Some customers are finding out their “It lasts forever!” LED lighting system, doesn’t just sound too good to be true. LED lumen depreciation is commonly expected, but is not the be-all and end-all of luminaire lifetime. There are just too many electronic systems to not expect “lights-out” failures along the way. Drivers seem to be particularly vulnerable to thermal stress and failures, making them a keystone to system lifetime and reliability estimates.

“If anything is going to fail, I think we’ve seen [the driver] be the first,” said Norma Frank, CEO of Colorado Lighting and chair of the Illuminating Engineering Society (IES) committee on lighting maintenance. Outdoors, “we’ve also seen voltage issues: 480V is not helpful in performance issues…. It’s all about surges and the type of surge protection you have.” Her anecdotal observations are backed up by the small group of members of the LED Systems Reliability Consortium coordinated by the DOE. They cited failures of power/driver components as the most frequently observed failure mechanism in LED systems, followed by failures on the LED board. Hammer testing by RTI International stressed LED luminaires beyond their design tolerances for temperature, humidity, electrical power, etc. The purpose was to accelerate failures in a short period to open lines of investigation into specific failure modes. A small sample of 6 inch downlights showed amazing durability under multiple environmental and electrical stressors, with failures typically in the driver.

A few initial failures are to be expected, but some customers have experienced high percentages of failures. “When you see a far-ranging failure, there’s either a flaw in manufacturing or there’s a flaw in compatibility in the system,” according to Frank. High-volume recalls of LED products were issued in 2015 by reputable manufacturers. Even with a solid 5 or 10 year warranty, the process of getting a replacement can be onerous. And the warranties are all different. “It’s really the onsey-twosies that take so much time,” said Frank, explaining the long process of documenting failures. “The manufacturer might not even be business anymore.”

These types of LED maintenance problems will inconvenience a customer and eat into the contractor’s (and the manufacturer’s) profit. “When you’re working on these system conversions, it’s a conversation you have to have with the owner. So they’re aware of what some of the issues might be,” she said.

What are you getting?

The marketplace is often described as “The Wild West” with an unsettling lack of standards. Those standards in place extrapolate from short-term testing. After all, we can’t wait years to complete testing each new product introduction. There currently is no industry-wide consensus protocol to estimate the life of an LED luminaire system. (LEDs, however, are proving themselves in the field. They are superheroes of long life and durability compared to high-maintenance legacy lighting products.)

Lifetime is a manufacturer’s statistical estimate of how long a product or system is expected to perform adequately under a very specific set of environmental, electrical and mechanical conditions. That single lifetime number most often corresponds to a pattern: a small number of initial failures, a long period of stability when failures are rare, followed by a steep upswing in failures: the so-called bathtub curve.

Performance of a system of electronic components is typically characterized by an initially high failure rate (infant mortality), followed by a long period high reliability, then a wear-out stage – commonly known as a “bathtub” curve. Predictability allows for more accurate maintenance planning.

Reliability estimates typically apply to that long period of stable operation – the useful life – expressed as mean time to failure (MTTF) or mean time between failures (MTBF). It describes an average failure rate, which is helpful in predicting maintenance for products that have a relatively constant failure rate over that useful-life period. Alternatively, IES describes a failures in time (FIT) metric for LED packages, expressed as a collective failure rate over a million (or a billion) of hours of operation. Again, these numbers are estimates based on specified operating temperatures, current and voltage.

It’s important to note that LED luminaires and integrated LED replacement lamps are complex systems that will only last as long as their weakest link. A faltering power circuit can cause a cascade of failures in a driver. A failure in thermal protection will curtail the life of the LEDs.

Addressing catastrophic failures is only part of the question. Parametric failures can also occur: lumen depreciation, of course, but also color shifts and changes in distribution patterns. If 20 percent of the emitters have failed but the product remains above L70, is that aesthetically acceptable? Failures in color stability over life and color consistency product-to-product may or may not be addressed quantitatively in a warranty, warns Frank. “A lot of people say, Just put a new fixture in. It’s very difficult to put a new fixture up and have the color be the same as the other fixtures in the area.”

Lumen maintenance

Early ratings of “LED life” were defined as the operating hours until the LED tested produced only a percentage of initial lumens: for instance, at L70-B50 lumen output is expected to have dropped to 70% of initial lumens in half of a population of LEDs. But that includes a portion of LEDs that have failed completely and are producing no lumens at all. This L70-B50 number can easily exceed acceptable light losses, the anticipated life of other components, even the expected life of an installation. An alternative method is to express the percentage of lumen maintenance at a given number of hours. A Lighting Facts Label will identify the calculated average lumen maintenance at 25,000 hrs.

Testing according to protocols developed by the IES will give these numbers, along with some information on color stability, for the LEDs. But because the useful life of an LED lighting product is dependent on other components in the luminaire and the overall system design, the US Department of Energy states clearly that “Lumen Depreciation is not a proxy for luminaire lifetime…. The LED package may not be dominant in determining product lifetime.” In fact, DOE recommends that customers avoid products that use only lumen maintenance numbers as an estimate of luminaire lifetime.

Appalachian Lighting Systems provided the DOE with data on failure modes in a family of outdoor luminaires. Overall failures were low, representing a >5 percent cumulative failure rate across 7 years. Different product types would likely show different relative instances of failure.

Factors such as stress on the boards and connections, degradation of onboard controls and optical components or corrosion of gaskets or seals may become even more impactful due to the decades-long operational lives of these products. DOE emphasizes that to fairly estimate lighting maintenance requirements, specifiers question the methods used to determine reliability or lifetime numbers, and stay abreast of testing methods established by industry consensus.

“The warranty is a critical part of the solution, and that warranty needs to be documented and spelled out forthrightly,” said Frank. “The manufacturer and the contractor should all be on the same page as to what that warranty covers.”