Circuit breaker contact resistance measurement — the 1.2×Ru acceptance limit as a maintenance indicator

The guide on contact resistance measurement with a micro-ohmmeter (DLRO) covers the general measurement method (4-wire Kelvin, high test current) for cable-lug, bolted and busbar joints. This article covers a specific application of that same measurement technique: the main contacts of a circuit breaker, for which IEC 62271-100 sets a concrete, normative acceptance limit instead of the general 'compare-against-similar- connections' approach.

The measurement: the same 4-wire Kelvin method, a specific purpose

Just as for a cable or busbar joint, the contact resistance of a circuit breaker's main contacts is measured with a 4-wire Kelvin method, at a DC test level of roughly 50 A up to the breaker's rated current. For a circuit breaker, however, the purpose is specific: tracking the wear of the main contacts caused by repeated switching (mechanical wear and arc erosion during current interruption) and by any contamination or oxidation of the contact surface.

The 1.2×Ru acceptance limit

For the type test of a new circuit breaker, IEC 62271-100 establishes a reference value Ru: the contact resistance as measured on the brand-new, unused device. For later routine (periodic) measurements on the same device, the acceptance limit is: the measured resistance must not exceed 1.2 times Ru — an increase of more than 20% relative to the original reference value points to relevant contact wear or contamination that warrants closer assessment. After a specific test involving making/breaking operations, a wider margin (up to a 100% increase) may apply, depending on the exact test conditions and manufacturer data.

Why this differs from the general DLRO comparison approach

For a cable-lug or busbar joint there is usually no universal absolute limit value, so comparison is made between similar joints within the same installation (see the DLRO guide). For a circuit breaker under IEC 62271-100, a concrete, normative reference value (Ru) does exist for each specific device type, established during the manufacturer's type test — so assessment here does not need to rely solely on comparison between devices, but can use an absolute, device-specific acceptance limit.

Trend matters more than a single reading

Besides the absolute 1.2×Ru limit, the trend across successive periodic measurements matters at least as much: a gradual increase in contact resistance across repeated measurements on the same device — even if the latest reading still falls (just) within the 1.2×Ru limit — is an early indication of progressing contact wear, and a reason to increase measurement frequency or schedule maintenance (cleaning, contact replacement) before the acceptance limit is actually exceeded.

Practical relevance

During periodic NEN 3140 inspection of circuit breakers in a switchboard (see also the guides on LSI(G) settings and the primary/secondary injection test), a contact resistance measurement of the main contacts belongs alongside the electrical function tests: the injection test shows whether the breaker trips on a fault, while the contact resistance measurement tracks the physical condition of the main contacts themselves — an aspect an injection test does not directly quantify.

Common mistakes

Applying the general 'compare-against-similar-connections' approach to a circuit breaker without using the specific Ru reference value and the 1.2× acceptance limit from IEC 62271-100, even though it is available.
Not recording a reference measurement (Ru or a first periodic reading) at commissioning, so later measurements cannot be assessed against a baseline.
Looking only at the latest absolute reading and ignoring the trend across multiple periodic measurements — a gradually increasing resistance that still falls just within the 1.2×Ru limit is already a warning sign.
Treating a contact resistance measurement as a substitute for a primary/secondary injection test (or vice versa) — both methods check a different aspect of the circuit breaker (physical contact condition versus protection operation) and complement each other.