Design engineers are busy people, and using shorthand techniques to turn out specifications is widely accepted. But there's a danger in applying quick and easy methods too often. If we're not careful, they become the accepted norm, without any real reason.
One particularly significant example of this is the use of 0.8 as the maintenance factor (MF) when calculating LED lighting specifications. In fact, this has become something of an elephant in the room where lighting design is concerned – the problem is there, and we all know it. But we have yet to tackle this issue head-on.
Maintenance factors deal with the fact that lighting installations will gradually reduce the amount of light they provide over a number of years, due to lumen depreciation, dirt, failures and so on. The MF is a percentage of the total light output at the start of the installation life, to which the output may eventually fall. It tells you how much you need to increase the light level at the start, in order to stay above the required level over the lifetime of the installation.
Thus, if the required light level over the lifetime of the project is 500lx, and the maintenance factor is set at 0.8 (or 80 per cent), then in order to maintain lighting at the level required, an initial 625lx should be installed to the space.
Here are the 10 things every lighting professional needs to know about maintenance factors:
1. Maintenance factors are recognised as crucial by the industry
Maintenance factors are an important consideration in the planning of lighting installations. The Society of Light & Lighting’s Code for Lighting states that: ‘The lighting scheme should be designed with an overall maintenance factor calculated for the selected lighting equipment, environment and specified maintenance schedule.’
2. An MF of 0.8 is not suitable for every project
Although 0.8 is a useful rule of thumb, it’s an arbitrary figure, and there is no reason why it should be used on every lighting installation project. Every project is different so the maintenance factor should be altered to suit the circumstances and the lighting technology being used.
If we consider what the key elements that go into the MF are, then we see a number of factors make up the final figure. The formula for the overall maintenance factor for indoor lighting is:
MF = (LLMF x LSF) x LMF x RMF
LLMF : lamp lumen maintenance factor
LSF : lamp survival factor
LMF : luminaire maintenance factor
RMF : room maintenance factor
SMF: surface maintenance factor
3. The lamp lumen maintenance factor is particularly important for LED
Factors such as RMF and SMF are affected by the location of the luminaires – whether an industrial warehouse or an office, for example. This would in turn affect the LMF in terms of dust-build up, which has an effect on light levels. Pollution levels for the luminaire and space can be viewed in the same way for LEDs as they would be for fluorescent lighting – dust and dirt will affect the quality of output from both.
However, with LED lighting the LLMF is particularly significant, and so is its impact on the overall maintenance factor. LEDs have a very long service life – in fact, it is highly likely that an office would be refurbished before the LED lighting required replacement.
4. Service life is key to the calculation in terms of energy use and cost
When specifying LEDs it is therefore very important to indicate the service life used in calculations because it will lead to decisions on the initial light level and the number of installed luminaires. This will greatly affect the amount of lighting required – and therefore have an impact on both capital and operational costs.
The power and lifetime of an LED is generally expressed as something like:
L80 B10 50,000 hours at 25C. Here, L is the service life; B is the gradual failure; and h is the hours of service life referred to.
5. The products may outlive the scheme they’re installed in
Having an accurate LLMF in order to find the correct maintenance factor for a lighting project has other benefits in terms of planning for client requirements. For example, with such extended LED lifetimes available, clients may want to consider if they need a 50,000-hour life in their lighting system.
Why plan for 12 to 15 years of operation, if the client is only taking on a seven-year lease for an office space? By changing this value, the LLMF will be altered – and the amount of light and number of luminaires could be greatly reduced. This will save the client money in the short- and long-term.
6. Comparing like-with-like is very important for designers and end users
One of the main challenges for those who plan lighting installations is that it is difficult to compare like-for-like when specifying.
For example, we find office luminaires that may be L80 B10 50,000 hours at 25oC all the way down to L70 B50 30,000h at 25oC. What’s more, the lack of consistency in comparative figures makes it difficult for clients to make a clear comparison.
Clearly, a lot of information needs to be gathered together in order to formulate an accurate maintenance factor.
7. New standards can help with comparisons
Some very useful sources of information have been developed for use across Europe to help lighting specifiers find the right figure quickly. One of these is the ENEC+ organisation which is developing standards for the validation of the lifetime and performance of LED-based products. The aim is to give users of the products confidence that what they are specifying and purchasing will perform as stated. The ENEC+ mark will also standardise the use of L and B factors, making direct comparisons much easier.
Another organisation that has carried out in-depth research in this field is ZVEI, a European body for the electrotechnical and lighting industries. It has produced a guide to reliable planning with LED lighting.
8. Just because the MF is above 0.8 doesn’t mean it’s wrong
Clients are right to be suspicious of high maintenance factor figures. But just because it’s higher than 0.8 doesn’t make it wrong.
Designers at the early stages of a project, whether new-build or refurbishment, often work without all the information. Some intelligent approximations are to be expected. However, with a standardised method for verifying LLMF and LSF available, and also guidance on how to compare these directly, there is no reason to use 0.8 because it has always been done that way.
Too often, designs that use an accurate maintenance factor of 0.88 can be returned with a request to use 0.8, even though that is an incorrect figure. This encourages the practice and should be stopped.
9. Poor design and overspeccing wastes the benefits of LEDs
As the SLL Code for Lighting points out: ‘The MF has a great impact on energy efficiency.’ If we specify too much lighting at the start of a project because of inaccurate maintenance factors, then the client will pay for that heavily through their electricity bills.
It has often been said that in the world of energy saving, lighting is the ‘low-hanging fruit’ – could this be because it has been over-specified too often and for too long?
10. We can all make things better
Manufacturers like Trilux are working to provide clear information on its products by engaging with projects such as ENEC+ and ZVEI. Designers should move away from applying 0.8 universally and treat each project as a one-off. And clients need to look carefully at the costs for lighting and make their requirements clear. Over-specifying is already a problem in other areas of building services such as pumps and fans. There is no need for lighting to take the same route.
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