What is Total Harmonic Distortion (THD) in LED Lights?

Q: Can I measure THD on my existing LED street lights?

Yes, with a power quality analyser (₹5,000–₹30,000 for professional models). The analyser clamps around the supply cable and displays THD reading directly. Consumer-grade options: Fluke 435B, Hioki PW3360. For a quick indication without equipment: if your meter room shows high neutral current relative to phase currents, high THD loads are likely present.

Q: Does high THD void any warranty?

No - THD is the driver's characteristic, not caused by external conditions. However, if the neutral conductor in the installation overheats due to high THD loads and damages fixtures, this is an installation design issue. Specifying ≤ 10% THD in the purchase order protects you contractually if poor-THD drivers cause electrical infrastructure problems.

Q: Is there a relationship between power factor and THD?

They are related but different. Power factor (PF) = real power ÷ apparent power. Displacement PF measures only the phase difference between voltage and current fundamentals. Total PF includes distortion - high THD reduces the true power factor even if the displacement PF appears good. A driver with 0.99 displacement PF but 40% THD has a true power factor of approximately 0.93. Always specify both PF ≥ 0.95 AND THD ≤ 10% in procurement documents.

What is Total Harmonic Distortion (THD) in LED Lights?

Total Harmonic Distortion (THD) is a measure of how 'cleanly' an LED driver draws current from the mains. High THD causes real damage to electrical infrastructure - particularly in Indian grids where many loads share a common neutral. It is a specification that every purchaser of LED street lights should understand.

By Xera Tech Team · Updated 01 Jun 2026 · 4 sections

What THD Means Simply

Ideal AC current is a perfect sine wave. An LED driver with high THD draws current in spikes and pulses (harmonics) rather than a smooth sine wave. These current spikes add on top of the fundamental frequency, creating distortion. THD is expressed as a percentage: 5% THD = very clean, 10% THD = acceptable (IS 16102 limit), 20%+ THD = poor, 40%+ THD = very bad (common in cheap LED drivers). A 40W LED driver with 40% THD draws harmonic currents equivalent to an additional 16W of reactive/distorted current - heating cables, transformers, and panels without doing useful work.

Why High THD is Especially Problematic in India

Indian electrical systems are predominantly 3-phase with shared neutral. When many LED drivers with high THD are connected: the 3rd harmonic currents (three times the mains frequency) add arithmetically in the neutral conductor - instead of cancelling. A neutral conductor designed for 100A balanced load can carry 150–200A when servicing many high-THD loads. This causes: neutral conductor overheating (fire risk in poorly maintained junction boxes), transformer neutral overheating (shortened transformer life), interference with sensitive equipment on the same feeder (generators, UPS systems, CNC machines in MIDC areas), and tripping of protective devices.

What to Specify

IS 16102 specifies maximum THD for LED drivers: ≤ 10% for luminaires above 25W. All BIS-certified LED drivers must meet this. However, meeting ≤ 10% THD is the minimum - quality drivers are at 5–7% THD. For government tenders: specify 'THD ≤ 10% at full load' and require test reports from NABL lab. For large industrial installations (factories, warehouses with hundreds of LED highbays): consider THD ≤ 5% specification to protect plant electrical infrastructure. For housing societies and commercial buildings: ≤ 10% THD per IS standard is adequate.

Frequently Asked Questions