Electrical faults on coastal plant tend to fall into two periods of the year: the plum rain and typhoon season, when the humidity of the air is high; and the period after the winter warming, when the difference between day and night makes condensation form repeatedly inside the cabinet. The symptoms are also typical: indicator lamps flicker, relays operate spuriously now and then, and the motor trips although the panel shows everything normal. Most of these phenomena point to the same cause: the insulation condition inside the cabinet has changed.
How salt fog acts inside the cabinet
Salt fog itself consists of solid particles deposited on the surface of the equipment, and it does not conduct by itself. The real problem starts once it takes up moisture. The relative humidity in coastal air is high in any case, and once the salt particles absorb water they form a film containing salt on the contacts and the terminals. The resistance of that film is much lower than in the dry condition. The current creeps along the film, which shows up as a fall in insulation resistance, as tracking marks, and as a rise in the contact resistance of the contacts followed by local heating.
The heating in turn accelerates the oxidation. Once the contacts are oxidised the contact resistance is higher, there is more heat, and the contact surface changes from bright to whitish and greenish, with a further loss of contact reliability. This is a self-reinforcing process, so once a cabinet starts to show problems it often deteriorates much faster than the corrosion alone would suggest. If an inspection only measures continuity and does not look at the colour of the terminals, the severity of the problem is easily underestimated.
What the cabinet protection and the door seal have to hold back
The protection rating of the control cabinet deals with external impact; it does not deal with condensation inside. On a coastal site what really works is usually three specific measures rather than the rating of the enclosure itself. When these three are in place the insulation condition inside the cabinet can be held through the wet season; the other way round, with a higher enclosure rating but a failed seal, condensation inside will still occur.
- Door seal: check for ageing, distortion and local flattening; an aged seal opens channels that are not easy to see.
- Cable entries: the point where a cable enters is the easiest path for moisture to move in and out, and a spare entry should be blanked off rather than left open.
- Structural sealing: how the internal partitions and the cable pass-throughs seat decides whether the condensation falls onto live parts.
The practice of cabinet heating and dehumidification
Fitting a heater inside the cabinet is the measure most used on coastal sites, and the principle is not complicated: keep the temperature inside the cabinet slightly above the outdoor dew point and the moisture cannot condense into droplets on the cabinet walls. The key is the control. The heater is usually switched by a humidity and temperature switch or a simple temperature control switch, and the set point has to be slightly above the temperature corresponding to the local dew point, not the higher the better.
A heater that stays energised all the time consumes power, but for a control cabinet that is the necessary price of holding the insulation condition. What really needs attention is the case of the heater itself failing: in many cabinets the heater has been working for a long time and no longer does anything, while the cabinet has no humidity or temperature indication, so nothing looks wrong from the outside until a humidity peak brings the whole thing out at once. Putting the cabinet heater on the regular inspection list is far more reliable than dealing with it after a fault.
In addition, heating and dehumidification deal with the moisture and have no effect on salt deposits that have already formed. The salt residue on the terminals has to be wiped down and cleaned; that is a maintenance action and cannot be replaced by the heater. Only the two together make a complete procedure: dehumidify first, clean afterwards. Reversing the order achieves little, because if the damp has not been driven off, wiping the salt off the surface simply returns it to the terminals.
| Symptom on site | Common cause | Direction of treatment |
|---|---|---|
| Droplets on the cabinet walls, liquid collecting at the bottom | Condensation from the temperature difference, a failed seal or a failed heater | Check the door seal and the heater, restore the dehumidification |
| Terminals whitened, greenish, showing rust | Corrosion after salt deposits have taken up moisture | Clean and treat the contacts, check the cabinet protection |
| Indicator lamps flashing, spurious operation now and then | A fall in insulation resistance or poor contact at the contacts | Measure the insulation section by section, locate the damp area |
| The motor trips although the panel shows nothing unusual | The contact resistance of the main circuit terminals has risen and is heating | Tighten and check the main circuit terminals with the power off |
Why the shutdown period is the one that needs dehumidification most
While the equipment is out of service the cabinet is not energised, the temperature follows the environment, and condensation forms most easily. On many sites the oil treatment equipment stands unused in a storeroom for several months, and by the time it is brought back into use there may be a layer of water inside the cabinet. Keeping the heater energised during the shutdown, or energising it periodically to dry the cabinet out, is the least troublesome approach. If the conditions do not allow that, at least carry out a full drying run before the machine is brought back into use, and inspect every terminal.
There is no permanent solution for electrical maintenance in a salt fog environment; the approach is to fix the inspection actions: look inside the cabinet for droplets and discoloration on every patrol, measure the insulation resistance once a quarter, clean the main circuit terminals whenever the power is off, and keep the working condition of the heater in the records. There are few actions, but whether they are carried out consistently depends on whether they are written into the patrol sheet rather than on who happens to remember.
The electrical reliability of a coastal site depends on the condition of the cabinet during the months it stands idle.