Why This Matters on Your Plant Floor
An unexpected UPS shutdown during a power cut stops your production. Assembly lines halt. Compressors lose pressure. Data in process controllers is lost. Equipment protection circuits may trip, requiring manual restart before the line runs again. Recovery time varies depending on the process.
For regulatory compliance, your insurance and audit requirements expect UPS systems to function as specified. If your UPS shuts down when mains fails, you have lost its primary function. This failure also flags a problem in your load calculation, battery maintenance, or equipment sizing that will recur until diagnosed.
On plant floors in south Gujarat, Daman and the Mumbai belt, unstable grid conditions and frequent outages make correct UPS operation critical. A shutdown during an outage exposes you to unplanned downtime, failed inspections and potential equipment damage.
The Three Fault Chains: Battery, Inverter and Load
UPS shutdown during mains failure follows one of three paths.
- Battery degradation or undersizing: Battery voltage drops below the inverter's minimum operating voltage requirement, triggering protection shutdown.
- Inverter module failure: The inverter cannot convert battery DC power to output AC, or control circuits fail to energise the transfer switch.
- Load mismatch: Connected equipment draws more power than the UPS can supply, causing overload protection to trip.
In an online UPS such as the Fuji Electric Falcon 8500 three-phase models, a rectifier converts incoming AC to DC during normal operation, and an inverter converts that DC back to clean AC. When mains fails, the inverter must draw power from the battery and maintain voltage to all connected devices. If any component fails or if the system is undersized for the load, the inverter shuts off to protect itself and the connected equipment.
Fault Chain 1: Battery Voltage Drop and Degradation
Battery degradation is the leading cause of unexpected UPS shutdown during mains failure. Most VRLA (valve-regulated lead-acid) batteries last 3 to 5 years under normal conditions. Lithium-ion batteries last 8 to 10 years. However, battery life shortens under heat, overcharge, deep discharge cycles, and poor maintenance.
When mains fails, the battery must supply current to the inverter. As the battery discharges, its terminal voltage drops. If the battery cannot maintain minimum voltage required by the inverter, built-in protection circuitry shuts down the inverter to avoid damage. This is not a fault: it is protection working as designed.
The battery voltage drop occurs in two scenarios:
- Battery capacity has degraded to 60% or less of original rating, so even under light load, voltage cannot be maintained for more than a few minutes.
- Battery is correctly sized, but the load exceeds battery discharge rate. The battery cannot deliver current fast enough, so voltage collapses.
Fuji Electric UPS systems display an alarm code BATTERY VOLTAGE LOW when battery discharge curves indicate reduced capacity. This is your first warning sign. Do not ignore it. Reduced runtime to 60% of the original manufacturer runtime requires immediate battery replacement.
To verify battery health, conduct a load test: measure actual runtime under realistic connected load and compare to the manufacturer's datasheet. Note that batteries perform poorly in high ambient temperatures. Batteries operating above 45°C experience faster discharge and thermal runaway risk. Ensure your UPS location has adequate cooling.
Fault Chain 2: Inverter Module and Control Circuit Failure
Transfer failures during mains outage typically result from inverter problems. These include IGBT (insulated-gate bipolar transistor) failure in the inverter power stage, control board faults preventing the transfer switch from engaging, or communication faults between battery monitor and inverter logic.
When the inverter shuts down due to internal fault, the UPS cannot supply backup power even if the battery is in good condition. The connected load loses power immediately.
Alarm codes from your UPS help identify this fault. Fuji Electric UPS systems include alarm messages for battery low load discharge, temperature alarms, and inverter shutdown events. Check your UPS manual for the specific alarm code displayed. If the code indicates inverter fault rather than battery voltage low, the battery may be serviceable but the inverter requires replacement or repair by the manufacturer's authorised service centre.
Do not attempt to restart or bypass the shutdown. This is protection working. Contact the brand's authorised service centre immediately.
Fault Chain 3: Load Exceeds UPS Capacity
When connected equipment draws more power than the UPS nameplate rating allows, the inverter triggers an overload shutdown to protect itself. This is not a failure: it is protection.
UPS capacity is quoted in volt-amperes (VA) and also in watts (W). The watt rating is the true power output available to your load, accounting for power factor and inverter efficiency. Many users mistakenly size UPS based on VA rating alone, leading to undersizing.
