You've hit on a nuanced topic in electrical grounding and bonding, particularly concerning signal cables in the presence of potential noise or interference. Here's a breakdown to address your questions

Key Concepts:
Main Ground (Equipment Grounding Conductor - EGC)

This is your primary safety ground. It's connected to the service entrance grounding electrode system (think grounding rod, water pipe, etc.) and is designed to carry fault current back to the source, tripping a breaker and clearing a fault quickly. In the NEC, this is what provides a low-impedance path for fault current. It is typically connected to metallic enclosures, conduit, and equipment.

Isolated Ground (Isolated Grounding Conductor - IGC)

This is a grounding conductor run specifically to reduce electrical noise. It's isolated from the normal equipment grounding system and is intended to provide a clean reference point for sensitive electronic equipment. It eventually ties back to the main grounding system, typically at the service entrance.

Shielding

Cable shields (foil, braid, or armor) are intended to protect the signal conductors from electromagnetic interference (EMI) and radio-frequency interference (RFI).

Regarding Armored Signal Cables

Here's the best practice and the "why" behind it:1.

Armor Grounding Point (Armor vs. Shield)



The Armor should generally be connected to the Main Ground (EGC). The armor's primary function is not signal integrity; it's mechanical protection and fault current carrying capability. If the cable is damaged and a live conductor shorts to the armor, you want that fault current to flow back to the source quickly and trip a breaker. Connecting the armor to the main ground provides that low-impedance path. The shield should be connected to the Isolated Ground (IGC) at one end only.2. Grounding Location (One End vs. Both Ends)



Armor: Both Ends: The armor should be grounded to the Main Ground at both ends of the cable run. This ensures a continuous fault current path along the cable's length. Shield: One End ONLY: Connect the shield to ground at one end only (the source end is often preferred, but consult the equipment manufacturer). Grounding at both ends creates a ground loop.

Why One-End Shield Grounding? Ground Loops

If you ground the shield at both ends and you have slightly different ground potentials at each end of the cable, current will flow through the shield. This circulating current can introduce noise into the signal conductors.

Effectiveness

Grounding the shield at one end is generally sufficient to provide shielding against EMI/RFI. The shield acts as a "Faraday cage," intercepting interference.

NEC and Code Considerations

The NEC doesn't explicitly mandate these techniques for signal cables in the same way it does for power cables. However, the principle of providing a safe fault current path is always paramount.

NEC 250.4(A)(5) - Effective Ground-Fault Current Path. The NEC emphasizes creating a low-impedance path for fault current. Bonding the armor to the equipment grounding system (main ground) fulfills this requirement. Manufacturer's Instructions

Always follow the manufacturer's recommendations for grounding and bonding signal cables. They may have specific instructions based on the cable type, application, and equipment involved.

Local Amendments

Be aware of any local electrical code amendments that might affect grounding practices.

In Summary

| Feature | Armor (Signal Cable) | Shield (Signal Cable)

--------------- | ----------------------- | -------------------------

Ground Type | Main Ground (EGC) | Isolated Ground (IGC)

Grounding Points | Both Ends | One End ONLY

Primary Purpose | Fault Protection, Mechanical Protection | Noise Reduction, EMI/RFI Shielding |

Important Considerations



Signal Type

The sensitivity of the signal being carried (analog vs. digital, low-voltage vs. high-voltage) influences how critical these grounding techniques are. Highly sensitive signals benefit most from proper shielding and grounding.

Environment

Environments with high levels of electrical noise (e.g., industrial facilities with heavy machinery) require more rigorous grounding and shielding.

Cable Type

Different cable constructions (e.g., shielded twisted pair, coaxial cable) may have specific grounding recommendations. Consult the manufacturer's documentation.

Bonding Jumpers

Ensure that all metallic parts (enclosures, panels, etc.) are properly bonded together with bonding jumpers to maintain a low-impedance ground path.

Example Scenario

Imagine an armored cable carrying a sensitive analog signal from a sensor to a data acquisition system in a factory.1.

Armor

The armor is connected to the metal enclosure of the sensor at one end and to the metal enclosure of the data acquisition system at the other end. Both enclosures are connected to the main equipment grounding system (EGC).2.

Shield

The shield wire inside the armored cable is connected to the isolated ground (IGC) terminal on the data acquisition system. The shield is not connected at the sensor end.This approach prioritizes both safety (fault protection via the armor's connection to the main ground) and signal integrity (noise reduction via the shield's one-point connection to the isolated ground).

Disclaimer: This information is for educational purposes only. Always consult with a qualified electrician or electrical engineer for specific installations and to ensure compliance with all applicable codes and regulations.* Flag for review