When the control cabinet runs warm, the first reaction is usually to fit a fan. But a warm cabinet leads to two very different outcomes, and the way forward is not the same for either. In one case the temperature inside the cabinet is above the dew point of the outside air, and moisture condenses on the inside surfaces. In the other case the cabinet is simply a little warmer than its surroundings and the heat cannot get out. The first affects insulation and wiring directly, the second only speeds up the ageing of components.
Where the heat in the cabinet comes from
The main heat sources inside the cabinet are the electrical components themselves: circuit breakers, contactors, the coils of relays when they pull in, the electromagnetic losses of an alternating contactor when it operates, and the losses of the control supply or of a transformer if one is fitted. These sources are scattered but continuous, and as long as the door is closed the heat can only leave through the steel of the cabinet and the gap around the door.
The power supply of the ZYD-100 is three-phase 380V / 50Hz and the total power is 72 + 5 kW; separating the electrical section from the main circuit is the normal arrangement. It is worth noting that a large current in the main circuit does not mean much heat inside the cabinet. What usually warms the cabinet is the control circuit and the auxiliary circuits, and their components have no direct relation to the capacity of the main load.
Part of the heat also comes from outside: the cabinet warms up as a whole in direct sunlight and the air inside is baked hot, and where the air ducting is not well designed the hot air circulates inside the cabinet, entering and leaving at the top, so the fresh air intake never actually exchanges the air.
Under what conditions condensation appears
The condition for condensation is simple: if the temperature of a surface inside the cabinet is below the dew point of the surrounding air, water forms on it. The environmental temperature range of the equipment is -20–45 °C. Where the temperature difference is large and the humidity high, the outside of the cabinet sits below the air temperature, the surfaces inside the cabinet on the cold side are just as cold, and the moisture condenses on that side.
Condensation therefore tends to appear at a few particular moments: the machine is moved from a cold environment into a warm and humid room, and after start-up the cabinet warms up while the outside surface is still cold; after rain or during a cold night the cabinet surface falls below the dew point; and the door is opened so that damp outside air enters the cabinet and beads immediately on the cold surfaces.
| State inside the cabinet | Likely consequence | Direction of action |
|---|---|---|
| Warm but dry, no traces of water | Component life falls and the accuracy of their operation degrades | Improve the ventilation and confirm that the ducting produces a real air change |
| Water droplets or rust on the inner walls | Insulation falls, terminals oxidise, the risk of a false operation rises | Switch off and dry first, then deal with the source of the condensation |
| Damp air appears as soon as the door is opened | Damp outside air enters and condenses on the cold surfaces | Restore the door seal and shorten the time the door stays open |
| Blackening and a temperature rise near the terminals | Contact resistance has increased and there is already a risk of overheating | Follow the terminal inspection routine: re-torque and measure the temperature |
What to inspect inside the cabinet
- Switch off and confirm that the circuits are dead before opening the door, then look for traces of water or rust on the inner walls, on the inside of the door and on the bottom of the cabinet.
- Look at the terminals: check for discoloration, blackening and deformed plastic parts, which are the traces of long-term exposure to moisture and heat.
- Check the door seal. If the seal has aged or come loose, or the door does not close properly, damp air comes straight in.
- Check whether the air ducts and filters inside the cabinet are blocked by dust. Once the ventilation fails, the temperature inside can only keep rising.
- Measure the temperature at several points inside the cabinet: the top, the terminal area and the bottom. A normal distribution is fairly even, and a point that is clearly warmer points to a heat source at that position.
- Check the door interlock and the protection circuits at the same time. The interlock that cuts the power when the door is open cannot rely on a warning alone; its purpose is to make live work impossible.
How the problem is worked through
The order for dealing with condensation is to switch off and dry first, then deal with the source, and only at the end to add equipment. There are usually three possible sources: a failed door seal, damp outside air getting in, and a cold surface that persists. A failed seal is replaced with a new one; too much time with the door open is a matter of working practice; a cold surface that persists calls for a heater inside the cabinet to lift the surface temperature, or for better insulation of the cabinet. A heating dehumidifier works well in many cases, but it consumes power and needs a reliable supply, so it comes last in the list.
Neither the temperature rise nor the condensation is a single accident; both accumulate on components and terminals. Writing the inspection results into the log is far less trouble than investigating after a trip or a false operation.
A warm cabinet is not the problem; a warm cabinet combined with damp air is what turns into an accident.
