The site uses Delta's R series 1–3 kVA UPSs and three 100 Ah, 12 V batteries. After the field engineer installs and starts the UPS, a low battery voltage alarm is generated, the low battery voltage indicator is on, the battery voltage measured at the battery cable connection point is 39 V DC, and batteries cannot supply power when the mains fails.
Based on the problem analysis concept, the following guide is provided:
1. Disconnect batteries from the UPS. Use the DC position of a multimeter to measure the battery voltage, which is found to be 39 V DC. Therefore, the low battery voltage is not caused by long-time battery discharge.
2. Refer to the manual to check the battery circuit breaker for damage and incorrect or loose connections, finding no fault. Therefore, the power supply failure is not caused by incorrect operations or connections.
3. Start the UPS that is disconnected from batteries. Use the DC position of a multimeter to measure the voltage of the battery connection point, which is found to be 0 V DC. In normal cases, the voltage should be at least 36 V DC. Therefore, it is determined that the fault is caused by the battery fuse damaged inside the UPS. Further, it is found that the subcontractor does not perform installation strictly by referring to the manual. The batteries short-circuit, which generates high transient currents burning the battery fuse (20 A, 250 V) inside the UPS.
The battery fuse is burnt inside the UPS. The subcontractor does not perform installation strictly by referring to the manual and reversely connects batteries. This generates high transient currents burning the battery fuse (20 A, 250 V) inside the UPS
Replace the fuse.