1. Basic UPS Construction
A UPS (uninterruptible power supply) consists mainly of four parts:
- Rectifier: converts the input AC power into DC power
- Inverter: converts DC power into 50 Hz AC power (sine wave or square wave) to supply the load
- Battery bank: floats on charge under normal mains conditions; when mains power fails or goes out of the allowable range, it supplies the inverter to keep the load powered without interruption
- Output transfer switch: performs automatic transfer between inverter and mains supply. Two types exist: contact switches (relays/contactors) and contactless switches (thyristors/SCRs, also called static switches, with no mechanical motion)
2. Five Working Modes
- Mains inverter mode: mains power is rectified and PFC-corrected before feeding the inverter circuit, which outputs continuous, uninterrupted AC power. Once the inverter starts, the charger automatically turns on to charge the batteries.
- Battery mode: when mains power fails, the system automatically switches to battery mode without interrupting the load; when mains power is restored, it automatically switches back to mains mode with a transfer time of 0 ms.
- Bypass mode: on inverter failure, overload or manual transfer, the load is switched from the inverter side to the bypass side without interrupting the supply.
- Maintenance mode (manual bypass): with the manual bypass switch closed, mains power flows directly from input to output without passing through the unit's internals, allowing UPS maintenance and repair while the load stays powered.
- Frequency converter mode: a stable 50 Hz/60 Hz output can be configured, with a mains input frequency range of 40 Hz–70 Hz. In this mode bypass is inactive; if mains power becomes abnormal, the unit automatically transfers to battery mode while maintaining the configured output frequency.
3. Operating Precautions
- Ensure the operating environment is well ventilated, conducive to heat dissipation, and kept clean
- Clearly label output sockets; do not connect unrelated loads or create short circuits
- Never connect inductive loads (money counters, fluorescent lamps, air conditioners), to avoid damage
- When a generator supplies power during a mains outage, the generator capacity should exceed 2 times the UPS rated power, and it must be connected only after starting up and stabilizing
- Turn on loads following the "largest first, smallest last" principle
- Keep the output load at about 80% of rated power; 30%–60% is the optimal operating range
4. Maintenance Cycles
Battery Maintenance Schedule
| Cycle | Inspection requirements |
|---|---|
| Monthly | 1. Check connections for looseness and corrosion; 2. Check casings for leakage and deformation; 3. Check around terminals and safety valves for acid mist and acid liquid |
| Quarterly | 1. Full cleaning; 2. Measure terminal voltage of the whole bank and of individual cells |
| Yearly | 1. Measure voltage drop on feeder busbars, cables and connection straps; 2. Verify discharge test; 3. Capacity test |
UPS Maintenance Schedule
| Cycle | Inspection requirements |
|---|---|
| Monthly | 1. Alarm indicators/display functions; 2. Grounding protection; 3. DC fuse voltage drop/temperature rise; 4. Relays/circuit breakers/fans; 5. Load sharing; 6. Cleaning |
| Quarterly | 1. Lightning protection; 2. Wiring terminal contact; 3. Contact of switches/contactors; 4. Neutral line current; 5. Automatic bypass performance |
| Yearly | 1. Verify discharge test; 2. Harmonic current and power factor tests |
Battery and Charging Essentials
- With the battery cabinet switch set to ON, charging starts automatically, reaching about 90% in roughly 10 hours
- Ideal temperature is 25 °C, with an allowable range of +5 °C to 35 °C; avoid direct sunlight and keep the area clean
- Never mix batteries of different brands, capacities or ages
- Batteries emit hydrogen — keep away from ignition sources and maintain ventilation
- Perform a charge–discharge cycle every 3–6 months; even idle batteries need charging every 3–6 months
- Normal service life is about 5 years; poor maintenance shortens it substantially
5. Key Operating Procedures
Pre-Start Checks
All screws tight and wiring correct; mains/bypass input/output/battery breakers all in the OFF position. For long-backup models, first configure the charging parameters and battery parameters; the bank count must match the actual number of battery banks (usually preset by the manufacturer — do not change it without authorization).
