electrical academy · lesson 1 · about 10 minutes
Keeping a switchboard dry
Damp air is a quiet enemy of electrical equipment. This lesson shows why panels collect moisture and how a few small heaters stop it.
Learning material, not a design or safety instruction. Electrical work must be done by qualified people following the maker's instructions and local rules.
Why moisture forms
- Air holds less water vapour as it cools. When a panel cools below the temperature at which the air in it is saturated, water appears on the cold metal. That is condensation.
- Big swings between day and night make it worse. A panel that sits idle, with no current flowing, cools right down and is most at risk.
- For outdoor panels, expect condensation. It always needs a plan.
What the standards expect
The general standard for low-voltage assemblies (IEC 61439) assumes indoor air that stays at or below about half relative humidity when the temperature is as high as 40 °C. Cooler air is allowed to be more humid.
As a working rule, if the daily average humidity is around 80 per cent or higher, plan for anti-condensation heating.
Two ways to keep a panel warm
Let the equipment do it: steady power losses of around 100 W from continuous current can hold a cubicle just above the surrounding air.
Add heaters: where losses are small, or current often stops for long periods, fit anti-condensation heaters. Size them for each board, using the manufacturer tables, the room temperature, the humidity and how much the enclosure cools by itself.
Where to put heaters
- Place heaters below the parts you want to protect, because warm air rises past them.
- Use several small heaters instead of one large one. The heat spreads more evenly and no single spot gets too hot.
- Keep heaters on a thermostat, and fit them where they cannot touch cables.
- In a panel with sliding or withdrawable units, put them low in both the equipment and cable compartments.
check yourself
Three quick questions
Think of your answer first, then open the question.
A panel is installed outdoors. What should you expect?
Condensation. Plan heating or other protection from the start.
Why is one big heater worse than several small ones?
Heat spreads unevenly with one heater and a hot spot can damage nearby parts.
Where should a heater sit relative to the parts it protects?
Below them.
Learning material, not a design or safety instruction. Electrical work must be done by qualified people following the maker's instructions and local rules.