What do I need to know about ‘LED Inrush Current’?
The term “LED inrush current” actually refers to the input current of short duration that flows into the LED driver on initial start-up, to charge capacitors on the input side.
It is an area of concern in the lighting industry as these inrush currents can be very high and have been known to continually trip circuit breakers, or cause damage to other devices such as sensors installed on the lighting system.
One of the advantages of LED lighting is that it responds so rapidly. This is the same thing that causes the high initial inrush. With a typical LED driver, once power is turned on the capacitors charge in less than a millisecond. It is this rapid charging that creates inrush current that can be a hundred times the LED driver’s continuous current rating. The inrush from LED drivers is six times the inrush current of the older style incandescent lamps, and four times the inrush current of the magnetic ballast used in fluorescent lighting – hence the concerns from the lighting industry!
How can I protect against LED Inrush Current?
Commonly protection is provided by two factors:
- Circuit Breakers – Typically a circuit breaker will trip when the current exceeds between 5 and 10 times its rating. On a common 10A breaker, therefore, it is typically designed to trip between 50 and 100 amps.
- Impedance of the Wires – All wires have both inductance and resistance, both of which will limit inrush current. The longer the wire and the smaller the cross-section, the greater the impedance and the higher the limiting effect on inrush.
Additionally, if a lighting control system is to be installed, safeguards can be programmed into the system to mitigate some of the effects of LED inrush current.
So how do I predict what the actual inrush current is going to be in a practical installation?
The answer, unfortunately, is that you can’t do this with any accuracy.
Every light fitting is different, variations occur in multiple quantities (with ballasts/drivers peaking at either the same time, or indeed different times on the phase, dependant on what is being used), and different lengths and cross-sections of wiring, all of which have an effect.
So is there nothing I can do?
The datasheets for reputable products will give a current rating, and sometimes current ratings for different kinds of load. Although this current rating is based on average load use, it also takes into account the average factoring for inrush current.
In our experience, we have found that a lot of people (including lighting manufacturers) have a relatively poor understanding of inrush current, and think that it can be easily quantified.
Our experience has found otherwise and has identified that there are many factors to actually take into account (as above). Our approach now is to use only products whose manufacturers use only high quality branded relays, and who subject their products to rigorous testing and are able to stand by the operating currents they state on their datasheets.
Such a product, for example, is unlikely to be that ‘cheap as chips’ range of sensors that can’t be beaten on price! However, the cost of replacing those sensors multiple times due to ‘inrush damage’ that is not covered by warranty, is destined to be more expensive in the long run!
If you’d like to know more feel free to give us a call
We have many years of experience in the design and installation of lighting control systems. Dealing with LED inrush current is just one of the multitude of potential issues that we work through on a daily basis. For example, we can programme lighting to turn on in a ‘staggered’ fashion, to ensure inrush from multiple fittings does not occur simultaneously.
