivery, 0.0495 V isn’t much, but on a millivolt signal, 49.5 mV could be anywhere between 50% and 166% of full-scale! Using larger cable doesn’t make sense in this kind of situation. Instead, we limit the distance between mV/V sensor and control equipment to 30 feet.

Now wait a minute, you say. Does this mean I can’t power a 4-20 mA sensor that’s 300 feet from my control equipment? Well, using the same math, with a 20 mA current, we get a voltage drop of 0.099 V. Most 4-20 mA sensors require a minimum of 10 VDC at the sensor. Since we’re dealing with volts for supply, and not millivolts, the loss of one tenth of a volt shouldn’t be a problem.

Since you mentioned it, however, it’s worth noting that a power supply set to match the low end of a sensor’s power requirement can lead to the sensor malfunctioning because of voltage drop. Sending more power to a sensor that is a long way from the supply source is easier than trying to troubleshoot a sensor malfunctioning because of low power.

Still have questions about cables and sensors? Contact us! We’d love to help answer your questions.

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top image credit: Dustin Gaffke via flickr cc cropped

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