"IP68" appears on most submersible pumps and on half the electrical gear sold today. The code is not decoration: it is a test certificate, and the two digits each certify something different.
Reading the code
The first digit rates protection against solids and touch, from 1 (large objects) up to 6 (completely dust-tight). The second digit rates water protection, from 1 (drips) through 7 (temporary immersion) to 8 (continuous immersion under conditions the manufacturer specifies).
So IP68 means: dust-tight, and tested for continuous immersion at a stated depth and time. That last part matters: IP68 is not a single universal depth. The manufacturer's declaration defines the conditions, typically a depth and duration the unit survived in the test tank.
What the rating does not cover
- Cable entries. The rating covers the enclosure, not the gland where the cable arrives. A bad cable joint is the usual path for water into a "waterproof" motor.
- Temperature and chemistry. Immersion rating says nothing about hot water, sewage or corrosive fluids.
- Damage over time. A seal that passed the test new is not guaranteed forever; depth ratings assume an intact unit.
For a pump that lives underwater, IP68 is the minimum you accept, with the cable joint given the same attention as the motor. The digits certify the product, not the installation.