What is this?
A brief dimming of lights when a large appliance starts up is caused by the sudden current draw pulling voltage down across the circuit. A slight, momentary dip is normal in many homes. However, if the dimming is severe, prolonged, or getting worse, it can indicate an undersized supply, a loose connection, or circuits that need separating.
Common causes
- High-draw appliance (kettle, electric shower, washing machine, tumble dryer) causing a temporary voltage drop on a shared circuit
- Undersized supply cable or weak incoming supply for the property
- Loose main neutral connection amplifying the voltage drop
- Lighting and socket circuits sharing connections that should be separate
- Ageing or corroded wiring increasing resistance and making voltage drops worse
- Consumer unit connections becoming loose over time
Is it dangerous?
A brief, slight dim when a heavy appliance starts is generally harmless and quite common. However, if the dimming is pronounced (lights visibly dim to half brightness or more), lasts more than a second, or is getting progressively worse, it suggests a wiring issue that could lead to overheating. A deteriorating loose connection can become a fire risk.
Can I fix it myself?
Note which appliance causes the dimming and how severe it is. If the lights only dip slightly for a fraction of a second when the kettle boils, this is likely normal. Try plugging the appliance into a different socket. If the dimming happens regardless of which socket you use, the issue is in the main supply or consumer unit connections. You cannot fix that yourself.
When to call an electrician
Call an electrician if the dimming is severe or prolonged, if it is getting worse over time, if it happens with relatively low-power appliances, or if the lights dim throughout the entire house. Also call if you notice warmth or burning smells around the consumer unit when appliances switch on.
What will an electrician do?
Measure voltage at the consumer unit and at the affected circuits under load
Test the main connections and supply tails for loose or high-resistance joints
Check the incoming supply voltage meets the BS 7671 requirements (230V ±10%)
Assess whether circuits need separating or upgrading
Tighten or replace connections that show signs of overheating
Liaise with the DNO if the incoming supply is inadequate
Typical cost
£80 – £250
Tightening connections is a quick fix at the lower end. If the supply needs upgrading or new circuits are required, costs will be significantly higher.
