Why Christmas Lights Can Get Hot: Electrical Safety During Holidays

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Metal screw base of a light bulb. Photo by Dhester.

Electronic Ballast

In CFLs the actual bulb does not connect directly to the electric circuit; the connection takes place through an electronic ballast. This is a small electronic circuit which regulates the amount of electricity which reaches the bulb. To switch on the lamp, for instance, it is necessary to have a relatively high amount of electricity for the light emission process to start; once the process is running, on the other hand, smaller amounts are necessary, to avoid the burn out of the lamp.

The circuit can be more or less complex, depending on the lamp; some key elements, however, are always present. These include capacitors (to store charge), electrical resistors, wires to transport the electricity and other electrical components.

The ballast is located between the bulb and the lamp base.

Metal Screw Base

A metal screw base is the one of the most common lamp bases. There are other types of lamp base, for instance the bayonet-style or those with two bottom pins.

Although these bases have different shapes and characteristics, they all perform the same function, which is to provide the connection between the lamp and the electric circuit.

Possible Causes of Overheating

There are three common causes of overheating in electrical devices:

  1. Overheating due to a poor connection – this generates heat when the current passing through a wire or component is too great for it. This commonly happens when the electrical resistance of a wire or component increases, due to the oxidation of a metal to an oxide; examples of this are rusting or corrosion. These phenomena reduce the capacity of that component to conduct electricity, causing resistive heating.
  2. Overheating due to excessive current – when too much current is flowing through part of a circuit. An example is an insulating cover on a wire wearing out; the wire can touch other elements of the circuit, causing a short circuit.  In this case the current bypasses the whole circuit and takes a “short-cut” through only part of it (e.g., only the lamp base), causing it to overheat.
  3. Overheating due to insulation breakdown – when the insulation on a wire fails completely, or a component breaks/bends/moves so that the current flows through another part of the appliance, not the circuit it should be in; this can cause massive overheating. Moreover, arcing of a current can occur; this is like a small lightning bolt between two conductors that are close but not actually in contact. This also causes a high increase in temperature.

Christmas lights may get hot, don’t use bulbs that get overheated. Photo by Seemann.

Christmas Lights: What Can Happen

In the case of the overheating of the Christmas lights bulb mentioned in the question, it is most likely that case one is happening (corrosion within the base/ballast), or case two (wearing out of the insulation on a component).

It is probably best not to use this lamp, as if there is such a problem it will only get worse with time, leading to ever-more overheating, and possibly causing a fire in the end. As other bulbs do not get hot, there does not seem to be a problem with the connections of the appliance or socket itself.

Resources

Korinek, C.W., Classification of overheating modesSynergy Technology LLC. Accessed November 2013.

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