LED Sign Trips at Switch-On: Understanding Inrush Current

1. Identify the trip and its timing before calculating

Record the device label and let a competent person identify whether it is an MCB, RCD, RCBO, fuse or another protective device. “The breaker tripped” is not enough: overcurrent and residual-current functions respond to different conditions, and an RCBO combines both. Do not repeatedly reset a device that will not remain set or that supplies damaged, wet or overheating equipment.

Time the event from the authorised switch command. An immediate trip when several supplies energise may make start-up current relevant; a trip after seconds or minutes needs a different load, thermal or fault investigation. Record whether the event occurs from a cold start, a rapid restart, after a long off period, or only in particular weather.

Stop and isolate if there are burn marks, staining, damaged cables, water ingress, smell or arcing. HSE says electrical equipment and the supply must be suitable and that unsafe equipment should not be used until corrected by a competent person.

2. Separate inrush current from sustained input load

Inrush is a short input-current transient, commonly associated with charging input capacitance at energisation. Normal input current is the current drawn during operation under stated voltage and load conditions. They use different values and time scales, so adding output watts or reading a clamp meter after start-up does not establish the peak.

Use the manufacturer’s definition. A datasheet may state peak current and a pulse width measured at a percentage of the peak, while another report may use T50. Those quantities cannot be compared or added correctly unless their conditions and definitions match.

A switch-on symptom is therefore not a generic instruction to use a different breaker curve. The electrician must assess the protective device, cable, fault protection, discrimination, input current, start-up waveform and installation requirements together.

3. Read one model-specific example without generalising it

MEAN WELL’s current XLG-100 series specification states, for that series, a typical cold-start inrush of 50A with a 300 microsecond width measured at 50% of peak at 230VAC. Its table also gives model-specific maximum quantities for particular 16A type B and C circuit breakers at 230VAC. Those figures are evidence for the stated series and conditions, not a universal rule for every 100W supply.

The official XLG-100-12 design verification report provides a more specific example: the captured result is 41.8A at 230VAC, 50Hz, full load and 25°C, with T50 of 264 microseconds. The difference between a specification value and one report result is precisely why the revision, variant, conditions and definition belong in the inventory.

A different family or wattage can publish a different peak, width and permitted quantity. Do not transfer XLG-100 data to an XLG-320, another MEAN WELL family, a Glowfy-labelled supply or an unidentified installed unit. Obtain the exact manufacturer document or mark the value unavailable.

4. Build the start-up circuit inventory

List every supply energised by the same switching event, including full part number, input rating, normal input current, inrush statement and load state. Add contactors, timers, photocells or controls that determine whether the units genuinely start together. A staged sequence changes the event but is an engineered control decision, not a wiring suggestion for this article.

Record the circuit and protective-device details through the competent person: voltage, frequency, device manufacturer and model, current rating, characteristic, breaking capacity where relevant, conductor information, other loads and upstream protection. Include fault or residual-current evidence rather than forcing every start-up trip into an inrush calculation.

State what is unknown. Missing T50, incompatible waveform definitions or an unidentified protective device means a safe quantity cannot be calculated from the available record.

• Circuit identifier and authorised isolation point.
• Protective-device manufacturer, model, function and complete marking.
• Every power-supply model, suffix, quantity and datasheet revision.
• Rated input current and documented inrush peak and width.
• Input voltage, frequency, load and ambient conditions.
• Cold start, hot start, restart interval and switching sequence.
• Other simultaneous loads, moisture, damage and residual-current evidence.
• Electrician’s calculation, onsite verification and final decision.

5. Treat arithmetic as a screening aid, not a breaker prescription

Suppose four identical supplies are documented to start together and the exact datasheet states a typical 50A peak under the relevant condition. Writing 4 × 50A = 200A records a theoretical coincident peak only. It does not predict the protective device response because waveform tolerances, point-on-wave, source impedance, pulse duration, device curve, temperature and simultaneous switching all matter.

Likewise, summing the supplies’ rated input currents helps check sustained load but does not replace the circuit design. MEAN WELL’s application guidance discusses both total rated input current and inrush when selecting protection, yet its worked values refer to named products and devices. The final assessment must use the chosen protective-device manufacturer’s data and the real installation.

Keep the calculation labelled with assumptions and a decision status such as “evidence incomplete”, “competent calculation required” or “onsite verification completed”. Do not turn the arithmetic into a public maximum-number table for mixed supplies.

6. Let the confirmed cause determine the remedy

Possible engineering responses may include correcting a fault, separating unrelated loads, selecting documented compatible equipment, or a manufacturer-supported start-up management solution. MEAN WELL publishes ICL inrush-current limiters, but their capacitive-load, rated-current, environmental and installation limits make them model-specific options for an engineer, not generic add-ons.

Never increase a fuse or breaker rating, change a trip characteristic, bypass residual-current protection or fit a limiter without the responsible electrical assessment. Protection must continue to suit the cable, equipment, fault conditions and applicable installation requirements.

After authorised work, verify cold start, normal running and representative switching under the electrician’s test plan. Keep the final circuit schedule, device data, PSU documents and results with the sign so a later replacement does not recreate the problem.