A medium-voltage motor starter acceptance test is a field protocol that proves the contactor, its fuses, its control circuit, and the auxiliary apparatus in the same cubicle are ready before a medium-voltage motor is started. It records the identification (manufacturer, model and catalog number, serial number, rated voltage and current, phase, control voltage, power fuse rating, and equipment designation). The inspection covers nameplate data, condition, anchorage and grounding, cleanliness, bolted-connection tightness, interlocks, barriers and shutters, the active components and indicating devices, contactor mechanical operation, contact gap, wipe, alignment and pressure, the overload protection rating and settings against the coordination study, lubrication, and a thermographic survey. The electrical work covers contactor insulation resistance closed and across each open contact, control wiring, an optional interrupter pressure test, an optional dielectric withstand test, vacuum bottle integrity, contact or pole resistance, blowout coil circuit resistance, power fuse resistance, energizing the contactor from an auxiliary source, and the referenced tests for the control power transformer, starting transformer, starting reactor, motor protection devices, instrument transformers, metering, the system function test, and the space heater. NETA ATS-2025 section 7.16.1.2 lists this as the standard acceptance sequence.
A medium-voltage starter is a small assembly rather than a single device, and most of what can go wrong in it is mechanical. The contactor closes and opens for every motor start, so contact gap, wipe, alignment, and pressure decide both whether it carries current without heating and whether it interrupts cleanly; on a vacuum design the bottle is the interrupting medium and a bottle that has lost vacuum looks perfectly normal until it fails to interrupt, which is why both an interrupter pressure measurement and a withstand test across the open contacts exist. The fuses are the short-circuit protection for the whole branch and a fuse that reads differently from its companions is either a mismatch or already damaged. And because the starter carries the motor's overload protection, checking the rating and the settings against the coordination study is what connects this cubicle to the study that protects the machine downstream.
Acceptance: after installation and before the motor is started, as part of the standard acceptance sequence, with the referenced tests for the transformers, reactors, relays, instrument transformers, and metering completed so the system function test can be run against a finished scheme. The same form carries a maintenance service type, since the contact and fuse resistance measurements, insulation resistance, and the mechanical checks are repeated on a periodic basis and after any contact or bottle replacement. Insulation resistance is taken before any dielectric withstand or bottle integrity test, and the vacuum bottle checks are repeated whenever the contactor has interrupted a fault.
With the starter de-energized, isolated, and grounded per site safety procedures and the element withdrawn where the design allows, the identification data is recorded and the inspection is worked through: condition, anchorage and grounding, cleanliness, bolted connections verified with a calibrated torque wrench, interlock operation and sequencing, barriers and shutters, the active components and indicating devices, contactor mechanical operation, and contact gap, wipe, alignment, and pressure against the manufacturer's data, along with the overload protection rating and settings against the coordination study. Contactor insulation resistance is measured for one minute three ways at the test voltage the standard prescribes: from each phase to ground and between phases, contactor closed, then across each contact with the contactor open, and control-wiring insulation resistance is measured where the circuit can tolerate it. Where they are in scope, each vacuum interrupter's pressure is measured and a dielectric withstand voltage is applied; vacuum bottle integrity is then verified by applying the manufacturer's withstand voltage across each bottle with the contacts open. Contact or pole resistance is measured and compared between poles with a low-resistance ohmmeter, blowout coil circuit resistance is measured, and power fuse resistance is measured across each fuse. The contactor is energized from an auxiliary source and the armature adjusted for minimal vibration and noise. The control power transformer, starting transformer, starting reactor, motor protection devices, instrument transformers, and metering devices are then tested under their own sections with each report referenced on this form, the cubicle space heater is verified, and the system function test is run under the commissioning standard.
The identification (manufacturer, model and catalog number, serial number, rated voltage and current, phase, control voltage, power fuse rating, and equipment designation); the result of each visual and mechanical item, including contact gap, wipe, alignment, and pressure, the overload protection rating and settings, and the thermographic survey; the insulation-resistance test voltage and the phase-to-ground, phase-to-phase, and across-open-contact readings; the control-wiring test voltage and reading; the interrupter pressure per phase; the dielectric withstand test voltage and result; the vacuum bottle test voltage and the per-bottle result; contact or pole resistance per pole; the blowout coil circuit resistance; power fuse resistance per fuse; the contactor energization result; the references for the control power transformer, starting transformer, starting reactor, instrument transformer, and metering device reports; the motor protection device, system function, and space heater results; and the ambient temperature, relative humidity, test equipment with calibration due date, comments, and deficiencies.
Contactor insulation resistance is judged against the manufacturer's published data, and where none exists the standard points this section at its transformer insulation-resistance table, with readings below the table or the manufacturer's minimum investigated rather than automatically failed. Control-wiring insulation resistance has a published floor of its own, and where solid-state components in the circuit would not survive the test voltage, the manufacturer's own recommendation replaces it. Each vacuum interrupter is evaluated per the test instrument manufacturer's instructions, and where the manufacturer publishes no value the standard gives both a pressure ceiling and an allowable spread between adjacent poles. The dielectric withstand test and the vacuum bottle integrity test each pass only when the specimen holds the applied voltage for its full duration with no sign of breakdown or insulation distress, at a test voltage from the manufacturer's data or, for the withstand test, the table this edition cites. Contact or pole resistance must not exceed the high end of the manufacturer's normal range, and without manufacturer data a reading well above the lowest of similar connections is investigated. Blowout coil resistance follows the manufacturer's published data, identical fuses are expected to agree with each other within the published margin, and the energized contactor must operate with minimal vibration and noise. Bolt torque follows the manufacturer's data with the standard's torque table used in its absence, and the referenced transformer, reactor, relay, instrument transformer, metering, system function, and thermography results are judged under their own sections. See the purchased NETA standard for the table values.
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