Symptom
Every indicator in the panel is dark; the controller and HMI are off; the main disconnect is on and the feeder is live; or the AC control is present but the DC side is dead.
The short answer
No Control Power in Panel
A panel with no control power has lost it somewhere between the disconnect and the control terminals, and the fault is found by measuring voltage at each point in order: the disconnect load side, the control transformer primary fuses, the transformer secondary, the secondary fuse, the control power breaker or switch, the emergency stop and safety relay outputs, the control terminal strip, and then the DC supply and its branch protection. The voltage is present at every point up to the fault and absent after it. The most common finds are a tripped control breaker, an open transformer fuse, a pressed or stuck emergency stop, and a failed DC supply, and a fuse that opened had a cause that must be found before a new one is installed.
Key points
- Measure from the source toward the load; the fault is between the last live point and the first dead one.
- Read the indicators first: the lights that are on say how far the power got.
- A fuse that opened had a reason; find it before replacing the fuse.
- The emergency stop chain and the safety relay are part of the control power path.
- AC present and DC dead is the DC supply or its branch protection.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Control power breaker or switch tripped or off | The breaker handle position; the load side voltage. Tripped means a fault downstream: find it before resetting. |
| Control transformer primary fuse open | Voltage on the line side and none on the load side of the fuse; indicating fuse holders show it. Opens on transformer inrush after an outage or on a transformer fault. |
| Control transformer secondary fuse open | Same test on the secondary fuse. Opens on a shorted coil, a ground fault, or an overload on the control circuit. |
| Emergency stop pressed, or the safety relay not reset | Every emergency stop released; the safety relay input and output indicators; press reset. A broken wire in the loop reads the same as a pressed button. |
| Control transformer failed | Primary voltage present, secondary absent with the secondary fuse good. Winding resistance with the power off. |
| Loss of a phase or the feeder | All phases at the disconnect load side. A control transformer on two phases dies when one is lost while motors on the others may still run. |
| Loose or broken wire at a terminal | Voltage at the terminal strip absent with everything upstream good; wiggle and re-terminate. |
| DC power supply failed or in current limit | Input voltage present, output absent or low; disconnect the load and see if the output recovers. Low output with the load connected is an overload or a short downstream. |
| DC branch breaker or fuse open | Each branch load side; a tripped branch is a short or a ground fault in that field circuit. |
| Power monitor or undervoltage relay dropped out | Its indicator; some panels drop control power on phase loss or low voltage until reset. |
The order of checks
- 1
Look
The control power light, the controller and supply lights, the safety relay indicators, the breaker positions, and the emergency stop buttons. What is lit tells you where to start.
- 2
Disconnect load side
All phases present at the expected voltage. Nothing here means the feeder, the upstream breaker, or the disconnect.
- 3
Primary fuses
Voltage on the load side of each. An open fuse is a finding, not a fix: note it and keep measuring downstream after replacing it only when the cause is known.
- 4
Transformer secondary
Control voltage at the terminals. Primary in and no secondary out is a failed transformer.
- 5
Secondary fuse and control breaker
Load side of each in turn.
- 6
Emergency stop chain and safety relay
Voltage through the chain; the relay inputs satisfied; reset pressed; outputs closed. A relay that will not reset has an open in its loop or a failed channel.
- 7
Control terminals
The distribution point. Present here means the AC control is fine and the problem is a load or the DC side.
- 8
DC supply and branches
Input present, output at voltage, each branch breaker or fuse good on its load side. Divide the branches to find a short.
What the pattern says
| Observation | Likely cause |
|---|---|
| Everything dark, disconnect on | Feeder or phase loss; primary fuse; transformer; control breaker |
| Control power light on, controller dark | DC supply, DC branch protection, or the controller power module |
| Everything died when a particular output was commanded | A shorted coil or a ground fault on that output circuit opened the secondary fuse |
| Everything died during a storm | A ground fault in the field, or a surge that took the transformer or the supply |
| Dead after a maintenance visit | An emergency stop left pressed, a safety relay not reset, a wire not landed, a breaker left off |
| Dead after a momentary outage | Primary fuse opened on transformer inrush; a power monitor relay waiting for reset |
After power is restored
- Confirm the controller is in run with the correct program, and check for lost retentive data if the outage was long.
- Confirm every mode selector is where it should be and the process restarts as the startup checklist expects.
- Record the cause and the fix; if a fuse opened, record the reason so the next person does not repeat the search.
Frequently asked questions
- The control breaker was tripped. Can I just reset it?
- Once, with everything downstream in a safe state, and while watching what happens. If it trips again, there is a short or a ground fault to find by dividing the circuit. If it holds, look for what drew the overload: a coil that failed, a circuit that was added, a wet field device.
- Why did the panel die after a brief power blink?
- The control transformer drew a large inrush when it was re-energized and opened a primary fuse that was sized without margin, or a power monitor relay dropped out and needs a reset. Time-delay fuses of the rating the transformer manufacturer specifies prevent the first; the second is a design choice that the operators need to know about.
- The safety relay lights show a channel fault. What is that?
- The two channels of the emergency stop loop did not agree within the allowed time, usually because one contact of a button or a switch is not making, or a wire in one channel is broken. Check each device in the loop for both contacts changing state together, and the wiring of both channels.
- How do I tell a failed DC supply from an overloaded one?
- Disconnect the output loads. A supply whose output returns to voltage with no load is being overloaded or shorted downstream; a supply that stays dead or low with no load has failed. Most supplies indicate current limit by cycling or a changed indicator.
Related topics
- No Control PowerThe control power path from the disconnect to the last relay, and how to work along it when the panel is dead: transformer and fuses, control breaker, emergency stop and safety relay, DC supply and distribution, what trips them, and design that makes it easy.
- Fuses in Control PanelsFuse classes, time-delay and fast-acting types, current limitation and what it does for the panel short-circuit rating, semiconductor fuses for drives, control circuit and DC fusing, and the habits that make a blown fuse a diagnosis instead of a mystery.
- Panel Power SuppliesThe 24 V DC supply behind everything in a modern panel: sizing with real inrush and duty, single versus redundant with diode modules, how a switch-mode supply behaves under a short, distribution and fusing per branch, monitoring, and the failure modes that take out a whole panel at once.
- Fuse Blows RepeatedlyA control fuse that opens again after replacement has a cause the replacement did not touch: a shorted coil, a ground fault in a wet field device, an overloaded circuit, the wrong fuse type for inrush, or an intermittent short. How the timing identifies it.
- Power Supply FailureA 24 volt DC supply whose output is gone, low, cycling, or noisy: an overload that has put it into current limit, a supply killed by heat and age, input voltage out of range, or surge damage. One test tells an overloaded supply from a dead one.
- Missing Equipment GroundAn enclosure, motor, or device with no effective equipment ground: how it shows up as a tingle, a fault that does not trip, surge protection that does nothing, or instrument noise; how to test the path safely; and why a ground rod is no substitute.
Direct contact
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