Power Management

How to Solve the Mysterious LED Electrical Network Problem

It has been quite a while since the invention of the first visible spectrum light emitting diode (LED) back in 1962. Although it took some time for the technology to develop, by the turn of the century that original low-powered expensive red glowing little orb had spawned a revolution in lighting.

Brighter LEDs in yellow and blue were developed in the 1990s, and improvements in manufacturing processes by then were making LED production less expensive. Demand grew and LEDs became more widely available (including in other colors); LED prices were dropping and their usage was greatly increasing.

Today LEDs can be found almost anywhere light is needed, from the single LED indicators on your home PC to the clusters of LEDs used for traffic signals, the light strips illuminating refrigerated coolers and freezers, and myriad other applications. (You probably already know it’s an LED that turns your smartphone into a handy flashlight.)

In recent years, LED lighting also has come into the mainstream for facility lighting as manufacturers have developed LED luminaires for many different applications, including offices, industrial facilities, street lighting and other applications, both indoor and outdoor.

Why this burgeoning interest in LED lighting? In many respects, LEDs are the proverbial “better mousetrap” of the lighting world – they use less energy to produce the same amount of light; they last much longer than any traditional lighting (especially compared to incandescent), which can mean significantly less maintenance is needed; and, they switch on and off very quickly. Beyond that, many LED luminaires are addressable, which opens up some wonderful lighting control possibilities.

In short, LED lighting is great in many different respects, but there is one caveat: In some application configurations, the inrush current associated with energizing LED luminaires can have unexpected results – not that it’s unpredictable, but most people just haven’t thought about what happens, electrically speaking, when you power up a large number of LEDs on the same circuit. To understand this situation, let’s look at inrush current.

Despite the fact that an LED uses very little power in its steady “on” state, at the instant the LED is energized it causes a brief but significant transient current. According to our measurements in the lab, this transient current draw – known as the inrush current – can be as much as 253 times the LED’s rated current. The inrush current typically lasts for a millisecond or less, after which the LED settles into its low level steady state operating condition.

For an individual LED, or even a bunch of LEDs working together in a single luminaire, this transient current generally is not a problem. And you get light quickly – the LED is already providing illumination while a fluorescent would just be getting warmed up.

The difficulty arises when large numbers of LEDs are wired together to create a luminaire, and then a number of those luminaires are installed to upgrade more traditional, energy-hungry fluorescent or incandescent bulbs and fixtures. Suddenly, the cumulative inrush current is something to be reckoned with.

To see what was happening in a real-world situation, we measured the electrical activity of a lighting installation in a commercial building where 25 LED luminaires, each rated at 56 watts, were installed on one 20-amp circuit. Although the rated power draw on the circuit was only 1400W, compared to the 4 amp draw from traditional lighting, energizing the LED luminaires produced a 237-amp inrush current over a period of 2.5 milliseconds. This tripped the circuit breaker, which is not a surprise once you realize what is going on.

Therefore, large retrofits of existing lighting systems designed to enable the use of energy-saving LED luminaires with existing electrical circuits must also address the issue of inrush current. In some cases, this could mean replacing the circuit breaker with a higher capacity unit, or the more difficult and costly approach of splitting the circuit into two or more smaller parts.

A third alternative is to use smart relays that energize the circuit precisely in sync with the waveform of the alternating current. By waiting to switch the LEDs on until the zero crossing of the voltage wave, this type of device minimizes the inrush current and makes it possible to use existing circuit breakers without derating them. The number of luminaires that can be powered by a single circuit is then limited only by the thermal capacity of the smart relay.

One important additional note with regard to LEDs is that they are more sensitive to voltage spikes – such as from lightning – than traditional incandescent lighting. We’ll address that issue in subsequent posts.

For more information on upgrading to LED lighting, download our white paper “Impact of LED Lighting on Electrical Networks.”

One Response to “How to Solve the Mysterious LED Electrical Network Problem”

  1. Peter Wachter IDSA IES

    Perhaps as commercial fixtures become smarter, those on the same switch and therefore pulling power from the same circuit breaker will have incorporated into the software on their PCBs a “courtesy queing” feature that recognizes that multiple fixtures are sharing the same circuit and they all voluntarily fire up a few milliseconds apart to attenuate the severity of the inrush current. Like Mr. Mathiou states they would all be up and running much faster than it takes the output “ramp up” inherent in fluorescents.

    Reply

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