The resistor acceptance test is an electrical and mechanical check performed on a resistor unit before it is first put into service. The test covers identification of the unit's manufacturer, model, serial number, and nameplate ratings (voltage, resistance in ohms, current capacity, and time or duty cycle); a visual and mechanical inspection for damage, corrosion, or loose fasteners; measurement of bolted-connection resistance at any accessible joints where the resistor connects to bus, cable, or ground; measurement of insulation resistance from the resistor terminals to ground under a specified test voltage; and direct measurement of the resistor's actual resistance value, which is then compared against the nameplate rating. NETA ATS-2025 Section 7.20.4 lists these as the core acceptance items for resistor units in electrical power systems.
A resistor is inserted into a circuit to limit current, dissipate energy, or protect other equipment from overcurrent or surges. Its nominal resistance value, its ability to hold that value under rated current and temperature, and its electrical isolation from ground all must be verified before energization. A resistor whose actual resistance differs significantly from its nameplate rating will produce incorrect limiting or dissipation, potentially allowing excessive current to flow downstream and damaging connected apparatus. Poor insulation to ground or corroded bolted connections introduce partial-arc paths or unexpected voltage drops that can trip protective relays or trigger thermal runaway in the resistor itself.
Acceptance testing is performed before a new or reconditioned resistor is first energized and connected into service. The test is ordinarily done at the factory or shop before shipment to the site, but may also be performed at the installation point if the unit has been stored, shipped, or exposed to a potentially damaging event. The identification, visual, and bolted-connection tests are done with the unit de-energized and isolated. The insulation-resistance and resistance-value measurements follow, using appropriate metering equipment and test voltages specified in NETA ATS-2025.
With the resistor de-energized, isolated, and grounded per safety procedures, the nameplate and any markings are read and recorded to establish the expected resistance, voltage rating, and current capacity. A physical inspection checks for mechanical damage, corrosion of terminals or hardware, loose bolts or fasteners, and any signs of thermal stress or burning. Using a dedicated low-resistance ohmmeter or connection-resistance tester, bolted joints at the resistor's connections to bus, cable, or ground are measured to ensure they do not exceed the contact-resistance acceptance limit. An insulation-resistance test applies a defined DC test voltage (typically 500 V, 1 kV, or higher depending on the resistor's voltage class) between the resistor terminals and ground, and records the resulting insulation resistance. Finally, a precision ohmmeter applies the standard four-wire or two-wire method to measure the resistor's actual resistance value, with the reading corrected to the standard reference temperature and compared against the factory acceptance-test baseline or the nameplate tolerance band.
For identification: manufacturer, model or catalog number, serial number, voltage rating, resistance rating in ohms, current rating, time or duty rating, and equipment designation. From the visual inspection: any defects or abnormalities observed. For bolted connections: the measured resistance in micro-ohms or milli-ohms at each accessible joint. For insulation resistance: the test voltage applied, the measured resistor-to-ground insulation resistance uncorrected and then corrected to a reference temperature. For resistance value: the factory acceptance-test baseline resistance in ohms, the measurement method, ambient temperature, and the measured value at each check interval. Instrument make, model, serial number, and calibration due date are logged alongside the readings.
The actual measured resistance value is compared against the factory acceptance-test baseline or the resistance tolerance specified on the nameplate. NETA ATS-2025 Section 7.20.4.D.3 establishes the framework for evaluating that measurement: the result should fall within the manufacturer's stated tolerance band or should not deviate from the factory baseline by more than the limit set by the applicable equipment standard. A result outside that window signals a possible drift, manufacturing defect, or thermal stress and requires investigation or unit replacement before the resistor is returned to, or left in, service. The insulation-resistance measurement must meet the minimum acceptance value prescribed in the same NETA table for the voltage class; low insulation resistance indicates moisture ingress or contamination and typically requires reconditioning or replacement. See the purchased NETA ATS-2025 standard for the complete acceptance thresholds and any resistor-type-specific qualifications.
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