Ensuring Signal Integrity with Insulation [Cable 101]

electrical-wire-insulation

Selecting the proper conductor is only the first step in building a cable.

Now that you’ve found the perfect conductor it’s time to add the electrical wire insulation.

Selecting the proper conductor is only the first step in building a cable.

Now that you’ve found the perfect conductor it’s time to add the electrical wire insulation.

In this post, we describe what insulation is, some of the characteristics to consider when choosing, and why you need it.

Please keep in mind that this post is meant to be an introduction into electrical wire insulation.

If you’d like to learn more, there are plenty of online (and offline) resources available for you to dive deeper into dielectrics, electrical properties, physical parameters, and much more.



What is electrical wire insulation?

Electrical wire insulation describes the material that surrounds the conductor.

Think of insulation as the sleeve through which a conductor is threaded.

Unlike shielding or jacketing, insulation is in direct contact with the conductor.

It serves to protect the conductor and its signal or power from neighboring conductors.

It also protects the end-user from interference with those signals or the voltage being carried over the conductors.


Why are wires insulated?

If you’ve read our post about conductors, you know the job of the conductor is to move electrons from one point to another.

Unfortunately, electrons don’t always behave.

Unlike a drinking straw where liquid can be transported from point A to point B rather smoothly and without incident, some of the electricity moving through a conductor can be lost to the environment.

So, one of the primary reasons wires are insulated is to prevent the loss of free flowing electricity from the conductor—helping to focus the electricity toward its intended destination.

Now, consider what would happen if two exposed conductors with electricity coursing through them were to touch.

You may (or may not) see a spark, but one thing will surely happen: they would interfere with each other’s current.

That being said, another one of the primary benefits of insulation is that it separates conductors and helps prevent them from interfering with each other—an important characteristic if you want your cable to work correctly.

However, it’s important to note that even fully insulated conductors can “cross-talk” and, depending on the level of voltage, can still short out.

So choosing the right insulator is critical to the application and environment in which it is to be used.


What is insulation made of?

As the primary purpose of insulation is to prevent signal loss and interference between conductors, it is important that non-conductive material is used.

There are semi-conducive materials that are used in special applications, but for this article we will focus on non-conductive.

So, metal is out of the question.

In general, the perfect insulation is air.

In fact, the best impedance, capacitance, and insertion loss is achieved with air.

For this reason, all other insulation compounds are measured against air.

Unfortunately, air is not a good fit for most applications as the wire will be subjected to outside forces, such as human touch or potential contact with other conductors.

As mentioned earlier, one of the chief benefits of insulation is that it keeps the conductors from making direct contact.

The only way to do this is to protect the individual conductors from each other or outside influences.

Air cannot accomplish this task, so most wires typically utilize various compounds.

Each compound has two primary ratings: effective dielectric constant and limiting oxygen indices.

Effective Dielectric Constant

Effective dielectric constant (“Dk”) rates the compound’s impact on capacitance relative to that of air.

Dk is the ratio of the capacitance of any given material when compared to air.

To find the Dk of a compound, a single naked conductor is measured for capacitance and then the same conductor is insulated with various compounds and the capacitance measurement is compared.

Capacitance is measured in picofarads (“pF”).

Given that air has an index rating of slightly over 1, the formula may look something like this: 12pF/m = 12/12 = 1.0.

Polyvinyl chloride (“PVC”) insulating the same conductor would have a capacitance of 60pf/m = 60/10 = 6.0 Dk.

Limited Oxygen Indices

In Principles of Polymer Engineering, limited oxygen indices (“LOI”) are defined as:

“[T]he minimum concentration of oxygen, expressed as a percentage, that will support combustion of a polymer.”

The authors further explain that LOI is measured by “[p]assing a mixture of oxygen and nitrogen over a burning specimen and reducing the oxygen level until a critical level is reached.”

Simply put, you can think of LOI as a rating for how many particular of oxygen are required to maintain combustion on a polymer.

The higher the number the more flame resistant (and, typically, the more expensive) the polymer.

For example…

Compare the LOI of a common household item like cotton with two commonly used polymers for wire and cable applications: polytetrafluoroethylene (“PTFE”) and PVC.

Cotton in its raw form is very flammable—its LOI is approximately 16-18.

PTFE, on the other hand, has an LOI of roughly 95 while the LOI of PVC is just under 40.

When combining LOI with Dk you can start to see the performance and cost impacts of one compound versus another.

To visualize this point, the following chart maps different compounds based on their Dk and LOI:

dk-loi-chart-electrical-wire-insulation
Comparison of the Dk and LOI of different polymers.

Which insulation compound is right for my device?

Most cables—especially those in the medical device realm—utilize some form of plastic electrical wire insulation also known as a polymer.

The type of polymer you use depends on a variety of factors.

For example…

In the medical device world, one key consideration is the application.

Is the cable going to come into contact with, or be inserted into, the body?

Does the cable need to withstand a certain number of autoclave cycles?

Another consideration is flexibility.

Does the cable need to be stiff so that it stays in place, or does it need to be flexible enough to navigate the arteries of the body?

Temperature rating and product lifespan are other examples of key considerations when selecting insulation for a device.

The list goes on and on.

However, below are some of the common electrical wire insulation materials we see used throughout the medical device industry today.

DEHP-Free PVC

PVC is a common choice for low-cost, disposable applications in which the cable will have limited exposure to the patient.

PTFE

PTFE is typically used in high temperature applications.

Though PTFE offers excellent relative permittivity, its relatively high manufacturing cost can make it unfeasible for certain applications.

Ethylene Propylene Rubber (“EPR”)

EPR offers excellent flexibility and durability.

Due to its strong insulative properties, it is commonly used in high-voltage applications.

Silicone Rubber

For applications that require many autoclave cycles and/or implantation, silicone is one of the only options available.

However, due to its relatively high cost, silicone is typically only used in applications where it is absolutely necessary.

For help finding the correct electrical wire insulation material for your application, we recommend speaking with an expert.


Additional resources.

Though the conductor and insulation are two very important components of every cable, there are other pieces (i.e., shielding and jacketing) that can impact the overall performance of the cable.

To learn more about what it takes to design a cable for your energy-driven device, download our free eBook.

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