Equipment tripping the supply on startup surge
Switching on a piece of equipment trips an upstream breaker or causes a momentary dip — the inrush/startup surge is exceeding what the protection or supply can ride through.
Safety first
Repeated switch-on attempts stress the supply and equipment. Limit attempts. Isolate before working and be aware other equipment may dip when this one starts.
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.
Apprentice mode
Full walk-through — the why, what readings mean, mistakes & safety.
Likely causes
Ranked from most to least likely.
- 1
High inrush current at switch-on
Most likelyTransformers, large capacitor banks, motors, and LED/SMPS loads draw a brief high inrush that trips fast protection.
- 2
Protection characteristic too sensitive
#2A breaker curve that doesn't ride through legitimate inrush trips on startup.
- 3
Several loads switching together
#3Combined inrush from multiple loads energising at once exceeds the protection.
- 4
Weak supply sagging on inrush
#4A weak supply dips on inrush, causing trips or resets nearby.
- 5
Genuine fault at switch-on
Least likelyA real short/earth fault that only manifests when energised.
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.
On the tools? Job Mode shows one test at a time with thumb-sized buttons — your place and readings carry straight over.
Enter Job ModeConfirm it's the inrush (trips only at the instant of switch-on, runs fine after) and rule out a hard fault.
Trips only at switch-on, then runs normally if it gets going.
Inrush pattern — look at protection characteristic and load grouping.
Trips and won't run / instant hard trip — test for a real short/earth fault.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Trips on startup surge
→ step 2 - 2decision
Does it trip only at switch-on (then run fine)?
Yes→ step 3No→ step 4 - 3decision
Is the protection curve suited and switching staged?
Yes→ step 5No→ step 6 - 4result
Test for a real short/earth fault at energising.
- 5decision
Does the supply hold during inrush?
Yes→ step 7No→ step 8 - 6result
Select a suitable curve / stagger switch-on.
- 7result
Re-confirm protection selection.
- 8result
Supply sags — consider capacity or inrush-limiting measures.
Common mistakes apprentices make
- Upsizing protection beyond the circuit's design to stop nuisance inrush trips.
- Not distinguishing inrush from a genuine switch-on fault.
- Switching many high-inrush loads simultaneously.
- Ignoring a weak supply that dips on inrush.
When to stop & escalate
Inrush mitigation (correct breaker curve, staged switching, soft-start, supply capacity) can be a design matter — plan it properly rather than defeating protection. A genuine switch-on fault must be found before re-energising.
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.
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Related faults
Motor trips protection on start but runs fine if it gets going
Protection trips during the start/inrush, but on the rare occasion it gets running it's fine — pointing at starting current, settings, or load inertia rather than a running fault.
MCB (circuit breaker) keeps tripping
A circuit breaker trips repeatedly — instantly on reset, or after a load runs for a while — and you need to tell a short from an overload from a faulty breaker.
Voltage drop on a long cable run
Equipment at the end of a long run misbehaves — dim lights, a contactor that won't hold, a motor struggling — because volt-drop along the cable leaves too little voltage at the load.
Learn the theory
How the gear and circuits behind this fault actually work.