An acceptance test for a low-voltage molded-case circuit breaker verifies the breaker's mechanical, electrical, and functional integrity before it is placed into service. The test battery includes identification of the breaker's manufacturer, model, serial number, frame size, rated continuous current, voltage rating, pole count, trip-unit type, and interrupting rating per NETA ATS-2025 7.6.1.1.1, paired with insulation-resistance measurement taken at 1-minute intervals: pole-to-pole (A-B, B-C, and C-A) and pole-to-ground readings taken with the breaker closed, and readings taken across each open pole with the breaker open. The insulation-resistance measurements are recorded in megohms, corrected for temperature, and compared against acceptance minimums that depend on the breaker's rated voltage.
A new or reconditioned low-voltage molded-case breaker must prove it can isolate one live conductor from another and from ground without leakage that would mask a fault or present a shock hazard during normal operation. Low insulation resistance between poles or across an open contact indicates moisture, contamination, or manufacturing defects in the breaker's internal structure that could cause nuisance tripping, failure to trip on a real fault, or an unintended ground path. Confirming identity and baseline insulation resistance before the breaker is first energized, and in the field rather than in the factory, ensures that damage during shipment, storage, or installation is caught before the breaker is relied upon.
Acceptance testing is performed on new or reconditioned low-voltage molded-case circuit breakers before they are energized for the first time, as part of the initial commissioning sequence. It may also be performed on replacement breakers, spare units removed from inventory for installation, or units returned after repair or reconditioning, to confirm they are fit for service in their intended location.
With the circuit breaker de-energized, isolated, and mechanically locked or tagged to prevent unintended operation, the breaker is inspected visually for shipping damage or obvious defects, and its nameplate data (manufacturer, model, serial number, frame size, voltage rating, poles, trip unit, interrupting rating) are recorded. An insulation-resistance test set energized at the voltage specified in NETA ATS-2025 Table 100.1 for the breaker's voltage class is applied with the breaker closed: readings are taken across each pole-to-pole pair (A to B, B to C, C to A) and from each pole to ground, with each 1-minute resistance reading recorded in megohms and temperature-corrected. The breaker is then opened, and insulation resistance is measured across each open pole (from the line side to the load side of the same pole) to confirm the open contacts isolate cleanly. The test connections are removed, the breaker is verified open, locked or tagged, and released for installation.
For each circuit breaker tested: the complete nameplate identification (manufacturer, model, catalog number, serial number, frame size, rated continuous current, voltage rating, number of poles, trip unit type, and interrupting rating); the insulation-resistance value in megohms for each pole-to-pole pair (A-B, B-C, C-A) and from each pole to ground, measured with the breaker closed; the insulation-resistance value across each open pole, measured with the breaker open; the ambient temperature at the time of test; and the identification (make, model, serial number, calibration due date) of the test instrument used. Date, time, and the initials of the person conducting the test are also captured.
NETA ATS-2025 Table 100.1 specifies minimum insulation-resistance values for low-voltage molded-case circuit breakers based on the breaker's rated voltage, with Table 100.14 supplying the temperature-correction factors applied to each reading. The minimums apply to pole-to-pole measurements and pole-to-ground measurements taken with the breaker closed, and to measurements taken across each open pole. Any reading below the value stated in the table for the breaker's voltage rating is grounds for further investigation before the breaker is placed in service. The corrected resistance value - adjusted to a standard reference temperature so that thermal effects do not mask true insulation degradation - is the basis for acceptance or rejection. See the purchased NETA ATS-2025 standard for the complete table and voltage-class breakpoints.
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