Engineering guide
Delta Modulon DPH alarm messages explained
A Delta Modulon DPH names its alarms in plain text on the LCD, such as BAT LOW WARNING or OUTPUT OVERLOAD SHUTDOWN, rather than with numeric codes. Chapter 10 of Delta's DPH user manual lists each message with its possible cause and solution. Some point to something an operator can check from the front, like a breaker left OFF or an overload; many end with contact service personnel. The tables below follow that chapter.
Key facts
- LOW BAT CUT OFF: battery voltage below the shutdown limit; the UPS stops supplying the loads to protect the battery
- BAT LOW WARNING: battery voltage below the warning limit; shut down critical loads if there is no backup power
- OUTPUT OVERLOAD WARNING: the UPS is overloaded; decrease some critical loads
- EMERGENCY POWER OFF: EPO is activated
How the DPH reports an alarm
The message appears on the LCD, and the manual's chapter 10 gives a possible cause and a solution for each one. Messages that start with PWR MODULE #n name the power module involved by its number. Note the exact message, the module number and the time before anyone resets anything.
Mains and bypass supply
| Message on the display | What the manual says | What to do |
|---|---|---|
| MAINS INPUT VOLT OR FREQ NOK | Main input breaker (Q1) is OFF, or the mains voltage or frequency is abnormal. | Check whether Q1 is OFF; if the mains is abnormal, wait for it to recover. If Q1 is on and the alarm stays, call service. |
| MAINS INPUT PHASE SEQ NOK | Wrong wiring. | Have the input wiring and phase sequence checked; call service if wrong. |
| BYPASS INPUT VOLT OR FREQ NOK | Bypass breaker (Q2) is OFF, or the bypass voltage or frequency is abnormal. | Check whether Q2 is OFF; if the bypass source is abnormal, wait for it to recover. |
| BYPASS INPUT PHASE SEQ NOK | Incorrect wiring. | Have the bypass wiring and phase sequence checked by service personnel. |
| OUT OF ECO RANGE | Bypass voltage or frequency is outside the ECO mode limit. | Check the bypass voltage and frequency. |
| INHIBIT ECO TRANSFER | Bypass source quality is not good. | Check the bypass source. |
When these appear together with a utility event or a generator start, they usually clear on their own once the source recovers. A phase sequence message after electrical work is a wiring correction for an electrician.
Battery messages
| Message on the display | What the manual says | What to do |
|---|---|---|
| BAT GROUND FAULT | Incorrect battery wiring, or a leaking or damaged battery. | Have the battery and its wiring checked. |
| BAT CABINET OVER HEAT | Battery temperature too high, or a battery has abnormalities. | Lower the room temperature; call service for replacement if a battery is abnormal. |
| BAT TEST FAIL | Wrong battery wiring or a battery abnormality. | Call service to check grounding and the batteries. |
| BAT LOW WARNING | Battery voltage below the warning limit. | With no backup power, shut down the critical loads now. |
| LOW BAT CUT OFF | Battery voltage below the shutdown limit. | The UPS stops supplying the loads to protect the battery until power returns. |
| BAT REPLACE REQUIRED | System date set wrong, or the battery replacement date is due. | Check the system date; if the date is due, plan replacement with service. |
| BATTERY BREAKER OFF | The battery circuit breaker is OFF. | Confirm whether it was opened for work; no battery backup until it is closed. |
| BATTERY FUSE OPEN | Battery fuse is blown. | Call service personnel. |
BAT LOW WARNING during an outage is a countdown: if no other source is coming, shut the critical loads down in order instead of waiting for LOW BAT CUT OFF.
