Article reference KA-01767
Occasionally, on products with Electro mechanical thermostats a blue flash, also known as “night flash”, can be seen coming from inside products. This is particularly more noticeable when the light levels are lower, or it is dark.
If the product is working correctly then this is nothing to be concerned about and is just a small electric arch jumping between the thermostat contacts as they open or close. When the contacts are fully open the gap between them is large enough to prevent this happening. This is something that can occur on any Electro mechanical contact or switch and can on occasion be seen on light switches.