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.
Full detail — causes, the why, and common mistakes.
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.
Confirm 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.
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.