Regular load and insulation testing protects life-safety power, uptime, and warranty. It proves the generator performs under real kW and temperature, and that windings and wiring still have the dielectric strength they need. ABM structures, executes, and documents tests so you can fix issues early, pass inspections, and keep emergency power ready.

Who needs this and how often

Facilities with emergency or standby power (healthcare, data centers, manufacturing, airports, stadiums, education, and commercial campuses) should exercise units routinely and perform formal tests on a defined cadence. Many sites run monthly exercise tests, annual load bank tests, and periodic insulation testing to meet OEM guidance, insurance requirements, and site standards.¹

Pre-test controls

Before any test, establish safe work conditions.

• Lockout/tagout and energized work boundaries documented
• Cooling airflow and exhaust routing verified; hot surfaces guarded
• Fuel level confirmed; spill kit and fire extinguisher staged
• Load bank cabling plan reviewed; ventilation and cable routing cleared
• Personal protective equipment staged for operators and observers
• Communications plan with operations and security, so no one is surprised

Documents and tools

Have these ready so the test runs smoothly:

• Generator nameplate data, single-line diagram, last service log, ATS sequence
• Calibrated instruments: clamp meters, IR camera, tachometer, power analyzer
• Insulation testers rated for the equipment (500/1000/2500 V), test leads, and temperature probe
• Data sheets to record RPM, VAC, Hz, kW, kVAR, power factor, temperatures, megohms, PI

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Part 1: Load testing

Load testing verifies real performance under kW and thermal stress and exposes control or cooling issues you won’t see at idle.

What you're validating

• Voltage and frequency stability and recovery after load steps
• Transient behavior on step loads and shedding events
• Thermal stability of the engine and alternator
• Fuel delivery, exhaust condition, vibration, leaks, abnormal noise
• Breaker or ATS behavior and alarms under changing load

Choose the right load bank

• Resistive, reactive, or combined, depending on what you need to prove
• Portable versus permanent, based on site access and frequency of tests
• Size and step plan to reach 25, 50, 75, and 100 percent of nameplate with appropriate dwell times
• Cable lengths, connection points, and ventilation path planned and inspected

Step-by-step-load procedure

• Pre-start checks on oil, coolant, fuel, batteries, belts and hoses, and air filters
• Start, warm up, and record baselines for RPM, VAC, and Hz
• Apply load in planned steps; at each step, record kW, kVAR if applicable, power factor, VAC, Hz, exhaust and coolant temperatures, and oil pressure
• Include at least one rapid step and one shedding event; document voltage and frequency dip and recovery time
• Observe alarms or nuisance trips, governor hunting, smoke, odor, or abnormal heat signatures
• Cool down under light load if required by the OEM, then safe de-energization
• Post-test inspection for leaks, loose lugs, discoloration, and insulation damage

What good looks like

• Voltage and frequency recover smoothly without prolonged hunting
• Temperatures stabilize in expected ranges; no coolant push-out or excessive exhaust smoke
• No nuisance alarms and ATS and breakers operate per sequence with clean transitions

Common load test findings and fast fixes

• Wet-stacking from chronic light loading; add periodic full-load runs and review minimum-load strategy
• Voltage or frequency instability; tune governor or AVR, verify sensor accuracy
• Overheating at higher loads; improve airflow, clean radiator, adjust fan belts, check shutters
• Cable lug hotspots; re-terminate and torque to specification, correct strand prep

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Part 2: Insulation testing

Insulation testing verifies winding health and wiring integrity, so you can spot moisture, contamination, or mechanical damage before they cause failure.

What you’re validating

• Stator and rotor insulation resistance to ground
• Harnesses and control wiring (where applicable)
• Trendable health metrics that predict failure risk

Test types and when to use them

• Insulation resistance at the correct voltage for the equipment rating
• Polarization index over ten minutes for absorption and cleanliness insight
• Dielectric absorption ratio, where a shorter window is needed
• Winding resistance balance as a complementary check

Preparation

• De-energize, discharge, and isolate circuits and confirm zero energy with a meter
• Record ambient and winding temperature so you can temperature-correct results
• Clean accessible surfaces; if moisture is suspected, schedule a dry-out or a loaded run before testing

Step-by-step insulation procedure

• Define test points, typically each phase to ground; include phase-to-phase if required
• Perform a one-minute insulation resistance test; record megohms and temperature
• Where appropriate, run a ten-minute polarization index and capture the curve
• If values are marginal, perform a conditioning run or dry-out and re-test
• Inspect terminations, harnesses, and conduit for abrasion, looseness, oil, dirt, or water ingress

Interpreting results

• Stable, high insulation resistance with a healthy polarization index indicates clean, dry windings; continue regular operation and trend annually
• Low resistance or a falling trend points to contamination, moisture, or damage; investigate and correct before returning to service
• Imbalanced readings between phases warrant targeted inspection and repair

Common insulation findings and remedies

• Moisture or condensation; schedule an extended run under load, use heaters, improve enclosure sealing
• Oil and dirt contamination; clean and improve filtration and housekeeping practices
• Mechanical abrasion at leads; add sleeving and strain relief, and re-terminate to spec

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Integrate ATS and system behavior

If site rules allow, observe ATS transfer during testing. Verify time delays, transition type, and annunciation to BMS or EPMS. Note any nuisance trips, alarms, or missequenced steps, and correct the programming or wiring before the next test.

Data you should always capture

• Nameplate versus measured VAC, Hz, kW, kVAR, and power factor at each step
• Transient dip and recovery times for voltage and frequency
• Coolant, exhaust, and alternator temperatures, including IR thermography of terminations and windings
• Insulation resistance and polarization index with ambient and winding temperatures
• All alarms and fault codes, with actions taken and follow-up tickets

After-action: fixes, documentation, and persistence

Convert findings into work orders with priorities tied to life-safety risk. Update the service log with data sheets, photos, and IR images. Schedule follow-ups such as radiator cleaning, control tuning, or cable re-termination. Store results where QA, operations, and compliance can review year over year.

How ABM makes generator testing repeatable and inspection-ready

After each ABM visit, you’ll receive a test plan and safety checklist, a step-by-step data workbook, IR photos, temperature-corrected IR and PI results, a punch list with priorities and target dates, and an audit-ready closeout.

Talk with a generator testing specialist

Put a schedule in place for load and insulation testing that’s repeatable, documented, and inspection-ready. Contact us and we’ll outline scope, timelines, and deliverables so you know exactly what comes next.

FAQs

What’s the difference between a monthly exercise and an annual load bank test?
A monthly exercise confirms the start and basic operation. A load bank test proves performance under real kW and temperature, exposes control issues, and burns off wet-stacking.

How do I know what insulation test voltage to use?
Use the OEM or equipment-class guidance for test voltage and duration. Match the tester to the generator’s insulation class and document the temperature so results can be corrected.

Can I test insulation right after a wet-stacking event?
Let the unit run under load to dry out, then shut down and isolate before testing. Moisture can depress readings and mask underlying issues.

Should I involve the ATS during testing?
If permitted, observe transfer to verify delays, transition type, and alarms. Many issues only appear during real changeovers, not during isolated generator runs.

How long should each load step be? ‍

Long enough to reach thermal stability and get a clean data set at each level. Many teams dwell several minutes per step, with at least one rapid step and one shedding event to capture transient performance.

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¹ Maintaining Your Emergency Power Supply System is Critical, Particularly During Hurricane Season

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