The test is conceptually a four-wire resistance measurement across the closed contacts of the breaker, taken pole by pole. Its purpose is to detect contact erosion, loss of contact pressure, contamination and connection problems before they become a temperature rise under load or a failure to carry fault current.

Why micro-ohm readings need repeatability

A micro-ohm measurement is dominated by the resistance of the connection between the test leads and the breaker terminals, not by the contact under test. Two measurements of the same healthy contact can differ by more than the change that would indicate erosion, purely because the connection was remade differently, because the current leads were routed differently, or because the voltage sense points were in a slightly different place.

The practical defence is to establish the scatter before interpreting the value. Measuring the same pole three times, breaking and remaking the connections between attempts, gives a spread that represents the measurement rather than the breaker. Any difference between outages smaller than that spread cannot be attributed to the contacts.

Repeatability also depends on where the voltage sense is taken. Four-wire measurement places the potential leads inside the current leads, and the further inside they sit, the less of the connection resistance is included in the result. For a breaker, the correct sense point is on the contact side of the terminal connection, which is why the connection arrangement should be described in the procedure rather than left to the technician.

Test current and its effect on the value

Contact resistance is not a fixed quantity. The interface between two mating contact surfaces conducts through a limited number of microscopic contact points, and the effective resistance depends on the number and size of those points, which in turn depends on the contact force and on the condition of the surfaces.

At very low test currents the measurement can be dominated by thin surface films and by instrument resolution. Raising the test current to a level representative of the contact’s operating range supplies enough energy to break down some of those films and produces a value that better represents service. Most dedicated instruments apply a current in the range that is appropriate to low-resistance measurement for this reason, and the value they report is specific to that current.

That dependency is why the test current belongs in the record. A measurement taken at a hundred amperes and one taken at ten amperes are not interchangeable, even on the same breaker on the same day, and a maintenance programme that changes instruments between visits creates a step in the trend that has nothing to do with the contacts.

Temperature correction for contact resistance

Contact resistance rises with temperature, because the resistivity of the contact material rises with temperature in the same way as any metallic conductor. The change is not as pronounced as in a transformer winding, where the effect is measured against a defined reference, but it is large enough to matter when a marginal result is being judged.

For breaker contacts the practical approach is consistency rather than correction. Readings are taken at a comparable temperature, usually at the start of an outage before the breaker has been carrying current and before the DC test current has warmed the contacts through repeated measurement. The temperature is recorded, and a reading is compared with earlier readings taken under similar conditions.

Where a correction is applied, it uses the same temperature coefficient reasoning as a winding measurement, with the material constant appropriate to the contact material. Applying a correction requires knowing the contact temperature, which for a breaker in an outdoor substation is not a single quantity: the enclosure, the contact and the terminal may all be at different temperatures. In that situation a corrected value carries more apparent precision than it has, and the uncorrected value with its temperature is the more honest record.

Contact erosion: how much is too much

Contacts erode as they interrupt current. The arc transfers material from one contact to the other, and the contact surface changes shape over the life of the breaker. The resistance change that results is not linear, because the contact force rises as the surfaces wear in some designs and falls in others, and because erosion changes the geometry rather than simply reducing the conducting area.

The useful question is not whether the value has moved, but whether it has moved faster than the pattern for that type of breaker. A breaker that has interrupted a large number of fault currents may show a higher value than an identical unit that has only performed load switching, and both may be entirely healthy.

The manufacturer’s value for the type, taken at a defined current and temperature, is the absolute reference where it exists. The unit’s own history is the more sensitive reference, and the operation counter is what gives the history meaning. A rise of a few micro-ohms over a period in which the breaker interrupted several faults is a different fact from the same rise over a period of no fault operations.

Comparing across poles and phases

Pole comparison is the most reliable check available, because all three poles of a breaker share a design, a temperature and, on most designs, a single operating mechanism. The spread between poles is therefore a comparison with very few confounding variables.

The comparison is invalidated by taking the three readings at different times. A pole measured first, before the DC test current has warmed anything, and a pole measured twenty minutes later after two other poles have been measured, are not measured under the same conditions. Keeping the measurement sequence short, and repeating any pole that reads anomalously, keeps the comparison meaningful.

Phase comparison across a switchboard, rather than across the poles of one breaker, introduces type and age differences. Two breakers of the same type and vintage make a reasonable comparison; a distribution breaker and a transmission breaker do not, and the comparison should be stated as what it is.

Interpreting a marginal rise

A marginal rise is the hardest result to act on, and the temptation is to escalate or to ignore it. Both responses have costs. The disciplined approach is to ask what the size of the rise is relative to the measurement scatter, what the operation history over the interval was, and whether the same rise appears in another measurement of the same quantity.

Where the rise is inside the scatter, the correct action is to record it and measure again at the next opportunity. Where it exceeds the scatter but remains inside the type limit, the correct action is to look at the operation count and the fault history for the interval, because a rise explained by duty is not a defect.

Where the rise is outside the limit, or where it is accompanied by a temperature measurement showing a hot spot in the same zone, the next step is physical inspection. The resistance measurement is a screening result; the inspection is what identifies the cause, and the two should not be confused in the report.

Repeat measurement discipline

Repeat measurement is a discipline rather than an optional extra. The procedure that produces usable results states how many times each pole is measured, how the connections are handled between repeats, how the breaker is operated between measurements where that is appropriate, and how the reported value is derived from the repeats.

Recording the individual repeats rather than only the average is what makes the record useful later. A mean of 45 micro-ohms with a spread of two and a mean of 45 micro-ohms with a spread of fifteen describe different situations, and the second is a reason to look at the connection or the contact seating.

Operating the breaker between measurements is a specific discipline with a specific purpose. Exercising the mechanism settles the contacts into their normal position and can change the reading on a pole that was not fully seated. Where the first and second measurements on the same pole differ after an operation, that difference is itself a finding.

What goes into the test record

The record should identify the breaker, the bay and the pole, and should state the test current, the measurement duration, the instrument and its calibration status, the ambient and contact temperature where available, and the connection arrangement used for the potential leads.

Alongside those conditions, the record should carry each individual repeat measurement, the derived value for each pole, the spread between repeats, and the comparison with the previous test and with the type value. The operation counter reading at the time of the test is the field that gives the comparison meaning.

Where the test forms part of an outage programme, the record should state the sequence used within the programme, because a contact resistance measurement taken after other tests that have warmed the breaker is not comparable with one taken first. The switching device requirements that define the rated characteristics the breaker is expected to maintain are set out in IEC 62271-100, with the common requirements for high-voltage switchgear in IEC 62271-1. Method documentation from instrument suppliers such as OMICRON covers the four-wire arrangements used for low-resistance measurement, and the mechanism-specific values are published by the switchgear manufacturers, including Hitachi Energy and Siemens Energy. The wider maintenance context is coordinated through CIGRE study committees.