Quick answer: When a wet cable trench connects to switchgear through open or poorly sealed entries, moisture enters in two ways: liquid water seepage and humid trench air rising into the cable compartment. Fix the trench first: drain it, find the water source, and seal the cable entries.
The trench is often cooler and wetter than the room, so its air has a high dew point. When it reaches cold metal in the cabinet base, it condenses, even if the switchgear room reads dry. Then deal with what remains in the cabinet with an anti-condensation heater or a cabinet dehumidifier. Heaters and dehumidifiers can’t keep up with a flooded trench.
Quick answer
- Two paths: liquid water and humid air through cable entries.
- A room sensor doesn't see trench air; measure the trench and cabinet base.
- Drain and seal first; then control moisture inside the cabinet.
Liquid water or water vapour?
| Finding | Likely path | First response |
|---|---|---|
| Standing water in the trench | Drainage, groundwater, rain | Remove water; find the source |
| Water marks below a cable entry | Liquid migration or ingress | Repair and reseal |
| Damp air but no visible leak | Vapour from the trench | Measure dew point; inspect openings |
| Droplets on several cold surfaces | Condensation | Compare surface temperature with local dew point |
| Repeated rust at the cabinet base | Persistent local moisture | Inspect trench connection, airflow, drainage |
Several paths can exist together. Pumping out the trench removes the water but leaves the vapour path open if entries are unsealed.
Why the trench is its own air zone
- Cooler than the room; damp concrete or standing water.
- Little ventilation; sometimes connected to outdoors or other buildings via long cable routes.
- Pressure changes from fans and weather move air between trench and cabinets.
A sensor at eye level in the room won't show the air entering at the cabinet floor.
What to inspect
Trench: water depth and location, recent rain or washdown, drain and sump condition, cracks and penetrations, temperature and RH at several points.
Cabinet bottom and cable chamber: gland plates and seals, gaps around cables, corrosion, tracking marks, wet insulation, the dew point just above the floor, and the temperature of the base plate and cable lugs.
Measure dew point and surface temperature
| Location | Measure |
|---|---|
| Trench air | Temperature, RH → dew point |
| Cable chamber air | Temperature, RH → dew point |
| Cabinet base plate, busbars, terminals | Surface temperature |
| Room air | Temperature, RH → dew point |
If surfaces in the cable chamber reach the trench air's dew point, condensation is expected. Log over days and nights.
Fixes, in order
- Drain the trench and remove the water source (groundwater, rain entry, blocked drain).
- Seal cable entries with suitable glands, sealing compound or cable transit systems.
- Ventilate or dry the trench where practical.
- Inside the cabinet: an anti-condensation heater raises surface temperature; a cabinet dehumidifier removes moisture. Choose with the heater vs ventilation vs dehumidifier guide.
- Air distribution: check the dry or warm air reaches the cable chamber, not only the upper compartment.
Verify after remediation
- Re-log trench and cable-chamber dew point and surface temperatures for several days, including after rain.
- Inspect for new water marks and corrosion.
- Check insulation resistance trends; see insulation resistance and humidity.
FAQ
Frequently asked questions
Can a cable trench cause condensation in switchgear?
Yes; humid trench air rises through unsealed cable entries and condenses on cold surfaces in the cabinet base.
Will a heater fix cable trench moisture?
It reduces condensation inside the cabinet but doesn't stop water or vapour entering; drain and seal first.
How do I seal cable entries against moisture?
Use glands, sealing compounds or cable transit systems rated for the enclosure and fire requirements.
Sources
- ASHRAE Handbook—Fundamentals, Psychrometrics.
- IEC 60529, Degrees of protection provided by enclosures (IP code).
