UPS service
UPS load-bank testing
A UPS load bank test connects the UPS to a load bank, a controlled artificial load, so it can be proven at the power it is rated for without risking the equipment it protects. We run load-bank tests at acceptance, after a battery replacement, after a major repair or relocation, and as part of a maintenance program. The report shows how the UPS regulated voltage, handled load steps and discharged its batteries.
Key facts
- Purpose: Confirm the UPS meets its own specification; validate system performance (Vertiv)
- Vertiv safe capacity test: Tests the full power train without an external load
- What the test proves on a UPS: Its ability to perform at differing power outputs, and for how long (ASCO)
- Resistive load bank: Real power (kW) only, at 1.0 (unity) power factor (ASCO)
- Full kVA test: Requires a reactive load bank (ASCO)
Why test a UPS under load
A UPS that runs at a fraction of its rating every day can hide problems that only appear near full load: a weak connection that heats up, a capacitor that cannot hold the DC bus, a battery string that collapses after a few minutes. Vertiv describes load-bank tests as the usual way to make sure a UPS can meet its own specification and to validate the operational performance of the whole system.
The stakes are highest on large systems, as this overview of three-phase UPS for mission-critical sites explains. Before choosing a test level, it helps to know the real load; a step-by-step guide to sizing a UPS shows how to work it out.
When to schedule a test
- Acceptance and commissioning of a new or relocated system, before it carries the critical load.
- After a battery replacement, to confirm the runtime of the new string.
- After a major repair, such as a power module, capacitor bank or inverter board.
- Periodically for sites that must prove runtime, such as hospitals and emergency services.
- Before a generator test, to check how the UPS and the generator behave together. See UPS and generator compatibility.
How the test runs
- Plan. The load is moved to maintenance bypass or the UPS is tested before it goes live. Connection points, cable runs and the load-bank size are fixed in advance.
- Steps. The load is applied in steps up to the test level while voltage, current, frequency and temperature are recorded.
- Thermal scan under load. Connections and components are scanned at high load, when defects show.
- Battery discharge. With the input opened, the UPS carries the load on battery to the target runtime or end voltage, with block voltages logged.
- Return and report. The load returns to the UPS and the results are written up.
Built-in test modes
Some large UPS systems can test themselves without an external load. Vertiv's safe capacity test circulates power through the unit's own rectifier and inverter so that the UPS can test its entire power train, including the rectifier, inverter, contactors, fuses, power busses, cabling, bypass static switch, magnetics and filter capacitors, without an external load connected. Systems with that feature, such as the Liebert EXL S1, need less rented equipment; a battery discharge test still needs planning around the site.
What the report shows
- Output voltage and frequency regulation at each load step.
- Response to load steps and transfers.
- Thermal images of anything abnormal under load.
- Battery runtime and the voltage of each block at end of test.
- Pass or fail against the acceptance criteria agreed before the test, and what to fix.
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Common questions
Does a load-bank test interrupt my load?
No, if it is planned. The load runs on maintenance bypass during the test, or the test is done before the system goes live. While on bypass the load has no battery protection, so the window is scheduled with you.
How often should a UPS be load-bank tested?
At acceptance, after battery replacement or a major repair, and periodically where runtime has to be proven, such as healthcare and emergency services. Routine maintenance visits cover the years in between.
Can the UPS batteries be tested without a load bank?
A runtime test on the real load is possible but risky, because a weak string drops the protected equipment. A load bank isolates the risk from production.
Do all UPS need an external load bank?
No. Some large systems have built-in test modes, such as Vertiv's safe capacity test, which exercises the power train without an external load.
What size of load bank do you bring?
One sized to the UPS rating and the test plan. We confirm the size, the connection points and the cable route during the planning visit.
What is the difference between a resistive and a reactive load bank?
A resistive load bank loads the UPS with real power (kW) at unity power factor. A reactive load bank adds an inductive or capacitive load, which is needed to test at full kVA rather than kW alone.
Sources, checked October 4, 2026:
- Vertiv white paper, Safe capacity test: Innovation and savings, pp. 3, 4
- ASCO Power Technologies white paper, Load Banks for Power System Testing, p. 2
- ASCO Power Technologies white paper, Application of Resistive/Reactive Load Banks for kVA Testing, p. 2
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