Grounding conductor and resistance of connection - 深圳市普威国际检测认证有限公司

The resistance of grounding conductors and connections is a core indicator in electrical safety testing for the continuity and reliability of the grounding system, directly affecting the effectiveness of the grounding protection function. It essentially refers to the sum of the resistance of the grounding conductors themselves (such as grounding wires and main grounding conductors) and the contact resistance at connection points between conductors (such as joints, terminals, bolted connections, etc.), collectively termed the "grounding loop resistance."

I. Core Definition and Function

Grounding conductor: Refers to the conductor (such as copper busbar, insulated grounding wire, flat steel, etc.) used to connect the exposed conductive parts of electrical equipment and devices (e.g., enclosures, metal frames) to the grounding electrode (or main grounding conductor), serving as the "pathway" for fault current and leakage current to flow into the earth.
Connection resistance: Refers to the contact resistance between conductors, between conductors and grounding terminals, and between terminals and grounding electrodes (additional resistance caused by loose connections, oxidation, corrosion, etc.).

The total resistance of both must be sufficiently low to ensure:

In the event of equipment leakage, fault current can quickly flow through the grounding loop into the earth, preventing the equipment enclosure from becoming energized (voltage must remain within safety limits, e.g., below 50 V);
Overcurrent protective devices (such as circuit breakers, residual current devices) can operate within the specified time (e.g., within 0.1 s), disconnecting the power supply and preventing electric shock accidents;
Abnormal currents from lightning strikes and surges can be discharged through the low-resistance loop, protecting equipment from damage.

II. Key Test Parameters and Limits

The resistance of grounding conductors and connections must meet the "low-resistance continuity" requirement, with specific limits varying by application:

General low-voltage distribution systems: The total resistance of grounding conductors and connections is typically required to be no more than 0.1 Ohm (this value represents the resistance of conductors and connection points, excluding the resistance between the grounding electrode and the earth). If this value is too high, it will cause the total grounding loop resistance (grounding electrode resistance + conductor and connection resistance) to exceed the limit.
Special applications:

IT systems in medical facilities (isolated power supply systems): grounding connection resistance required to be no more than 0.2 Ohm;
Explosion-proof equipment and precision instruments: may require no more than 0.05 Ohm to prevent sparks or interference caused by excessive connection resistance.

Note: The resistance between the grounding electrode and the earth (i.e., "ground resistance") is typically required to be no more than 4 Ohm (e.g., TN systems, TT systems), and the resistance of grounding conductors and connections serves as a "prerequisite safeguard" — if this portion exceeds the limit, it will directly cause the total ground resistance to be non-compliant.

III. Test Tools and Methods

1. Common Tools

Milliohmmeter (low-resistance tester): Used for measuring small resistances (0.001 Ohm to 10 Ohm), suitable for testing conductor body resistance and connection point contact resistance;
Ground resistance tester (four-wire method): Uses the "current electrode - voltage electrode" separation method to eliminate test lead resistance interference, enabling precise measurement of total grounding loop resistance (including conductors, connection points, and grounding electrode resistance);
Loop resistance tester: Designed for grounding connections of high-voltage equipment (such as switchgear, circuit breakers), measures low resistance by applying a high DC current (e.g., 100 A) to simulate fault current conditions.

2. Test Procedure

Disconnect unrelated circuits: Before testing, disconnect the equipment from the power supply to ensure the grounding loop is isolated, preventing interference from other circuit currents;
Clean connection points: Remove oxide layers and oil/grease from connection points (sandpaper may be used) to ensure good contact;
Connect test leads: When using a milliohmmeter, connect the test clamps to both ends of the grounding conductor (or both sides of the connection point); when using a four-wire method tester, wire according to "E (grounding electrode), P (voltage electrode), C (current electrode)";
Read data: Apply the test current (e.g., 1 A DC), wait for the reading to stabilize, then record the resistance value and compare against the standard limit to determine compliance.

IV. Common Causes of Non-Compliance and Hazards

1. Causes of Non-Compliance

Insufficient cross-sectional area of the grounding conductor (excessive resistance);
Loose, oxidized, or corroded connection points (increased contact resistance);
Broken conductor, poor crimping (e.g., terminal not fully compressed during crimping, resulting in a false connection);
Use of substandard conductors (e.g., copper material with excessive impurities, resulting in elevated resistance).

2. Hazards

Excessive resistance -> fault current cannot be quickly diverted -> equipment enclosure becomes energized, leading to electric shock accidents;
Unstable connection resistance (e.g., looseness) -> intermittent failure of grounding protection, increasing safety hazards;
During lightning strikes/surges, high resistance prevents energy dissipation -> equipment burnout (e.g., motors, distribution boxes).

V. Relevant Standards

China: GB 50169-2016 Code for Construction and Acceptance of Grounding Devices in Electrical Installation Engineering (specifies resistance requirements for grounding connections); GB 7251.1 Low-Voltage Switchgear and Controlgear Assemblies (specifies grounding connection resistance requirements for enclosures).
International: IEC 60364-5-54 Earthing Arrangements and Protective Conductors; UL 467 Grounding and Bonding Equipment.

Summary

The resistance of grounding conductors and connections is the first line of defense for the "effectiveness" of a grounding system — its value must be sufficiently low and stable. Through periodic testing (e.g., after equipment installation and during scheduled maintenance), issues such as loose connections and conductor aging can be promptly identified, ensuring reliable grounding protection and preventing safety incidents including electric shock and equipment damage.