For example, a 10 kVA UPS might deliver only 8 kW (kilowatts) of usable power when accounting for 0.8 power factor and 90% inverter efficiency: 10,000 VA × 0.8 × 0.9 = 7,200 W. If your total connected load draws 8,500 W including inrush current during startup, the UPS will overload and shut down.
| Load Check | Method | Expected Result |
|---|---|---|
| Nameplate comparison | Sum all equipment power ratings (in watts) and compare to UPS watt rating | Load should be 70-80% of UPS rated watts for headroom |
| Inrush current | Measure or consult equipment manual for motor/compressor startup current | Peak inrush may be 3 to 5 times running current; UPS must handle this spike |
| Three-phase balance | Measure current on each phase; for three-phase UPS, load should be balanced across phases | Phase currents should not differ by more than 10%; severe imbalance triggers shutdown |
| Runtime test | Turn off mains and measure time to UPS shutdown | Should match or exceed manufacturer datasheet for connected load |
To calculate whether your load exceeds UPS capacity, list every connected device with its rated power consumption in watts. Include the UPS's own losses (typically 10%). Multiply motor power by 1.3 to account for inrush current. Sum all values and compare to the UPS watt rating.
Worked example: A three-phase UPS rated 30 kVA (24 kW) supplies a CNC machine (8 kW), a hydraulic pump (4 kW), compressed air compressor (5 kW), control panel (2 kW), and lighting (1 kW). Total continuous load is 20 kW. Inrush current during compressor startup could spike the air pump to 6.5 kW (5 × 1.3), bringing peak load to 21.5 kW. This is within the 24 kW UPS rating with 2.5 kW headroom. However, if the plant later adds a second pump (another 5 kW continuous plus 6.5 kW inrush), the load becomes 25 kW continuous and 26.5 kW at startup. The UPS will overload and shut down.
How to Diagnose the Root Cause
Follow this sequence to identify which fault chain is causing shutdown.
Step 1: Check alarm codes. On mains failure (or during a test with mains disconnected), observe the UPS display. Fuji Electric UPS systems show alarm codes for battery voltage low, end of battery discharge, battery low load discharge, and temperature alarms. Write down the exact code. This tells you whether the shutdown is due to battery, inverter, or overload.
Step 2: Measure actual runtime. With mains disconnected and all normal loads connected, time how long the UPS runs before shutdown. Compare this to the manufacturer's datasheet for the same load. If actual runtime is less than 60% of the datasheet value, battery capacity has degraded and requires replacement.
Step 3: Check load balance on three-phase systems. Using a clamp meter, measure current on each of the three phases. If one phase carries significantly more current than the others, load imbalance may trigger protection shutdown even if total load is within capacity. Rebalance loads across phases.
Step 4: Calculate connected load. If alarm codes and runtime do not explain the shutdown, your load may exceed UPS capacity. List every device and its power rating. Multiply the largest motor or compressor by 1.3 to account for inrush current. Sum all values in watts and compare to the UPS nameplate watt rating (not VA rating). If total exceeds 90% of UPS rating, you have an oversized load.
If none of these steps reveal the cause, the inverter or battery management electronics may have failed. Contact the brand's authorised service centre.
Standards and Compliance
UPS systems for industrial use are designed and tested under IEC 62040. Refer to the current edition of IEC 62040 and confirm with your licensed electrical contractor that your UPS specification, installation and maintenance comply with this standard. This standard covers safety, performance, environmental conditions and testing procedures for uninterruptible power supplies.
UPS Supply and Load Assessment
Vimal Electric Corporation supplies Fuji Electric UPS systems including the Falcon 8500 series (30 to 300 kVA, three-phase) and other online UPS systems from its full product catalogue, for industrial and data centre applications. These systems include alarm monitoring, battery discharge display and transfer switch diagnostics to help you identify faults before they cause production loss.
To specify the correct UPS for your load, send us a load list (nameplate power ratings for all connected equipment) or your single-line diagram. We can calculate the correct capacity accounting for inrush current, power factor, and ambient conditions on your plant floor. Use the UPS size calculator for a quick estimate, or submit a detailed enquiry with your equipment schedule and we will confirm the correct model and battery configuration for your installation.