If lights flicker, breakers trip for no obvious reason, or a transformer runs hot, harmonics are a prime suspect. You can't see them, but a power quality survey measures them precisely.
What are harmonics?
A healthy mains supply is a clean 50 Hz sine wave. Harmonics are additional currents and voltages at whole multiples of that frequency — the 3rd harmonic at 150 Hz, the 5th at 250 Hz, the 7th at 350 Hz, and so on. When they add to the fundamental, they distort the smooth sine wave into a jagged shape. The total amount of distortion is expressed as Total Harmonic Distortion, or THD.
What causes them?
Harmonics are created by non-linear loads — equipment that draws current in sharp pulses rather than smoothly. Modern buildings are full of them:
- Variable speed drives (VSDs) on pumps, fans and HVAC
- Uninterruptible power supplies (UPS)
- LED and fluorescent lighting drivers
- Rectifiers, chargers and switch-mode power supplies
- EV charge points
- Induction and arc furnaces, welding sets
Why do they matter?
Harmonics waste capacity and generate heat where you don't want it:
- Overheated neutrals — triplen (3rd, 9th…) harmonics add up in the neutral of a three-phase system, which is often undersized for the load.
- Transformer losses — extra heating shortens transformer life and can force de-rating.
- Nuisance tripping — distorted current confuses protection and trips healthy circuits.
- Wasted capacity — distortion inflates apparent power (kVA), so you pay for headroom you can't use.
- Equipment faults — sensitive electronics misbehave on a dirty supply.
What the standard says: ENA G5/5
In the UK, Engineering Recommendation G5/5 (published by the Energy Networks Association) sets the planning and compatibility limits for harmonic voltage distortion a customer may inject into the distribution network. When you connect large non-linear loads, the DNO expects you to stay within these limits — and a harmonic study demonstrates whether you do.
How do I know if I have a problem?
You can't diagnose harmonics by eye — you measure them. A power quality survey connects a Class A analyser to your supply for a week or more and records the full harmonic spectrum against G5/5 and BS EN 50160. If distortion is high, the same survey pinpoints which loads are responsible so the fix is targeted.
What's the fix?
- Harmonic filters — tuned passive or active filters that absorb specific harmonics
- Line reactors on drives to smooth the current they draw
- Detuned capacitor banks for power factor correction that won't resonate
- Load segregation — keeping clean and dirty circuits apart
- Neutral and cable upgrades where triplen harmonics have overloaded them
Think harmonics are costing you?
A survey measures your distortion against G5/5 and tells you exactly what to do about it.
Book a power quality survey