Using LED drivers for a bespoke lighting installation can be confusing due to the range of different types and possible variations for their connection that are available. This article from the experts at NICEIC will describe the function of LED drivers, highlight where and how they may be used, and also advise on which requirements of BS 7671 apply.
LED lighting is a popular choice for many installations due to their long lifespans, energy savings, instant operation at switch-on and the bespoke lighting schemes and methods for control that are available.
A typical example for a bespoke lighting installation, as shown in Fig 1, may include alterations to the method of control of an existing luminaire into multiple luminaires having means to be remotely controlled via an App and/or other sensors.
It will usually be necessary to install an additional smart device1. However, reference should be made to the manufacturers’ literature for compatibility between different products.
The smart device may be incorporated in equipment such as a smart GU10 lamp, which then communicates with a hub or controller connected to the local area network (LAN)
An LED driver is a self-contained power supply whose output is typically matched to the electrical characteristics of the LED(s) it is supplying.
However, the stability of the LED and associated driver is likely to be dependent upon the method of installation, including the ability to dissipate heat and the circuit configuration for the type of LEDs, which may be constant current or constant voltage.
Suitable precautions must be taken to prevent overheating when installing LEDs and their drivers to avoid the occurrence of, thermal runaway.
Thermal runaway occurs when an LED module exceeds its operating temperature, for whatever reason, and more current is consumed due to a reduced resistance in the silicon than it needs for a particular light output. This reduces the life expectancy of both the LED and the driver.
There are two main types of drivers; those that use extra-low voltage DC input power (generally 5-24 V DC) as shown in Fig 2, and those that use low voltage AC input power (generally 90-277 V AC) as shown in Fig 3.
Constant voltage drivers
Where a constant voltage driver (or power supply) is used, the LEDs will be connected in parallel and typically operate at a voltage suitable for the LED, for example 12 V DC or 24 V DC.
A constant voltage driver receives a line voltage (120-277 V AC) and converts this AC voltage to an extra-low direct current voltage. The driver will always attempt to maintain a constant voltage no matter what current load is placed on it.
Constant voltage drivers are commonly used for under-cabinet lighting and other LED flexible strips, as shown in Fig 4, but are not limited to those categories.
Constant current drivers
Where a constant current driver is used, the LEDs will be connected in series. This is usually the option selected when installing small high-powered LEDs such as those found in plinths, external decking areas and driveways, used as marker lighting typically covering a larger area, as shown in Fig 5.
The main advantage of using a constant current driver is that the same current will flow through each individual LED whilst maintaining a constant level of brightness and over a greater circuit length. Furthermore, because the load current is constant, the risk of thermal runaway (discussed earlier) is reduced.
The constant current will typically be in the range 350 mA (0.35 A) to 2,000 mA (2 A).
Requirements of BS 7671
As for other types of lighting installations, LED lighting schemes forming part of the fixed installation must comply with the general rules of BS 7671 and any particular requirements of Section 559 for luminaires and lighting installations and, for extra-low voltage lighting, Section 715.
Where LEDs and their associated drivers are installed in special locations, as referred to in Part 7 of BS 7671 including locations containing a bath or shower (701), swimming pools or other basins (702), or outdoor lighting installations (714), the requirements of the particular section must also be met.
It is important that the LED lighting components such as the individual drivers or controllers meet an appropriate product standard (Regulation 511.1).
Where an LED lighting product is not covered by a British or Harmonized Standard, as with any product, it may still be installed provided that such usage gives the same degree of safety as that afforded by compliance with Regulation 511.2. Such a departure from BS 7671 shall be recorded on an appropriate Electrical Installation Certificate (Regulation 133.5).
It should be remembered that electrical equipment, including those products used for bespoke LED lighting installations, which is secured on or in the building fabric is by definition ‘fixed equipment’ and any wiring installed to a point in wiring for connection of such equipment should be considered as forming part of the fixed wiring of the installation.
For electrical products that are pre-wired and supplied with a cable and moulded plug for connection to an appropriate point in wiring such as a socket-outlet, the wiring is generally considered as not forming part of the fixed wiring, regardless of whether the equipment is secured to the building.
This article considered different types of LED drivers and their suitability depending on the circuit configuration. There are generally two types of LED driver – constant current and constant voltage.
Constant voltage drivers may be used for flexible strip lighting to maintain the voltage, as the length of strip is changed to meet requirements.
Constant current drivers are considered to be the preferred driver to use for high powered LEDs. The LED will only draw the current it needs, allowing the light output to remain constant over fluctuations in supply voltage whilst improving thermal stability.
BS 7671 makes no distinction between a typical installation and a bespoke LED installation. All areas of the LED lighting installation and its associated products, such as drivers, shall comply with any relevant requirements detailed within that standard.
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