Christmas Tree Light Bulb Checker

What The Checker Decides

6 symptoms cover almost every failure you will see on a household light set: 1 dead set, 1 dark bulb, a dark section, a dim or discoloured glow, flicker or cut-out, and heat. The checker reads the symptom together with the type of set — sealed LED, plug-in incandescent mini, or large screw-in C7/C9 — because the same symptom means a different fault on each of those 3 types. It also runs a separate load check: watts per string multiplied by the number of strings, divided by 230 V, then compared with the 690 W a 3 A fuse allows or the 1,150 W a 5 A fuse allows.

Why One Bulb Switches Off The Whole String

Incandescent mini sets are wired in series, so 1 bulb's filament sits in the path of every other bulb on that run — break it and the run goes dark. Each bulb carries a shunt that bridges the break, which is why 1 dead bulb often leaves the rest glowing while the shunt holds the circuit together. A shunt is a holding measure, not a repair, so replace the dark bulb in the same session: the 2nd filament to fail may meet a shunt that has already given up. LED strings behave the other way round, because the LEDs sit in parallel, so 1 dark LED leaves the other 99 on a 100-LED set running and a whole dark section points at the cable rather than at a bulb.

The Plug Fuse Does Two Different Jobs

A BS 1363 plug takes 1 of 2 cartridge fuses in ordinary use: 3 A, which caps the circuit at 690 W at 230 V, or 5 A, which caps it at 1,150 W. The 13 A fuse reaches 2,990 W at 230 V, and it belongs in plugs stamped 13 A rather than in a plug marked 3 A. The fuse protects the cable, not the lights: it opens once the current passes its rating, before the flex reaches a temperature that starts a fire. That distinction decides the diagnosis — a set that is hot, buzzing or smells of burning can sit far below 690 W and still be dangerous, because the fault there is a joint rather than the total.

Chained Strings And The 230 V Load

Power in watts is voltage multiplied by current, so 230 V at 1 A is 230 W, at 3 A it is 690 W and at 5 A it is 1,150 W. The label on the set states both the watts for 1 string and the maximum number of sets allowed in 1 chain — the 2 numbers that together decide whether your setup sits inside the fuse you have. Enter them and the table shows the same chain at 1, 2, 3 and 4 strings: at 40 W a set, 4 strings draw 160 W and 0.70 A, which is 23% of a 3 A ceiling, so the fuse is not your problem. A chain that trips its fuse the moment you switch on is reporting a fault somewhere in the run, not asking for a bigger fuse.

Replace With The Same Bulb, Not A Similar One

Buy spares matching the type and the wattage printed on the original box, because a replacement with a different rating changes the share of the 230 V each bulb in a series run receives. Screw-in C7/C9 bulbs match by base and wattage, while mini bulbs match by base shape and the voltage marking on the glass — the 2 figures a shop assistant needs. Mixed bulbs in 1 series run show up as a set that glows unevenly or burns through its spares inside a single season. Keep the box: the watts per string and the maximum chain length are usually printed on it, and those are exactly the 2 numbers the load check above requires.

Method and limits

Method. The fault verdict is a lookup rather than a measurement: your 5 answers select 1 row of the published fault table, and that row supplies the cause, the safety reading, the time and the parts. The load check is arithmetic — strings × watts per string, then ÷ 230 V for amperes, compared against fuse × 230 V for the ceiling. Both appear in the Calculation Breakdown so you can audit them by hand.

• Assumes the wattage entered is for 1 string and is printed on the set's own label — a figure carried over from a different set understates the whole chain.
• Assumes the sets are linked end to end into 1 socket — an extension lead, a second socket or a timer puts more cable between the plug and the lights, and none of that is visible here.
• Does not model a controller box, a transformer-fed indoor set, a solar set or a 12 V low-voltage set — those fail differently, and none of the 6 symptom rules was written for them.
• Does not test your lights — it cannot see a cracked cable, a scorched contact or water sitting in a connector, so any of those outranks the verdict shown.
• Does not override the maximum chain length on the label, which is a manufacturer limit this tool has no way to read.
• Does not decide product compliance: CoryVu is independent, is not affiliated with any manufacturer or retailer named on this page, and cannot confirm whether a set meets any safety standard.

Figures checked 25 October 2026.

When To Stop Using The Set

Heat, a burning smell, a scorch mark, a cracked plug or a tingle when you touch the flex ends the diagnosis: unplug at the socket and replace the set, because neither a 3 A nor a 5 A fuse will stop a joint that has already started to burn. The HSE advises an RCD rated at no more than 30 mA for portable and outdoor supplies, and that device is the backup that limits the energy in a shock rather than a reason to keep a damaged set running. Trimming a cable, taping a flex or fitting a larger fuse to keep a set alive removes the protection the plug was marked with, and none of those 3 repairs is one you can undo at the end of the season. Bring outdoor sets indoors while the connectors are wet: water in a socket turns up as flicker, a dim glow or a tripping circuit, and all 3 of those readings mean inspect before reuse.