Starting in Mains Mode
- Close the input and bypass breakers; after power-up the UPS automatically enters bypass
- Tap the "Power On/Off" icon; the first startup requires the startup password (obtain it from customer service), then tap "Transfer to Inverter" to enter mains mode
- Close the battery breaker (long-backup models) and the output breaker; normal operation begins
Maintenance Bypass Procedure
- Entering: confirm transfer to bypass mode on the LCD → close the maintenance bypass breaker → open the output and battery breakers → open the input and bypass breakers
- Exiting: close the input and bypass breakers → manually start bypass; the charger turns on automatically → close the output and battery breakers → open the maintenance bypass breaker → select "Transfer to Inverter" and wait 30 seconds for the unit to return to mains mode
Safety note: when using maintenance bypass, strictly follow the sequence "transfer to bypass first, then close the maintenance breaker, then open the internal breakers"; when exiting, restore internal supply before opening the maintenance bypass, to avoid an artificial power outage.
6. Common Fault Handling
| Symptom | Possible cause | Remedy |
|---|---|---|
| Power failure alarm | Tripped breaker/poor contact/abnormal voltage/damaged switch/circuit fault | Check switches and wiring, correct the voltage or start the generator, check fuses, check circuits |
| Load loses power after mains failure despite normal voltage | Low battery/failed charger/overload transferred to bypass/poor connection/failed inverter | Start the generator to charge, add a voltage stabilizer, check the charger, replace batteries, reduce load, check the inverter |
| Unit unusable | Battery deeply discharged from long storage/AC-DC not properly connected/startup circuit fault/internal damage | Fully charge, check contacts, check the startup circuit, check the power supply and inverter |
| Sudden audible alarm | Excessive load/short circuit/inverter fault/circuit protection | Check power capacity, check for short circuits, check the inverter, check voltage, load and circuits |
| Unit works normally but load equipment misbehaves | Neutral-to-ground voltage too high/load ground not at same point as UPS ground/interference | Parallel a 1–3 kΩ resistor between neutral and ground at the output, connect UPS ground and load ground to the same point, restart the load |
Typical Cases
- Dual-source transfer failure (metal rust): during a planned outage maintenance, the dual-source transfer switch failed and would not respond. Disassembly revealed that fully rusted contacts caused the transfer failure. Remedy: reinstall the switch and recover manually, with dedicated personnel performing periodic rust-removal maintenance. Keep wiring compact and disassemble with care.
- Unexpected transfer to inverter (grounding issue): with mains power normal, the unit kept switching to battery supply with continuous beeping. Investigation found the air conditioner's neutral line bonded to the ground line; separating them resolved the issue. Lesson: neutral-ground mixing introduces interference that causes the UPS to misjudge mains status.
7. Fault/Alarm Handling Reference (Excerpt)
| Category | Fault/alarm | Solution |
|---|---|---|
| Battery | Battery overcharge | Turn off the charger and contact customer service |
| Battery | Battery low voltage | Test the battery/charger; verify battery settings match the actual configuration; when on battery supply, shed non-critical loads promptly |
| Battery | Battery not connected | Check the external battery breaker; check battery wiring |
| Charger | Fault/not connected/overtemperature/short circuit/Buck soft-start failure | Turn off the charger and contact customer service; confirm whether it is enabled; no action needed; check the wiring |
| UPS | BUS overvoltage/undervoltage/imbalance/short circuit | Check three-phase mains balance and fluctuation; power off for 30 s and restart; if the problem persists, contact customer service |
| UPS | Output short circuit (phase R/S/T and phase-to-phase) | Check output wiring and load for short circuits |
| UPS | Overload fault | Shed non-critical loads, redistribute to balance the three phases, power off completely for 30 seconds and restart |
| UPS | Overtemperature fault | Keep ambient temperature within range; power off for 30 seconds and restart |
| UPS | Mains/bypass phase sequence error | Check the input power supply and three-phase input wiring |
| UPS | Maintenance panel open | No action needed in maintenance bypass mode; in other modes, check whether the panel screws are tightened |
| UPS | Emergency shutdown | Confirm whether the EPO button has been pressed |
| Communications | CAN bus/synchronization/communication fault | Check that communication cables are properly connected |
Pages 31–34 of the original document contain roughly 40 fault/alarm entries; printing them and posting them at the equipment room duty station is recommended. This guide is compiled from implementation materials of a municipal project equipment room UPS (EA990 series, online double-conversion).