You're right to be concerned. Using conductor shielding as a current-carrying conductor, especially in a manner that relies on the shielding's ampacity being a fraction of the main conductor's, is highly unusual and potentially dangerous. Here's a breakdown of the problems, why it's unlikely to be done, and potential reasons behind the claim (however misguided)

Why this is a Bad Idea (and likely a violation of electrical codes):
Shielding is Not Designed for Current Carrying

Shielding on power cables is primarily designed for:

Electromagnetic Compatibility (EMC)

To contain electromagnetic fields generated by the conductor, preventing interference with nearby equipment.

Fault Current Path

To provide a low-impedance path to ground in the event of a fault (short circuit). This allows protective devices (circuit breakers or fuses) to trip quickly and safely.

Reducing Voltage Stress

In some medium and high voltage cables, shielding helps distribute voltage stress more evenly, preventing insulation breakdown.

Ampacity Mismatch and Overheating

Shielding is almost always a much smaller cross-sectional area than the main conductors. Running a continuous load of 1/3 the conductor's capacity through the shielding would almost certainly cause it to overheat, leading to:

Insulation Degradation

Heat weakens the insulation around the main conductors and the shield itself, increasing the risk of short circuits and ground faults.

Fire Hazard

Overheated shielding could ignite surrounding materials, leading to a fire.

Reduced Cable Lifespan

Repeated overheating significantly shortens the cable's useful life.

Impedance and Voltage Drop

Shielding typically has a higher impedance than a dedicated conductor designed for current carrying. This would lead to:

Increased Voltage Drop

The voltage at the load would be lower, potentially affecting equipment performance.

Uneven Current Distribution

It's unlikely that the current would distribute evenly across the three shields, potentially overloading one shield while others are underutilized.

Grounding Issues

Using the shield as a neutral or ground return complicates grounding practices. Proper grounding is critical for safety, and this configuration could compromise the effectiveness of the grounding system.

Code Violations

Virtually every electrical code (NEC in the US, IEC standards internationally) would prohibit this type of installation. These codes mandate specific conductor sizes and types based on the load, and using shielding in this way would not meet those requirements.

Difficulty in Testing and Certification

Manufacturers do not design or test cables with the shielding being used as a primary current-carrying conductor. There would be no independent certification (e.g., UL, CSA) for such an application.

Why You're Not Finding Information Online



It's Not a Standard Practice

The reason you're not finding information online is because it's not a recognized or safe engineering practice. There's no legitimate use case for deliberately loading a cable's shielding to that extent.

Potentially Dangerous

No reputable source would promote a method that increases the risk of fire and electrical shock.

Possible Explanations (Even if Misguided) for the "Testing" Claim



Misunderstanding of Fault Current Capacity

The person making the claim might be confusing continuous current-carrying capacity with the short-time fault current capacity of the shielding. Shielding is designed to carry a large current for a very short duration during a fault, but that's a completely different scenario than continuous loading.

Erroneous Testing

Perhaps the testing was flawed or misinterpreted. It's possible that the test setup was not representative of a real-world installation.

Desperation to Cut Costs

The idea might stem from an attempt to reduce the cost of materials by eliminating a conductor. This is an extremely misguided attempt, since it will almost certainly lead to more expenses due to maintenance, and ultimately replacement, of the installation.

Misapplication of Signal Cable Practices

Shielding in low-voltage signal cables can sometimes be used for low-power grounding purposes. This practice is not applicable to power distribution cables.

What to Do

1.

Challenge the Claim

Politely but firmly challenge the assertion that the shielding can carry 1/3 of the conductor's ampacity. Ask for detailed documentation of the testing that supports this claim, including: Test Setup Cable Specifications Measured Temperatures of the Shielding and Conductors Duration of the Test Ambient Temperature During the Test Independent Certification of the Test Results2.

Consult a Qualified Electrical Engineer

Engage a licensed and experienced electrical engineer to review the proposed design and provide an independent assessment. They can point out the safety and code compliance issues.3.

Demand Code Compliance

Insist that the design comply with all applicable electrical codes and standards.4.

Document Your Concerns

Keep a written record of your concerns and the responses you receive. This documentation could be valuable if problems arise later.5.

Escalate if Necessary

If your concerns are not adequately addressed, consider escalating the issue to the relevant city authorities or electrical inspectors.

In summary, using conductor shielding as a primary current-carrying conductor in the way described is a dangerous and code-violating practice. It's crucial to challenge this approach and ensure that the electrical system is designed and installed according to established safety standards. Flag for review