What is this?
A mains hum is a low-frequency drone or buzz caused by the alternating current (AC) in your electrical system vibrating at 50Hz (or its harmonics at 100Hz and 150Hz). Every home has some level of mains hum, but it is usually inaudible. When it becomes noticeable, it can be caused by equipment within your home, your incoming supply, or external sources like street transformers. Some people are more sensitive to these frequencies than others.
Common causes
- Transformers in the home (doorbell, low-voltage lighting, phone chargers) vibrating
- External supply transformer or substation transmitting vibration through the ground
- Fluorescent light ballasts resonating at mains frequency
- Consumer unit busbars vibrating under heavy load
- Large appliances (fridge, freezer, boiler pump) transmitting vibration through the building structure
- Poor power quality on the local supply network causing excessive harmonics
Is it dangerous?
A general mains hum is usually not dangerous — it is a normal physical phenomenon of AC electricity. However, if the hum is new, suddenly louder, or localised to a specific fitting or point in the house, it could indicate a developing fault. A hum from the consumer unit or a socket specifically is more concerning than a general background hum.
Can I fix it myself?
Start by identifying the source. Turn off circuits one at a time at the consumer unit and listen after each one. If the hum disappears when a specific circuit is off, you have narrowed it down. Check small transformers (doorbell, LED drivers, chargers) as these are the most common domestic sources. Placing rubber pads under vibrating appliances can reduce structure-borne hum.
When to call an electrician
Call an electrician if the hum is localised to the consumer unit or a specific socket, if it is accompanied by other symptoms (heat, burning smell, flickering), if it has recently appeared or worsened, or if you cannot identify the source yourself.
What will an electrician do?
Perform a systematic circuit-by-circuit isolation to locate the source
Inspect the consumer unit and incoming supply for abnormalities
Check the mains supply quality with a power quality meter if needed
Replace noisy transformers, ballasts, or other identified components
Advise on vibration isolation for appliances transmitting through the structure
Typical cost
£80 – £200
Diagnosis and a simple fix (replacing a transformer or tightening connections) is at the lower end. If the source is external (supply network), the electrician can liaise with the DNO at no extra cost.
