Why Does My Heater Switch Spark?

Just flipped your home’s water heater switch and saw a small spark? Now you may be wondering if this is due to an electrical wiring fault but wait a minute… what exactly causes this spark?

To understand why sparks in switches occur (especially with water heater switches), we need to understand the following:

Dielectric strength: The maximum electric field that a material can withstand without breaking down. The dielectric strength of air is 30kV/cm.

Dielectric breakdown: When an electric field is applied across a material more than its dielectric strength, a dielectric breakdown occurs and you may see a spark.

In the case of a household switch in Singapore, we assume operation of our appliances at 220V. To get the minimum distance for a chance that the spark may occur, we divide the voltage (220V) by the dielectric strength of air (30,000V/cm). This gives us 0.0073cm. With a gap of anything less than or equal to 0.0073cm between the terminals of a switch (this is usually the case), there is a sudden dielectric breakdown of air molecules, conduction occurs in the air and during this period of time and we see a spark. 

Why do water heater switches tend to spark more than other switches?

Water heaters tend to draw a much higher current than many other household electrical appliances. For instance, a television runs at 200 watts, an air conditioner runs at 1500 watts while a water heater runs at 3000 watts. The higher the wattage of the appliance, the higher the current. The higher the current, the larger the spark.

Don’t want to see that spark?

Sparking may be normal, but it is not desirable. Using a larger rocker switch will hide the spark when it occurs. Also, it is best to have the power switch on the appliance off before hitting the rocker switch, especially on those drawing higher currents such as water heaters. This avoids the accumulation of high resistance between the receptacle contacts.

Other articles

What causes a small spark in an outlet? https://ths.gardenweb.com/discussions/2648729/what-causes-a-small-spark-in-outlet

Electrical heater sparking at plug socket, why? https://boards.straightdope.com/sdmb/showthread.php?t=502487