The standard behind the contact resistance test on high-voltage switchgear and circuit breakers is IEC 62271-1. It doesn’t use the phrase “contact resistance” — it calls the test “measurement of the resistance of the main circuit.” The method is a DC measurement across the contacts, taken as the average of three readings, at a test current of at least 50 A. On routine tests, the measured resistance must not exceed 1.2 × Ru, where Ru is the reference value from the type test.
If you run a micro-ohmmeter across a breaker in the field, this is the standard your acceptance value ultimately traces back to.
Table of Contents
What contact resistance testing is, and why it matters
Every bolted joint, every breaker contact, every disconnector blade adds a small resistance to the current path. In a healthy connection that resistance is a handful of microhms. In a bad one — a loose bolt, an oxidized contact, a burnt arcing tip — it climbs.
That matters because power dissipates as I²R. A joint carrying 2000 A at 100 µΩ burns 400 watts. At 500 µΩ it burns 2000 watts, in one small spot, forever. The joint overheats, the heat accelerates oxidation, the resistance climbs further, and the connection eventually fails — often as the hottest point in the whole installation.
Contact resistance testing catches that early. You inject a known DC current, measure the voltage drop, and calculate the resistance in microhms. It’s the cheapest, fastest health check there is for a current-carrying connection.
What the standard calls it, and where it sits
IEC 62271-1 is the common specification for high-voltage switchgear and controlgear — the umbrella standard that breaker standards like IEC 62271-100 build on. Inside it, the contact resistance test appears in three places:
- As a type test (Clause 7.4.4) — the full procedure, run once to qualify a design.
- During the temperature-rise test (Clause 7.5.5) — a resistance measurement taken before the continuous-current test, which becomes the reference value.
- As a routine test (Clause 8.4) — run on every unit before it ships, checked against that reference.
So the same measurement does three jobs: it qualifies the design, it sets the baseline, and it screens every production unit against that baseline.
The measurement method
The procedure in Clause 7.4.4 is specific, and worth following even in the field because it’s what makes readings comparable.
DC, not AC. The measurement is made with direct current. That removes any inductive component and gives you the pure ohmic resistance of the joint.
Measure across the nearest accessible points. You take the reading at the closest reachable points on either side of the contacts or connections being checked — not across half the busbar. The shorter the span, the less the fixed conductor resistance dilutes the contact reading.
Average of three. A single reading isn’t enough. You take three measurements and average them. If the device has moving contacts, you run a no-load open/close cycle between each reading; if it has removable parts, you remove and replace them between readings. That captures the real variability of the contact, not one lucky seating.
The test current rule. This is the part people get wrong. IEC 62271-1 sets it by the rated normal current (Ir) of the equipment:
- If Ir ≤ 50 A, measure at the rated current (with a −20% to 0% tolerance).
- If Ir > 50 A, measure at any convenient value between 50 A and Ir.
So 50 A DC is effectively the floor. In practice, field micro-ohmmeters (DLRO or “ductor” testers) inject 100 A or more, which sits comfortably inside the standard’s range and helps burn through minor surface films for a stable reading.
Ambient, and consistent. Before-and-after measurements are made at ambient temperature, within 10 K of each other. Resistance rises with temperature, so a reading only means something when you know the temperature it was taken at.
The acceptance criteria
There are two different pass marks, depending on which test you’re running.
Routine test (every unit)
The measured resistance must not exceed 1.2 × Ru.
Ru is the resistance measured on the type-tested unit before its continuous-current test — the reference the design was qualified at. In plain terms: a production breaker can be up to 20% higher than the qualified reference and still pass. Go above that and something in the assembly isn’t right. For assemblies, the expected resistance may have to be calculated from the relevant type tests rather than read off a single reference.
Condition check (after another test)
When resistance is measured as a health check after some other type test — a short-circuit test, a mechanical endurance test — the pass mark is a limit on how much the resistance increased:
- 20% increase is the default ceiling for most tests. Above 20%, a continuous-current test is required to prove the device can still carry its rated current.
- 100% increase is the ceiling specifically for the condition check after making and breaking tests, where some contact erosion is expected.
The logic: the resistance you started with is the reference, and the test must not have degraded the contacts beyond a defined margin.
How this maps to field testing
Here’s the honest bridge. IEC 62271-1 defines the test in a factory and type-test context. When you run a contact resistance test during commissioning or maintenance, you’re doing the same physical measurement, but your acceptance value usually comes from one of:
- The manufacturer’s declared value for that specific breaker — always the first reference. The 1.2 × Ru principle is why manufacturers publish a maximum.
- A commissioning baseline — the value recorded when the equipment was new, which you trend against over the years.
- A maintenance spec such as NETA ATS/MTS, which sets field acceptance limits (commonly “not more than a set percentage above the manufacturer’s value, or above similar units”).
The measurement discipline still applies: DC, a healthy injection current (100 A+), the reading taken close across the contacts, and the temperature noted. A rising trend across successive tests is the real warning sign — a single number in isolation tells you less than its history.
FAQ
What standard covers the contact resistance test?
For high-voltage switchgear and circuit breakers, IEC 62271-1 covers it, under the name “measurement of the resistance of the main circuit.” Breaker-specific standards such as IEC 62271-100 refer back to it.
What test current does IEC 62271-1 require?
DC. If the rated normal current is 50 A or less, measure at the rated current. If it’s above 50 A, use any convenient value between 50 A and the rated current. Field instruments typically inject 100 A or more.
What is the acceptance criterion for contact resistance?
On a routine test, the measured resistance must not exceed 1.2 × Ru — 20% above the type-test reference. As a condition check after another test, the resistance increase must stay within 20% (or 100% after making and breaking tests).
Why is the test done with DC and not AC?
DC removes the inductive component of the circuit, so you measure the pure ohmic resistance of the contacts and connections without reactance distorting the result.
Is “contact resistance” the same as “main circuit resistance”?
In IEC 62271-1 the formal term is main circuit resistance, which includes the contacts plus the connections in the measured path. “Contact resistance” is the common field name for the same measurement.
Key takeaways
- The governing standard is IEC 62271-1; the test is “measurement of the resistance of the main circuit” (type test 7.4.4, routine test 8.4).
- Method: DC, measured close across the contacts, average of three readings, at ≥ 50 A (or the rated current if that’s below 50 A).
- Routine acceptance: ≤ 1.2 × Ru — up to 20% above the type-test reference.
- Condition-check acceptance: ≤ 20% increase (≤ 100% after making/breaking tests).
- In the field, compare to the manufacturer’s value and your own baseline, and watch the trend, not just the single reading.