LED lighting - constraints and recommendations

In order to understand the impact that LED technologies will have on existing electrical networks, it is important to analyze the behavior of all key elements in the network. Below is a list of potential risks to consider and also some recommendations for mitigating those risks

The risk related to circuit breaker selection

The choice of circuit-breaker characteristics depends on the nature of the load powered. The rating depends on the cross section of the cables to be protected and the curves are chosen according to the loads' inrush current.

When switching on LED luminaires, very significant inrush currents occur up to 250 times the nominal current according the type of driver for a very short time (< 1 msec). Normative curves according to the “standard” (curves as defined in NF EN 60898 and NF EN 60947-2) used for circuit-breaker certifications (which characterize fault currents of a duration exceeding 10 ms) give the circuit breakers' tripping threshold for currents maintained for 10 ms or more. For transient currents of duration less than 10 ms, no normalized curve exists. The peak value of the total current at switching on depends on the energizing time, the number of luminaires forming the lighting circuit, and the short-circuit power and architecture of the network.

Recommendations

In order to address this risk an appropriate choice of the circuit breaker (rating, curve) must be done during the design phase of installation, according to the recommendations given by the manufacturer.

Another option, very useful in the case of replacing conventional lighting by LED lighting on existing large installation, is to implement a remote control including zero crossing function in place of standard device. That will limit the total inrush current in the order of 4 to 5.

The risk relating to Earth leakage protection device

The leakage current is at maximum for switching on at the voltage peak. The frequency of this transient current is high (about 100 kHz). For switching on at zero voltage, the leakage current is practically zero.

Recommendations

The permanent earth leakage current at 50 Hz is generally less than 1 mA for a luminaire. Given that lighting circuits are protected by earth leakage protection devices of 300 mA rating in commercial application, a large number of luminaires can be installed downstream of a protective device. For a frequency of 100 kHz, the current is not detected by the earth leakage protection devices.

The risk for remote control device

The standardized categories of use (according to NF EN 60947-4-1 and IEC 61095) stipulate the current values that the contactor must establish or cut off. These depend on the nature of the load controlled and the conditions under which circuit (closing and breaking) is performed. Only lighting loads employing conventional technologies are covered by this standard, and no test is required to certify contactors for controlling luminaires that employ LED technology. For switchgear and control gear, the main constraints of the LED lighting technology are the high transient currents which can generate premature wear of contact pad materials.

Recommendations with standard relays

Contactor and impulse relay deratings given by manufacturers must be taken in account in the design phase in order to obtain the right coordination level with LED lighting. That performance will determine the maintain of the electrical endurance and lifetime given by manufacturers.

Solution with smart relays - smart contactor using zero crossing principle

A technique exists to limit the current peak on energization of circuits with capacitive behavior (magnetic ballasts with parallel compensation, electronic ballasts, driver). It consists of ensuring that powering of lighting occurs at the moment when the line voltage passes through zero (called “zero crossing function”).

The use of remote control device including zero crossing function will reduce dramatically the inrush current generate at switch on (in the order of 4 to 5 times). Up to now, only solid state switches with semi-conductors offer this possibility but with the constraints to heating generated few compatible with conventional electrical distribution system.

The operating principle of the static relay consists of the following: when the control voltage is applied to the relay input, an internal static component performs the switching function at zero crossing of the voltage wave. The accuracy at switching (connection to the network) is excellent. The inrush current is then reduced (see Figure N51). As a result, it is possible to use circuit breakers without derating. The number of luminaires that can be powered by a single circuit is limited only by the thermal withstand of the smart relay.

Fig. N51 – Current at switching "on" according to voltage angle (zero crossing and 90°)

More recently, hybrid technology devices have been developed that combine a solid state switch (activation on voltage passage through zero) and an electromechanical contactor short-circuiting the solid state switch (reduction of losses in the semiconductors) (see Figure N50a).

For three-phase circuits (power supply of luminaires between a phase conductor and the neutral conductor), switchgear and controlgear of the three-pole type is preferable to a control device of the four-pole type. Not switched the neutral pole will help to prevent a harmful voltage surge at power frequency from being applied across the terminals of the luminaire if the neutral conductor fails to close.