Lights dimming briefly when a large appliance starts is usually caused by high inrush current — it's mostly normal, but severe or sustained dimming indicates a wiring or supply problem.
🏠Homeowner view
A brief dim when your fridge compressor starts, or when your washing machine begins a cycle, is completely normal. Large motors and heating elements draw a surge of current the moment they switch on, and this brief load causes a small voltage drop across the property's wiring, momentarily dimming lights. This usually lasts less than a second. If the lights stay dim while an appliance runs, or if dimming is severe (noticeably dark), there may be a problem: the supply cable from the meter may be undersized, the incoming fuse (cut-out) may be corroded, or there may be a loose connection somewhere. LED lights can show this more visibly than older bulbs because they respond faster to voltage changes. If dimming is significant or affects multiple circuits, have an electrician check your incoming supply and main earthing.