Inrush Current
Definition
Inrush current is the surge of current a fixture draws when it is switched on, before it reaches its steady state, while the capacitors of its switch-mode power supply charge from zero. For an LED driver the peak can exceed a hundred times the steady-state current, but it lasts only a few hundred microseconds. ABB cites a 150 W driver that draws 0.7 A in operation and 78 A for 195 µs at switch-on, and driver datasheets give the actual value for each model. Under IEC 60898-1, a Type B breaker must not trip instantaneously below 3× its rated current and must trip in less than 0.1 s at 5×; the corresponding limits for a Type C are 5 and 10×. In North America, miniature breakers built to UL 489 are also offered with trip curves, most often Z, C and K, and some manufacturers use B and D in place of Z and K. Staggering the power-on sequence avoids simultaneous inrush.
In the field
A breaker that trips at power-on but never during normal running usually indicates inrush current. The remedies are to power fixtures up in groups and, where the installation rules allow it, to use Type C breakers on lighting circuits that feed many fixtures. Some LED driver manufacturers publish the maximum number of drivers per breaker type.
Related lessons
Sources
- IEC 60898-1 : zones de déclenchement instantané B (3 à 5 In), C (5 à 10 In), D (10 à 20 In) ; température de référence 30 °C · se.com
- UL 489, Molded-case circuit breakers, molded-case switches and circuit-breaker enclosures : norme nord-américaine des disjoncteurs de branchement · shopulstandards.com
- Panel Builder US, An introduction to UL 489 current-limiting miniature circuit breakers and their trip curves : courbes Z, C et K, ou B et D selon les fabricants · panelbuilderus.com
- ABB, How to protect lighting circuits for LED lights : driver de 150 W et 0,7 A, courant d'appel de 78 A pendant 195 µs, nombre de drivers par disjoncteur C16 · new.abb.com
- Wikipédia (pointeur secondaire), Inrush current : charge des condensateurs d'entrée des alimentations à découpage · en.wikipedia.org