A lot of enclosures intended for electrical equipment incorporate a small heater more applicable to complex or expensive equipment that is vulnerable to damp.

This heater may be either thermostatic, or perhaps a PTC type that maintains a constant temperature. Not normally for boxes that house domestic cut outs and kwh meters.

Here is a random example, self regulating PTC type, many others are available.https://undercontrol.co.uk/heating-elements/enclosure-heaters/mini-semiconductor-cabinet-heater/?attribute_wattage=13w&utm_term=&utm_source=google&utm_medium=cpc&gclid=Cj0KCQiA6rCgBhDVARIsAK1kGPKUjodIjtJ5JQRbtGlutznDhiefnWxek5JbqwTf6ggYlaF6WJBJy1AaAgMDEALw_wcB

Those little heaters look a lot like the power resisters we had at work in some test gear except they had connections at both ends not just one. I remember they seem to have mies of wire inside.

Kelly Marie Angel said:

hose little heaters look a lot like the power resisters we had at work in some test gear

Pretty much all they are.

The older type of enclosure or box heater was indeed a simple resistor. Newer products tend to use a PTC heater these maintain a constant temperature at any voltage within reason, indeed the same unit can often be used on both 110 volt circuits and on 240 volts. The wattage used and the temperature reached will be the same on either voltage.

The self-regulating heaters are quite interesting, the same basic technology is used for heaters, heating tapes and self-resetting fuses.

A large amount of carbon black is mixed with a polymer suitable for the intended temperature range to make it conductive (semi conductive). This is then configured in various ways, extruded over two parallel conductors to make heating tape, sandwiched between two conductors to make a self-resetting fuse or in some form of enclosure with embedded conductors for a heater. The resultant product is then crosslinked, usually with an electron beam, so it will not melt.

When a voltage is applied a current flows through the conductive polymer causing heating. The increase in temperature causes the polymer to expand reducing the contact between the carbon particles and increasing the resistance. This reduces the current flow and hence the heating. The polymer will then cool down and contract decreasing the resistance and increasing the heating again.

The cleverness in this is choosing the exact grade of carbon black and the correct concentration to get the required temperature/resistance characteristic. A self-resetting  fuse will have a steep curve so that the remaining current flow when it has opened is enough to keep the temperature and the resistance high. A heat trace cable will be chosen to give a balance between the required temperature and the ambient heat loss.

Self Regulating Heat Trace Cables | Heat Trace Technology | nVent RAYCHEM