Loop Resistance VS Contact Resistance

1.What is "Loop resistance" in substation and why is it called that?

(Loop Resistance) refers to the total resistance of all conductive parts within a complete current loop.

In substations or high-voltage systems, this “loop” usually refers to the current path that starts from the circuit breaker main contact, passes through conductive clamps, busbars, connection lines, current transformers, isolating switches, and finally returns to the other end, forming a closed loop.

The term “loop” is used because the current starts from the power source, flows through the load or conductive path, and then returns to the source — forming a closed circuit.
Loop resistance measurement focuses on the total resistance that hinders current flow within this loop. It encompasses the resistance of internal conductive components, connection points, and conductors themselves — a key electrical parameter in high-voltage circuits.

2.Components Included in Substation Loop Resistance

1.Conductors: Intrinsic resistance of conductive materials such as high-voltage busbars, cables, and leads.

2.Connection Points: Contact resistance at circuit breaker contacts, isolating switch contacts, terminals, bolted joints, and bushing connections.

3.Internal Conductive Parts of High-Voltage Equipment: Intrinsic resistance of transformer windings, conductive rods in circuit breaker arc chambers, and primary conductive parts of current transformers.

Are Loop Resistance and Contact Resistance the Same?

Loop resistance (Loop Resistance) and contact resistance (Contact Resistance) are often used interchangeably in practice, but they are not identical.
They differ in definition, composition, measurement range, and application scope (see table below):

Project	Loop Resistance	Contact Resistance
Definition	The total resistance of all conductors and connection points in a complete conductive loop.	The local resistance between two conductor contact surfaces caused by microscopic imperfections and oxide layers.
Measurement Scope	Covers all conductive parts such as contacts, cables, busbars, and connecting bolts	Limited to contact points or connector contact surfaces
Test Purpose	To evaluate the conductivity of the entire loop	To evaluate the quality of contact

3.Loop Resistance:

Includes: Conductor resistance + Connection resistance + Contact resistance

Measurement target: a complete closed circuit, e.g. the “main circuit of a circuit breaker” or “busbar + isolating switch + cable” section.

Contact Resistance: Focuses only on the microscopic contact surface between conductors, such as:

1.Between moving and fixed contacts of a circuit breaker

2.Between isolating switch contacts

3.Between plug and socket connectors

In short:

Contact resistance is part of loop resistance.
Loop Resistance = Conductor Resistance + Contact Resistance

Contact resistance is "Local", while loop resistance is "overall".

• High contact resistance indicates poor contact between conductors.

• High loop resistance indicates a problem anywhere along the conductive path (contact points, connectors, or conductors).

4.Differences Between Loop Resistance and Contact Resistance Measurement

Items	Loop Resistance Test	Contact Resistance Test
Test Current	Generally100A~600A DC current (according to IEC standards)	Relatively small, generally tens of amperes or even milliamperes (depending on the object
Tester	Loop resistance tester	Micro-ohmmeter or dedicated contact resistance meter
Wiring Method	Connecting both ends across the entire loop	Directly connected to both sides of the contacts.

5.Significance of Measuring Loop Resistance in Substations

1. Identify Connection Defects:
   Excessive contact resistance (due to oxidation, loosened bolts, etc.) can cause local overheating, melting of contacts, or even arcing faults.

2. Ensure System Stability:
   Over-limit loop resistance causes excessive voltage drop, reduces power transmission efficiency, and may trigger protection misoperation or equipment derating.

3. Verify Equipment Condition:
   Pre-commissioning or post-maintenance testing confirms installation and repair quality; periodic testing tracks equipment aging and wear trends.

6.Key Safety and Testing Precautions

1. Strict Power Isolation:
   Disconnect all power sources, perform grounding and voltage verification before testing to prevent electric shock or equipment damage.

2. Clean Contact Surfaces Thoroughly:
   Use sandpaper or cleaning agents to remove oxidation and grease, ensuring firm contact between test probes and the test points.

3. Use Dedicated High-Voltage Instruments:
   Employ a high-current loop resistance tester (typically 100 A / 200 A DC output) for accurate measurement; ordinary instruments lack sufficient precision.

4. Isolate Non-Tested Branches:
   Disconnect unrelated branches or parallel components to prevent current division that would cause abnormally low readings.

5. Maintain Safe Distance:
   Keep an appropriate safety distance from energized high-voltage equipment to avoid induced voltage hazards.

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