No control voltage in the panel
Nothing in the control circuit will operate — contactors won't pull in, indicators are dead, the PLC may be off. The control voltage that should be there simply isn't.
Safety first
A dead control circuit does not mean the panel is dead. Incoming power and the load side can still be fully live. Prove what's isolated before working.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
Blown control fuse or tripped control MCB
Most likelyThe most common cause — a fuse or small breaker protecting the control circuit has opened, killing the whole control supply.
- 2
Failed control transformer or supply
#2A control transformer or DC power supply has lost output (failed device, open primary, blown internal fuse).
- 3
Missing incoming supply to the control source
#3The control transformer/PSU isn't getting its input because an upstream isolator, link, or supply is open.
- 4
Open in the control wiring / loose terminal
#4A broken wire or loose terminal between the supply and the rest of the control circuit drops everything downstream.
- 5
An interlock/main contact upstream of the control feed is open
Least likelyA master control relay, e-stop, or main contact that feeds the control rails is open, removing the supply by design.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
Measure for the expected control voltage at the main control rails/terminals.
Rated control voltage present across the control rails.
Control voltage is actually present — the fault is downstream of the rails, not the supply.
Confirmed missing control voltage — work back toward the source.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
No control voltage
→ step 2 - 2decision
Is rated control voltage present at the control rails?
Yes→ step 3No→ step 4 - 3result
Supply is fine — the fault is downstream of the rails. Trace from there.
- 4decision
Is the control fuse/MCB intact and closed?
Yes→ step 5No→ step 6 - 5decision
Does the control transformer/PSU have rated output?
Yes→ step 7No→ step 8 - 6result
Blown fuse / tripped breaker — find why it opened, then restore.
- 7result
Output present but rails dead — open wiring/terminal or an open interlock between them.
- 8decision
Does the supply have its rated input?
Yes→ step 9No→ step 10 - 9result
Input present, no output — failed control transformer/PSU. Replace.
- 10result
No input — problem is upstream (isolator/link/incoming feed) or an open interlock.
Common mistakes apprentices make
- Assuming a dead control circuit means the whole panel is safe to touch — incoming and load sides can be live.
- Replacing a blown control fuse without finding why it blew.
- Measuring at the wrong reference point and chasing a 'missing' voltage that's actually present elsewhere.
- Overlooking an e-stop or master control relay that's intentionally removing the control supply.
When to stop & escalate
If a control fuse keeps blowing, stop and find the fault current source rather than upsizing the fuse. An e-stop or safety relay holding the control supply off must be understood against the machine's safety design before any reset or bypass.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
Related faults
Contactor has voltage at the coil but won't pull in
You measure the rated control voltage (e.g. 24V) across the coil terminals, but the contactor refuses to energise — no clunk, no pull-in, contacts stay open.
Timer relay not switching its output
A timer relay is powered but its output contact never changes state — the delayed action (start, changeover, stop) never happens, or it switches at the wrong time.
Three-phase equipment single-phasing (lost a phase)
Three-phase equipment is misbehaving — motors humming, struggling, overheating, or tripping — because one phase has been lost somewhere between the supply and the load.
Learn the theory
How the gear and circuits behind this fault actually work.
Transformer
Transfers electrical energy between circuits by magnetic coupling, stepping voltage up or down.
HMI (operator panel)
A touchscreen/operator interface that lets people monitor and control a machine or process.
AC vs DC
Alternating current reverses direction many times a second; direct current flows one way. Why it matters on site.
Control vs power circuits
Low-power control logic decides what happens; the power circuit carries the load — kept separate for safety and clarity.
No-volt release
After a power cut, equipment stays off until deliberately restarted — preventing dangerous auto-restart.