How to protect lighting circuits for LED lights

In 2017, the 21 million buildings in Germany accounted for 36 percent of the country’s total energy consumption. When you consider that 18 percent of the electrical energy in non-residential buildings is consumed by lighting1, making sure that you install the most efficient lighting can have a significant impact on energy consumption and costs.

Consuming 75 percent less energy than traditional incandescent lights, Light Emitting Diodes (LED) lights can have a dramatic impact on reducing consumption.

Characteristic of LED lighting systems

If you decide to install LED lights in your electrical installation, it is important to consider the electrical parameters of the LED light, not just the power consumption.

To better understand the parameters, here is an overview of the components of a LED luminaire:

LEDs are semiconductor light sources which emit light when a current of electricity flows through it. To emit light, the LED needs direct current, which is provided by an electronic device called a driver. The driver converts the power of the low-voltage AC network into the DC voltage and current for the LED luminaire. All drivers contain capacitors. This causes inrush currents when switching on. In a steady-state condition, harmonics occur and the current of the LED luminaire is not perfectly sinusoidal.

Impact of the electrical parameters of LED luminaire on the electrical network

By installing a Residual Current Device (RCD) in the circuit, the earth leakage current must be considered. For luminaires, the typical earth leakage current is less than 1 mA. This means that a large number of LED luminaires can be installed.

You also need to consider the distortion of the sinusoidal waveform. The non-sinusoidal waveform has an impact on the tripping current of an RCD type AC, sensitive to alternating current only. As a result, an RCD type A, sensitive to alternating current and/or pulsating current with DC component, should be used.

Another element to consider is that the selection of the Miniature Circuit Breaker (MCB) characteristic depends on the characteristic of the load. This includes the cross-section and length of the cables as well as the inrush current. When switching on LED luminaires, high inrush currents occur due to the capacitor in the driver. It can be observed that the MCB trips when LED luminaires are powered on.

The following example depicts this situation:

  • In one circuit, twelve 150 W, 0,7 A LED drivers are installed.
  • The data sheet of the LED driver states an inrush current of 78 A for 195 µs.
  • The circuit is protected by an MCB C-curve, In = 16 A.
  • When switching on the circuit the MCB trips. For a rough estimation, the peak currents are additive. However, the network impedance, the cross-section and length of the cables have a significant effect on the peak value and duration of the inrush current.
  • The impulse tripping diagram shows for a peak duration of 195 µs a factor k = 13.
  • The MCB C-curve, In = 16 A, does not trip up to an inrush current of 13 x 5 x 16 A = 1.040 A. This calculated value is higher than the total inrush current of the twelve LED drivers, which is 936 A - but the MCB did trip. This is caused by the longer duration of the sum of the inrush currents of the LED drivers.
  • The total inrush current was measured and the duration of the envelope of the inrush currents is 250 µs.
  • The MCB C-curve does not trip at 250 µs up to 11 x 5 x 16 A = 880 A. This shows that, for this specific installation, eleven LED drivers can be protected by one MCB C-curve, In = 16 A.

The peak value and duration of the inrush current of a LED driver is shown in the technical datasheet. The effect of multiple drivers, the network impedance, the cross-section and the length of the cables on the peak value, and the duration of the inrush current is not known. For a calculation of the number of LED drivers to be connected to one MCB, the impulse tripping diagram can be used. For a safety margin the result should be multiplied by 0.8.

For the example above, the calculated inrush current is 1.040 A multiplied by 0.8 the maximum inrush current is 832 A.

Using LED lights in your electrical installation can significantly reduce energy consumption and resultant costs.

When thinking about your installation, you need to consider the impact of the inrush current when switching on the LED lights. Check the inrush currents of the LED drivers you want to install in one circuit in the technical datasheet. This inrush current must be lower than the impulse tripping current of the MCB protecting this circuit (available among the technical data). It is also important to consider using a safety margin of 80 percent of the tripping value.

By considering all these points, you will make sure that you have comfortable and safe lighting with the lowest possible costs.

Source: 1 dena-Gebäudereport 2018, www.dena.de

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