Why Your LEDs Are Flickering — And It’s a Bigger Grid Problem Than You Think

Abstract
Flickering in LED lighting has become a growing concern in educational and institutional buildings, particularly where lighting upgrades coincide with distributed energy installations and increased use of electronic or inductive appliances. This paper examines how Class II LED luminaires, which lack earth reference, interact with harmonics and voltage disturbances induced by brushless DC motors and rooftop solar photovoltaic (PV) systems. The analysis identifies systemic weaknesses in Class II drivers under disturbed power conditions and offers engineering mitigation strategies.

1. Introduction
Educational buildings are increasingly retrofitted with LED lighting and solar PV systems to improve energy efficiency. However, power quality issues such as voltage fluctuations, harmonics, and electromagnetic interference (EMI) have led to increased reports of visible flicker in classrooms and corridors. This paper investigates the underlying causes and focuses on installations using Class II LED luminaires with single-phase drivers.

2. Class II LED Fittings and Flicker Susceptibility
Class II luminaires, defined by double insulation and no protective earth (PE), rely on internal filtering and isolation to meet EMC requirements [1]. Unlike Class I devices, Class II fittings cannot use earth as a sink for common-mode currents or a reference for internal noise filtering. As a result, they are more susceptible to EMI, harmonics, and common-mode interference, particularly from non-linear loads [2].

3. Harmonics and Voltage Instability from Brushless DC Equipment
Large switching appliances such as industrial vacuums, polishers, and HVAC units often contain brushless DC (BLDC) motors driven by variable speed drives (VSDs). These introduce high-frequency harmonics and voltage dips during startup, which can cause sensitive LED drivers to malfunction or flicker [3]. Class II drivers, particularly those without active power factor correction (PFC), are less capable of rejecting these disturbances [4].

4. Influence of Rooftop Solar PV on Voltage and Harmonics
Rooftop solar PV systems, especially in lightly loaded networks, can contribute to voltage rise and harmonic distortion [5]. Rapid fluctuations in solar output, such as during cloud cover transitions, can induce voltage flicker, particularly when solar exports exceed building demand [6]. PV inverters that are poorly tuned or lack advanced filtering can further compound harmonic issues, affecting LED lighting.

5. Harmonic Handling in Modern LED Drivers
Modern LED drivers, whether Class I or Class II, typically include harmonic mitigation features such as active power factor correction (PFC) and differential-mode filtering. Active PFC circuits shape the input current to align with the voltage waveform, significantly reducing total harmonic distortion (THD). Class I drivers can go further by leveraging the earth connection for improved common-mode noise suppression and enhanced electromagnetic compatibility (EMC) performance. In contrast, Class II drivers must rely solely on internal capacitive filtering and galvanic isolation, limiting their effectiveness under harmonic stress. Therefore, while both classes can comply with standards like IEC 61000-3-2, Class I drivers generally perform better under disturbed power conditions [4].

6. Typical LED Driver Circuit Designs
Class I and Class II LED driver designs share core components but differ significantly in their treatment of noise and fault protection:

• Class I Driver Circuit: Includes a protective earth (PE) connection. The circuit typically incorporates input EMI filtering (with common and differential mode filters), a bridge rectifier, active PFC stage, and isolated DC-DC conversion. Y-capacitors and grounding of the driver chassis enhance common-mode noise rejection and surge tolerance.

• Class II Driver Circuit: Lacks a PE connection and instead uses reinforced insulation and floating architecture. It also includes input filtering, PFC (if present), and isolated DC-DC stages, but without earth-referenced capacitive filtering. It depends more heavily on internal shielding and design layout to meet EMC requirements.

These design differences explain the generally superior performance of Class I drivers in electrically noisy or harmonically distorted environments.

7. Combined Impact in Educational Buildings
In schools, lighting circuits often share distribution boards with general-purpose outlets powering equipment such as vacuums or heaters. If LED fittings are Class II and solar generation is active, any switching event from appliances can lead to visible flicker due to:

• Voltage sags from inrush current.

• Harmonic injection from VSDs.

• Elevated background voltage from PV.

• Lack of earth reference in the driver.

8. Mitigation Strategies
To reduce flicker in such environments, the following engineering measures are recommended:

• Use Class I luminaires with PE-connected drivers to improve EMI immunity [1].

• Segregate lighting circuits from heavy inductive loads and power conditioning equipment [7].

• Install power quality filters or surge suppressors at the board level.

9. Conclusion
The combined effects of non-linear appliances, rooftop PV, and floating Class II luminaires present a real challenge in educational facilities. While energy efficiency goals drive the adoption of these technologies, attention to power quality and driver design is essential. Class I luminaires with robust filtering and proper circuit segregation remain best practice for sensitive lighting environments.