Common UPS Failures: Analysis and Solutions

I. Preface

1.1 50-70% of load power supply failures are caused by quality issues, installation problems, human operation, or maintenance issues in other parts of the power distribution system. These issues can lead to UPS malfunction or even failure.

1.2 According to data analysis, battery failure accounts for approximately one-third of UPS failures returned for repair. Therefore, we must pay special attention to battery failures.

1.2 UPS equipment is only one factor affecting power system availability. Improving UPS reliability alone cannot fundamentally address the safety and reliability of the entire power system.

1.3 Preventive maintenance of equipment is a means to prevent the occurrence, escalation, and spread of failures.

II. Typical Failures

2.1 UPS Failure to Start

Because the Smart-UPS is powered by DC, it will fail to start if the battery is not connected, is low, or has a problem. There are several similar situations:

Case 1: A newly installed UPS fails to start.

Check the battery connector on the UPS rear panel to ensure it is connected. New batteries tend to self-discharge during storage, so the UPS may not start if the battery is low. In this case, connect the UPS to the battery and the mains power supply. Press the Test button on the UPS front panel. Although the UPS panel indicator light will not illuminate, the UPS will charge the battery. After charging for a while, press the Test button again and the UPS will start.

Second Situation: The UPS will not start after operating as an inverter for a period of time.

This is also due to low battery power and requires charging.

Third Situation: The UPS will not start after the battery is about two years old.

According to most customer experience, batteries typically experience some degree of capacity loss after two years of use. If the battery is no longer functioning properly, it should be replaced.

Fourth Situation: The voltage of each battery is normal, but the UPS will not start.

This is likely due to a problem with the battery-to-battery connection or the battery-to-UPS connection, such as a loose connection or oxidation. In this case, remove the oxidation and reconnect. The fuse connecting the UPS to the battery may be blown. If so, simply replace it.

The cable connecting the UPS to the battery is very long, thin, or has a connection point, resulting in a large voltage drop, preventing the UPS from starting.

2.2 The UPS cannot switch to inverter mode after a mains power outage.

Run the UPS on mains power. Set a multimeter to the voltage range, connect the test leads to the inside of the ANEN plug on the back of the UPS, and directly measure the DC voltage reaching the UPS. One person should observe the multimeter display while another person unplugs the UPS input cable and observe the multimeter display at the moment of power outage. If the voltage drops significantly, it indicates a problem with the battery. If the connection problem can be eliminated and the battery pack is about two years old, consider replacing the battery pack.

2.3 The UPS inverter time is short and does not meet customer requirements.

First, the battery parameters for long-duration Smart-UPS models must be configured at the time of installation. Failure to do so will result in a short inverter time.

Second, the battery parameters have been configured, but the UPS's inverter time is still short. You can measure the battery voltage when the UPS low-battery alarm sounds. If the measured value indicates that the battery is indeed low, you need to replace the battery. If the measured value indicates that the battery is not low, you need to perform a charge and discharge calibration. Note that during the charge and discharge calibration, the battery must be fully charged and discharged with approximately 50% load.

Third, you have installed the PowerChute Plus software. Because the default setting is to shut down the computer 5 minutes after a mains power outage, you need to modify the software parameters.

2.4 Mains Power Environment

1. Grid Interference:

Severe grid interference, such as voltage sag, can cause UPS power outages and other malfunctions. Below are some examples of such mains interference. You can install the PowerChute Plus software and use the software's event log to determine if there is any grid interference. If you see many such events in the event log, it indicates that your mains power grid is experiencing significant interference, which can also reduce the battery life. If conditions permit, we recommend replacing the mains input or modifying the power grid.

2. A leakage protector is installed at the UPS input:

The UPS has a high leakage current, which can cause the leakage protector to trip when the UPS is turned on. If you need to install a leakage protector, you need to connect it to the UPS output line.

3. The circuit breaker at the UPS input has tripped:

This phenomenon may be caused by the circuit breaker at the UPS input having a small capacity. Because the UPS's startup current is relatively high, the circuit breaker at the front end must have a sufficiently large capacity.

4. The UPS frequently switches between inverter and online states:

First, it may be caused by mains power fluctuations.

Second, this can occur if you are using a generator.

5. Ground Fault:

Typical Fault:

The neutral-ground voltage at the UPS output is too high (>2V [GB50174-2017]).

An open neutral line may cause the load input power module to burn out.

During UPS maintenance and testing, pay special attention to the neutral line connection.

There are four main causes of high neutral-to-ground voltage in power systems:

The neutral-to-ground voltage of the power system is inherently high, or the impedance of the neutral and ground wires is high. Therefore, even a low grounding resistance does not necessarily mean a low neutral-to-ground voltage.

Severe IT load imbalance.

Large H3 harmonics in the load current cause a corresponding increase in neutral harmonic current.

The neutral-to-ground voltage is also related to the power system's transmission impedance: wiring, power distribution distance, cable selection, and grounding system design.

2.5 Human Fault

UPS input phase sequence is reversed, causing the input fuse to blow.

Battery maintenance, improper operation, causing a short circuit or reversed polarity.

The UPS is not operating in normal mode (forgetting to close the battery breaker, not starting the inverter, etc.).

The UPS is not started or stopped according to the specified procedures.

2.6 Cooling fan failure

Related to environmental cleanliness.

Related to normal maintenance: Regularly check the fan bearing temperature and clear any blockages.

Some fans are affected by mains voltage: Excessive mains voltage can damage the fan.

End of life indicator: Mandatory replacement after 5 years.

Fan replacement cost: 5% of the UPS price.

2.7 Failures Caused by AC/DC Capacitors

-Current failure: open circuit or short circuit;

-Mechanical damage: safety valve activation, top cover cracking;

-Performance deterioration:

Core temperature Tc > 105°C (85°C product)

Capacitance change > ±10%;

ESR > 2 times the initial value;

Impedance Z > 3 times the initial value;

Leakage current > safety limit. Capacitor replacement cost - 10% of the main unit price

2.8 Failures caused by improper maintenance

1. Unclear definitions of monitoring information and panel indicators, or lack of understanding of the physical meaning of UPS electrical performance indicators, leading to suspected failures and unintentional downtime;

2. Incorrect or loose input and output wiring, leading to unintentional downtime;

3. Lack of understanding of machine failure symptoms and warning signals, leading to delayed downtime;

4. Long-term lack of necessary maintenance, leading to downtime due to environmental factors;

5. Failures caused by improper system configuration (e.g., inaccurate or unadjusted settings of leakage protectors, contactors, or circuit breakers);

6. Unintentional failures caused by lax shift change;

7. Overly complex systems and cumbersome maintenance, leading to operational errors, etc.