Where this starts
Flicker is voltage varying at the lamp, and voltage varies either because the load changed or because something in the path has resistance it should not have. Scope tells you how far back along the path the problem is.
MAP IT FIRST. Walk the house while it happens. ONE fitting flickering with everything else steady is local: the lamp, the holder, the dimmer, or the connection at that fitting. A group of lights on ONE circuit is that circuit — a connection at a switch, a ceiling rose, a junction, or the breaker. EVERY light in the house dipping together is upstream of all of them: the main switch, the meter tails, the supply head, or the network.
THE TRIGGER IS THE SECOND READING. Flicker that coincides with something switching on — an electric shower, a kettle, a washing machine heating, a motor starting — is voltage drop under load. A small dip when a large load starts is normal and universal. A pronounced dim, or lights visibly brightening and dropping repeatedly, is not.
WHY A LOOSE CONNECTION IS THE SERIOUS CASE. Resistance at a joint turns current into heat exactly at that joint, and the heat degrades the connection further, which raises the resistance again. That is a runaway, it happens inside an enclosure where nobody can see it, and it is a principal cause of electrical fires. The symptoms are exactly these — flicker under load, sometimes a smell of hot plastic, sometimes a warm faceplate or a discoloured socket.
LED LAMPS HAVE ADDED A WHOLE FAMILY of flicker that is not a fault in the installation at all: an LED on a dimmer designed for filament lamps, an LED retrofit in a fitting with a transformer meant for halogen, or a very low load on a circuit with a neighbouring switch that has an illuminated indicator. Those flicker at a specific lamp and are a compatibility question rather than a wiring one.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
LED lamp and dimmer or transformer incompatibility
Most commonFlicker at one fitting or one dimmed circuit only, beginning when LEDs replaced filament or halogen lamps, often worse at low brightness.
How to check: Whether it started when the lamps changed. An LED on a dimmer or transformer meant for filament is a compatibility issue, not a fault.
A loose connection on a circuit
Most commonA group of lights on one circuit flickering, sometimes with a warm or discoloured faceplate, a smell of hot plastic, or intermittent complete loss.
How to check: Whether the affected lights share a circuit. A joint with resistance heats at the joint and gets worse, not better.
Voltage drop when a large load starts
CommonThe whole house dipping briefly when an electric shower, a kettle, a pump or a motor starts, recovering immediately and consistently.
How to check: Whether it coincides with a specific appliance. A brief dip is normal; a pronounced or sustained one is not.
A failing lamp or lamp holder
CommonOne fitting only, often with the lamp visibly flickering at its end of life, or a holder with a loose or corroded contact.
How to check: Whether swapping the lamp changes it. A single fitting is the cheapest thing on this page to rule out.
A problem at the supply, the meter tails or the head
CommonWhole-house flicker with no local trigger, sometimes affecting neighbours too, sometimes worse in bad weather.
How to check: Whether neighbours see it as well. A street-wide symptom belongs to the network and is reported rather than repaired.
A neutral fault
Less commonLights in one part of the house BRIGHTENING while others dim, appliances behaving oddly, and the effect changing as loads switch.
How to check: Whether anything gets BRIGHTER rather than dimmer. That is a specific and dangerous pattern and is not a lamp problem.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It locates the problem along the path in one walk, and the three answers lead to a lamp, a connection and the supply respectively.
- 2
Flicker tied to a shower, a kettle or a motor starting is voltage drop under load, and how pronounced it is separates normal from not.
- 3
LED retrofits on dimmers and transformers built for filament lamps account for a large share of modern flicker complaints and are not installation faults.
- 4
A warm or discoloured faceplate, or a smell of hot plastic, is a loose connection heating at the joint — which is the one finding here that should stop everything else.
- 5
A street-wide symptom belongs to the distribution network, is reported rather than repaired, and is free to establish.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Immediately, where anything is warm or discoloured, or there is a smell of hot plastic — a loose connection heats at the joint and is a principal cause of electrical fires.
- Immediately, where some lights BRIGHTEN while others dim, which indicates a neutral fault and can damage appliances as well as being dangerous.
- Where a whole circuit flickers, since the fault is at a connection somewhere along it and finding it means opening accessories.
- Where whole-house flicker has no appliance trigger, because the question is then the main switch, the tails or the supply and none of those is a householder's to open.
- Where an RCD trips as well as the flicker, which indicates a fault to earth rather than a connection problem.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.