Last week, a customer in the Philippines sent me some photos.

They bought a batch of low-voltage power cables from another company, and used them for about eight months. One of them experienced voltage fluctuations, so they replaced it with a new one and then cut it for an "inspection." In the photos, the surface of the copper conductor had a dark black layer.

The customer asked directly: "Is this recycled copper? Will yours be like this too?"

I asked him to first send me photos of both ends of the black copper core cable.

Sure enough, it was as I suspected.

The conclusion is this: In most cases, blackening of the copper core is not due to a problem with the copper itself, but rather a problem with the insulation material or the operating environment.

More precisely: Blackening of the copper does not necessarily mean the cable is about to fail, but you need to learn to distinguish between "harmless surface discoloration" and "fatal internal corrosion."

Two most common causes of "copper blackening":

First: Oxidation caused by sulfur released from the insulation material.

This is the most common situation. Some rubber or PVC materials contain sulfur. During cable heating, this sulfur is released and reacts with copper to form cuprous sulfide—a black substance.

This phenomenon even occurs in many international brand cables.

Does it affect conductivity? Slightly.

Will it cause the cable to burn out immediately? No.

However, it is a warning sign, indicating that:

The insulation material is not a high-purity, sulfur-free formula.

The actual operating temperature of the cable may be higher than normal.

The second type: Moisture ingress + electric field action → electrochemical corrosion.

This is the truly worrying situation.

If the cable joint is not properly sealed, or the sheath has minor damage, moisture slowly seeps in. Under the influence of an electric field, copper and water undergo an electrochemical reaction, producing black copper oxide and cuprous oxide.

The characteristics of this corrosion are:

A shiny black color and brittle texture.

Pits appear on the surface of the copper wire.

Continuing to develop, the cross-sectional area of ​​the copper wire decreases → localized heating → eventual burnout.

Here's a method you can use without instruments.

I've transformed the question a customer asked into a simple method you can use.

Step 1: Determine if the blackening is "attached to the surface" or "infiltrated into the copper."

Gently scrape the blackened copper surface with a blade:

If it can be scraped off, revealing shiny copper → it's most likely a harmless sulfidation reaction.

If it remains black after scraping, or even if the surface is rough and powdery → be wary, it may be corrosion.

Step 2: Check if the blackening is "uniformly distributed."

If the copper core of the entire cable is uniformly black → it's more likely due to oxidation from the insulation material.

If it's black in sections and shiny in others, or particularly severe near the joints → it's highly likely due to water ingress/moisture.

Step 3: Measure the resistance (the most reliable method).

Take a one-meter length of the same cable and measure its DC resistance. If the resistance value is still within the allowable range of the national standard (IEC 60228), it means the copper core itself has not been severely worn.

An experience I learned from a customer:

"Blackened copper doesn't necessarily mean it's bad, but if the resistance is too high, it must be replaced."

What happened to the customer's cable later?

I had him measure the resistance.

It was still within the standard range.

Then I had him scrape the surface.

It came off. The final assessment was that the discoloration of the copper sulfide surface was not the cause of the voltage fluctuations. The real problem lay elsewhere – the crimping quality of the intermediate joints.

If they had replaced all the cables simply because of the "blackened copper," the cost would have been at least three times higher, and the problem wouldn't have been solved at all.

Three practical suggestions for you:

1. If you see blackened copper cores, don't immediately blame the supplier.

First, perform the three-step assessment mentioned above. Often, blackening is a problem with the application environment, not the copper itself.

2. What you should really be wary of isn't the color, but rather "powdering" and "increased resistance." If the copper core is brittle, the surface sheds powder, and black powder comes out when rubbed between your fingers – in these cases, the cable must be replaced.

3. If you are working on export projects, you can add a sentence to the procurement terms:

"Copper conductor discoloration alone shall not constitute grounds for rejection unless accompanied by measurable resistance increase or corrosion-induced cross-section reduction." This sentence can help you avoid most unprofessional claims.

Finally, let me be frank:

Of the cables we've dissected, less than 5% actually fail due to "impure copper."

Most complaints about "blackened copper" turn out to be caused by either excessive operating temperature, water ingress into the joints, or normal sulfidation—which doesn't affect usability at all.

The problem is: many people, upon seeing black, assume it's "inferior recycled copper."

This lack of understanding leads to unnecessary expenses.

If you happen to have a cable with blackened copper core and are unsure, you can use the method I described above to test and verify it.

Not for any other reason than that I myself have been misled by the same issue.