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The operating temp of ATV312 is - 10 to + 50 °C without derating.
Below -10°c, a lot of frost is forming inside the product. After that, when you supply the product, the temperature is increasing, and some dewdrop is forming inside the product.
It is really dangerous in this case because it can make some short circuit inside the product.
Recommendation: to install heaters in the panel is the best solution if the product is used below -10°C.