I. A Commonly Confused Concept

In cable and conductor selection, a set of concepts is often used interchangeably: bare conductor, covered conductor, and insulated conductor.

Many purchasing and technical personnel treat "covered conductor" and "insulated conductor" as the same thing when requesting quotes. However, in the standard definitions of electrical engineering, these three are fundamentally different.

Choosing the wrong one can either result in unnecessary expenses or create safety hazards.

Let's clarify these three definitions first.

II. Standard Definitions: What are the differences between the three?

Bare Conductor – Conductive material without any covering or electrical insulation. It is simply a pure metal wire exposed to the air.

Covered Conductor – Covered with a dielectric material, but this material does not have a rated dielectric strength. In other words, it has a "cover," but this cover is not designed and certified as insulation.

Insulated Conductor – Covered with a dielectric material with a defined dielectric strength rating. It has a complete insulation class and can withstand specific voltages without breakdown.

This distinction has a significant impact on practical engineering.

III. Bare Conductor: The simplest form, but not "unsafe"
Bare conductors are the most traditional and widely used form in overhead transmission lines.

Common types of bare conductors include:

ACSR (Aluminum Stranded Wire with Steel Core) – Aluminum stranded wire is wound around a galvanized steel core, combining high conductivity and high mechanical strength, making it the mainstay of long-distance high-voltage transmission. The steel core bears the mechanical tension, while the aluminum stranded wire carries the current.

AAC (All-Aluminum Conductor) – Lightweight and highly conductive, but with low tensile strength, suitable only for short spans or low-voltage networks.

AAAC (All-Aluminum Alloy Conductor) – Higher tensile strength and better corrosion resistance than AAC, without the complex structure of a steel core.

Copper conductors—with the highest conductivity (approximately 58 MS/m), have been largely replaced by aluminum in high-voltage transmission due to their weight, high cost, and susceptibility to theft.

The "insulation" of bare conductors is air. Air is used as the insulating medium by maintaining sufficient phase-to-phase and phase-to-ground distances. As long as the physical spacing is sufficient, it is safe.

IV. Covered Conductors: A "skin" on the surface, but not "insulation"

The "covering layer" of a covered conductor is a thin layer of dielectric material. It does not have a rated dielectric strength in standards.

What does this mean?

It means you cannot treat it like a real insulated cable, assuming it can touch other objects or be grounded without problems.

The primary design purpose of covered conductors is not "insulation," but rather:

Reducing short circuits caused by vegetation contact—a tree branch touching a covered conductor does not necessarily cause an immediate phase-to-phase fault.

Reducing the risk of electric shock to wild animals—the covering layer provides some protection when animals such as squirrels and snakes simultaneously come into contact with both the covered conductor and the grounding electrode.

Reducing the frequency of line faults—under certain conditions, the covering layer can prevent transient faults from developing into permanent faults.

The key point is: coverings are not safety barriers.

As a technical report from EPRI (Electric Power Research Institute) points out: "Safety is sometimes cited as a reason for using covered conductors, but these systems do not necessarily provide a safety advantage; in some ways, covering is actually a disadvantage."

V. Hidden Risks of Covered Conductors This may be something you haven't realized before: covered conductors can cause more insidious and destructive faults than bare conductors.

First, faults are harder to detect.

When a bare conductor fails, it is usually a direct short circuit, producing a visible arc and tripping. Protection devices act quickly, and the fault is immediately isolated.

The situation is different with covered conductors. A small defect—such as a pinhole or crack in the covering—can lead to a high-impedance fault. The current in this type of fault is not large enough to trigger conventional overcurrent protection devices, but the arc and localized heating persist. The fault may persist for hours or even days, slowly worsening and eventually developing into a more serious failure.

Second, damage is hidden by the covering.

Surface damage to bare conductors—corrosion, wear, broken strands—can be detected through visual inspection. Maintenance personnel can see the problem from the ground using binoculars.

Damage to the covered conductor lies beneath the covering layer. Even professional inspections often fail to detect internal conductor corrosion, broken strands, or other degradation. One analysis revealed that over 70% of field faults in covered conductor lines involve insulation breakdown due to UV exposure, mechanical abrasion, or animal contact—problems that would be much easier to detect and repair on bare conductors.

Third, the fire risk may be higher.

This is a counterintuitive conclusion: a fallen covered conductor touching the ground can pose a greater fire risk than a bare conductor.

The reason is that a bare conductor grounding typically produces a noticeable arc and sparks, which would actually "warn" nearby people to stay away. The covering layer of a covered conductor suppresses the arc, making it less noticeable, but the fault current still exists, and the continued localized heating could ignite surrounding dry vegetation.

The 2018 Woolsey Wildfire in California is a prime example. The investigation determined that a steel guy wire came into contact with a live 16kV covered conductor under high wind conditions. The covering layer failed to prevent the arcing, ultimately causing a fire that burned nearly 97,000 acres and destroyed over 1,600 buildings.

VI. When to Choose Bare Conductors vs. Covered Conductors

This information is not meant to deter you from using covered conductors. Covered conductors have clear value in specific scenarios. The key is choosing the right solution.

Scenarios where bare conductors are preferred:

Long-distance high-voltage overhead transmission lines (69kV and above)

Lines that are easy to maintain and inspect

Cost-sensitive projects (bare conductors are cheaper)

Situations where direct power outages are acceptable in the event of a fault

Scenarios where covered conductors should be considered:

Areas with dense vegetation and frequent tree branch contact faults
Areas with high wildlife activity and high rates of electric shock faults
Critical power lines with high requirements for transient fault tolerance
Lines in fire-risk areas (requiring additional protective measures)

VII. Three Core Engineering Reminders

1. Covered Conductors ≠ Insulated Conductors
Do not assume that covered conductors can be directly exposed to other objects or buried underground like insulated cables. The dielectric strength of the covering layer is not rated and certified. 1. For true insulation protection, choose standard-compliant insulated conductors (such as XLPE insulated cables).

2. Covered conductors must not reduce safety clearance requirements.

NESC explicitly requires that covered conductors be treated as bare conductors in terms of safety clearance. Do not reduce phase-to-phase or phase-to-ground distances simply because of a covering.

3. If using covered conductors, appropriate detection methods are essential.

Hidden faults in covered conductors pose a real risk. Lines using covered conductors should be equipped with early fault detection methods such as partial discharge monitoring, infrared thermal imaging, and ultrasonic testing. Otherwise, faults may slowly worsen unnoticed, eventually leading to larger accidents.

VIII. Summary
The core difference between bare conductors and covered conductors lies in whether the covering layer has a rated dielectric strength.

Bare conductors rely on air insulation; their fault modes are simple and direct, making them easy to detect and maintain.

Covered conductors have a thin dielectric layer, which can reduce transient faults caused by external factors, but it can also hide damage, conceal faults, and increase the difficulty of detection.

Insulated conductors, on the other hand, have a complete insulation class and can withstand specific voltages without breakdown.

The key to selection decisions is not "which is better," but "which is more suitable for your specific application conditions."

If you are unsure whether a line should use bare conductors or covered conductors, first compile the following parameters: voltage level, line length, corridor environment (vegetation density, wildlife availability), maintenance conditions, and acceptable failure rate. This information will guide you to the correct choice.