There’s more to electric insulation than you might think

Article By : Bill Schweber

As with all simple-sounding materials and components, there’s much more to electrical insulation than just its most-obvious performance characteristics.

Most design engineers take wire for granted and don’t give much thought to its basic wire-covering insulation. And why should they? In most cases, the insulation fulfills its role of preventing bare conductors from contacting anything else, does it well, and doesn’t cause problems (Figure 1).

Figure 1 Wire is widely available in diverse types, and generally, designers need not pay much attention to its insulation. Source: The Home Depot

Today’s fabrication technology makes coating wire with insulation of various types generally a low-cost, high-speed process. That’s in sharp contrast to the early days of electricity when investigators such as Michael Faraday had to manually wrap silk around copper conductors (Figure 2).

Figure 2 In the early days of this mysterious thing we now call electricity, experimenters had to insulate their own wire with wrapped silk or cotton. Source: YouTube

Of course, “in most cases” is not the same as “in all cases.” As with just about any simple-sounding function or component in our technical world, just because it seems easy and straightforward doesn’t mean it really is, and that’s true for insulation too.

What you worry about with insulation depends on where you are on the voltage and frequency spectrum. In the low-voltage world of single-digit potentials, insulation is often a relatively minor consideration. Still, insulation has parameters that affect its ability to isolate conductors and relate to RF, fire, mechanical/abrasion, and chemical-resistance performance.

Obviously, the ability of insulation to meet its primary function of doing what its name defines is what most applications need to look at first. If you are not sure about it, check out the 24-page application note “The Complete Guide to Electrical Insulation Testing” from Megger. They have been in business for 130 years and the name Megger for their megaohm tester has become a generic term for that class of instrument, much like Band-Aid or Q-tip have become for their products (Figure 3).

Figure 3 This hand-cranked Megger, which is still available, has four selectable test voltages of 100, 250, 500 and 1,000 VDC, and can measure insulation resistance up to 2000 MΩ. It has an analog meter for displaying results. Source: Test Equipment Depot

Three insulation tests

There are three basic electrical insulation tests:

  1. The high-potential or hipot test, formally called a dielectric strength test, stresses the insulation and ensures that no current flows from one conductor to another. It’s the opposite of a continuity test and is usually done following the International Electrotechnical Commission (IEC) standard IEC 60950.
  2. The partial discharge (PD) test finds small electrical sparks, the localized dielectric breakdowns that occur within the insulation of medium- and high-voltage conductors. These discharges result from electrical breakdowns or ionization in air pockets within insulating layers, which may be due to microvoids.
  3. The insulation resistance test uses a megohmmeter—the “megger”—to impose a high DC voltage between conducting layers and then measures the associated resistance.

There is some overlap among these tests, but each also provides unique insight on the insulation’s present condition as well as early clues of possible problems, especially as voltages and other stresses increase.

Insulation issues don’t end with DC or AC power performance. As designs migrate to the RF world of hundreds of megahertz and tens of gigahertz, the more subtle properties of insulation such as its dielectric constant—usually designated by εr or κ, and also called relative permittivity—become important.

But why stop there? Once you worry about subtle properties such as dielectric constant, you also have to think about the temperature coefficient, which can become a serious issue in and above the 1-10 GHz zone. Vendors of these RF cables specify both the nominal dielectric constant and temperature coefficient. In general, you’ll have to take their word for both parameters as they are hard to measure accurately and consistently.

Insulation in counterfeit cables

There’s another insulation factor that designers should be aware of, even if they are not directly responsible for it. It turns out that counterfeit cables are a big headache, especially for in-building data wiring such as for Ethernet. Some of these counterfeits start at the conductor by substituting cheaper aluminum for copper, which is easy to spot, or they go one step better and provide copper-clad aluminum. The result is substandard data-rate performance. In the case of power-over-Ethernet (PoE), it can also lead to overheating and can even initiate a fire in higher-current IEEE 802.3bt applications. IEEE 802.3bt was previously called PoE++.

The conductor is literally only half the story. These counterfeits also use cheaper insulation with the needed high-resistance performance but don’t meet the strict flammability tests required for cabling placed in walls. The cabling standards ensure that the insulation does not sustain a flame or give off noxious gases. No surprise: the fake-insulated cabling comes on reels or boxes with all the needed approval markings and works fine—until a fire breaks out.

In almost all cases in which cables failed to meet applicable fire- and flame-resistance standards established by Underwriters Laboratories (UL) and the National Fire Protection Association in the United States, they were marked as UL-certified.

Do a search on “counterfeit” at the website Cabling and Installation Maintenance, and you’ll get a good sense of the magnitude of the problem. In some cases, substandard cables had to be ripped out and replaced literally. There have been lawsuits regarding negligence and lack of due diligence when fires did break out and insulation contributed to damage and injury. Some contracts now require the contractor to re-test each cable reel to assure it does meet the UL and other standards regardless of what’s marked.

When someone casually remarks, “Hey, it’s no big deal, it’s just insulation,” it’s mostly true, however, it’s not true in all cases. Its quality, reliability, and performance in the areas of electrical resistance, chemical resistance, abrasion resistance, dielectric constant, and even outright fraud can make this simple-sounding component a performance and safety concern.

Have you ever encountered insulation that was inadequately specified or tested for your application? Even worse, have you ever been “burned” by substandard or counterfeit insulation? Share your experiences, good and bad.

This article was originally published on EDN.

Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.

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