Periodic testing of a UPS installation — load test, leakage current and terminal contact

The guide on UPS battery types (VRLA vs lithium) covers the chemistry and characteristics of the battery itself, and the guide on UPS emergency-power continuity covers the design of the bridging chain. This article covers a different question: how is an existing UPS installation tested periodically, and why does the regular NEN 3140 leakage current and insulation resistance measurement of protection class I equipment not suffice on its own?

What a regular NEN 3140 inspection does and does not cover

For the regular NEN 3140 inspection, a UPS is first and foremost just connected equipment: protection class, protective conductor continuity, insulation resistance and leakage current are checked the same way as for any other fixed or portable equipment (see the guide on protection class I/II/III and the guide on insulation resistance testing). Those measurements, however, say nothing about the function the UPS exists for: bridging a mains failure for the specified autonomy time. A UPS that measures electrically safe (correct insulation, no leakage current) can still fail functionally during a real mains outage if the battery no longer holds its rated capacity.

The battery load test

The only way to establish whether the battery can actually deliver the specified load for the stated autonomy time is a load test: the UPS is run (or run for part of the time) genuinely on battery under representative load, while the terminal voltage of the battery cell(s) is tracked until the discharge limit is reached. A capacity check that only measures the battery's resting voltage, or only checks the charging current, often does not reveal gradual capacity ageing — only under actual load does it become apparent whether the battery still reaches the specified time.

Terminal contact: an underrated failure source

Besides the battery itself, the terminal connections between battery cells and between the battery and the UPS are a common, easily overlooked failure source: a loosened or corroded battery terminal raises the contact resistance, which can lead to local overheating during the high discharge current of a load test (or a real mains outage) — a known cause of fires in battery rooms alongside the battery chemistry itself (see also the guide on ventilation of stationary battery rooms for the associated gassing risks with VRLA batteries). A visual inspection for corrosion and a check of the battery terminal torque should therefore be a standard part of the periodic UPS inspection, alongside the electrical measurements.

Checking the transfer function

Besides the battery itself, the automatic transfer function (from mains to battery, and the bypass function in the event of an internal UPS fault) must also be periodically function-tested — a UPS that is electrically sound and has a good battery offers no protection if the transfer itself does not act correctly during an actual mains outage.

NEN 3140 sets no fixed frequency

NEN 3140 itself sets no fixed testing interval for UPS installations (or for equipment in general) — the person responsible for the installation determines the testing frequency based on a risk assessment (usage intensity, environment, criticality of the load the UPS protects). For a UPS bridging a critical process, a shorter cycle including a load test is common compared with a UPS feeding an office workstation.

Practical relevance

When setting up a testing regime for a UPS installation, it must be established that a periodic battery load test, a visual check of the terminal connections and a functional test of the transfer, are included alongside the regular electrical safety measurements — the regular NEN 3140 measurement alone does not reveal a functionally deficient battery.

Common mistakes

Performing only the regular leakage current and insulation resistance measurement and assuming the UPS is thereby "tested" — this says nothing about the actual autonomy time of the battery.
Assessing battery capacity ageing only from resting voltage instead of with an actual load test — an aged battery can show a normal voltage at rest and still sag rapidly under load.
Not visually checking terminal connections for corrosion or a loosened fixing — increased contact resistance on a battery terminal is a known fire and failure cause that an electrical safety measurement does not necessarily reveal.
Not function-testing the transfer/bypass function, assuming that an electrically approved UPS will automatically switch correctly during a real mains outage.