Overload messages
| Message on the display | What the manual says | What to do |
|---|---|---|
| OUTPUT OVERLOAD WARNING | The UPS is overloaded. | Decrease some critical loads. |
| OUTPUT OVERLOAD SHUTDOWN | The UPS is overloaded. | Decrease some critical loads before restarting. |
| BYPASS STS OVER CURRENT | The UPS is overloaded. | Decrease some critical loads. |
| REDUNDANCY LOSS | Overload causes redundancy failure. | Decrease some loads; redundancy is reset after that. |
Heat and fan messages
| Message on the display | What the manual says | What to do |
|---|---|---|
| FAN FAIL | STS module fans abnormal, or foreign matter stuck in them. | Call service personnel. |
| PWR MODULE #n PFC OVER HEAT WARNING | Fans abnormal, or foreign matter stuck in the fans. | Check from outside that no vent is blocked; if fans are at fault, call service; if not, reduce some loads. |
| BYPASS STS OVER HEAT | Fans abnormal, foreign matter in the fans, or the UPS is overloaded. | Check the vents and the load; call service if the fans are at fault. |
| SYSTEM FAN FAIL | STS module fans abnormal, or foreign matter stuck in them. | Call service personnel. |
Check vents and room temperature from outside the cabinet only; fans and filters inside the modules are service work.
Breakers, EPO and latches
| Message on the display | What the manual says | What to do |
|---|---|---|
| EMERGENCY POWER OFF | EPO is activated. | Find the cause; once the emergency is cleared, restart with the turn-on procedure. |
| ON MANUAL BYPASS | The manual bypass breaker (Q3) is ON. | Check whether maintenance is under way; Q3 is turned off as part of the return-to-normal procedure. |
| OUTPUT BREAKER OFF | The output breaker (Q4) is OFF. | Confirm whether it was opened on purpose; call service if it is on and the alarm stays. |
| MAINS INPUT BREAKER OFF | The main input breaker (Q1) is OFF. | Confirm why it is open; call service if it is on and the alarm stays. |
| BYPASS BREAKER OFF | The bypass breaker (Q2) is OFF. | Confirm why it is open; call service if it is on and the alarm stays. |
| BYPASS STS REPAIR SWITCH OPENED | STS module latch not properly locked. | Have the latch checked by whoever last serviced the module. |
A breaker or latch message after maintenance usually means a step of the return-to-service procedure was missed. Only someone trained on the DPH procedure should operate Q1 to Q4.
Messages that need a technician
| Message on the display | What the manual says | What to do |
|---|---|---|
| PWR MODULE #n PFC FUSE OPEN SHUTDOWN | Power module PFC fuse is blown. | Call service personnel. |
| PWR MODULE #n GENERAL FAULT | Power module control circuit abnormal (auxiliary power, soft start). | Call service personnel. |
| SYSTEM GENERAL FAULT | System auxiliary power abnormal. | Call service personnel. |
| PWR MODULE #n DC BUS NOK | DC bus voltage too high or too low. | Call service personnel. |
| UPS OUTPUT FAULT SHUTDOWN | Inverter output voltage too high or too low. | Call service personnel. |
| INV OVER CURRENT | The output might have a short. | Call service personnel. |
| BYPASS STS FAIL | Bypass static switch or its driving circuit damaged. | Call service personnel. |
| BYPASS STS GENERAL FAIL | STS module internal fault. | Call service personnel. |
For all of these, the manual's answer is to contact service personnel. Leave the cabinet closed and the load where the UPS put it. Our Delta UPS service team reads the DPH event history on site and can swap a failed power module, and the modular UPS overview explains how module swaps keep the load protected during that work.
Related pages
Common questions
Does the Delta DPH use numeric alarm codes?
The user manual lists alarms by their LCD message, such as BAT LOW WARNING, each with a cause and a solution.
What does OUTPUT OVERLOAD WARNING mean on a DPH?
The UPS is overloaded. Delta's solution is to decrease some critical loads.
What should I do on BAT LOW WARNING?
If there is no other backup power, shut down the critical loads connected to the UPS right away.
What does ON MANUAL BYPASS mean?
The manual bypass breaker Q3 is ON. If maintenance is finished, the return-to-normal procedure turns it OFF.
Which DPH alarms need a technician?
Any message whose solution is to contact service personnel, such as PFC FUSE OPEN SHUTDOWN or BYPASS STS FAIL.
Sources, checked October 4, 2026:
- Delta Modulon DPH Series, Three Phase 25-75 kVA, User Manual, chapter 10 Troubleshooting, p. 10-2
- Delta Modulon DPH Series, Three Phase 25-75 kVA, User Manual, chapter 10, pp. 10-2 to 10-7, 10-3 to 10-7, 10-4 to 10-6, 10-5 to 10-7
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