IP65 and IP66 are quoted interchangeably in tender documents, and the difference looks like rounding. It is not. The two grades describe jets that differ by a factor of ten in flow, and which one an installation needs is decided by where the nearest hose is.
Reading the two digits
The code is two numbers with separate jobs.
First digit: solids. 5 is dust protected, meaning dust cannot enter in a quantity that interferes with operation. 6 is dust tight, no ingress at all. In practice both 5 and 6 are acceptable for outdoor industrial work, and most gas detectors claim 6. This digit rarely decides anything.
Second digit: water.
| Code | Test | What it means on site |
|---|---|---|
| IPX4 | Splashing from any direction | Rain, roof drip, occasional spill |
| IPX5 | 6.3 mm nozzle, 12.5 L/min, 3 m, 3 minutes | A hose used to rinse a floor nearby |
| IPX6 | 12.5 mm nozzle, 100 L/min, 3 m, 3 minutes | A pressure washer aimed at the enclosure |
| IPX7 | Immersion, 1 m, 30 minutes | A pit that floods |
| IPX8 | Immersion, depth and duration by agreement | A sump, permanently |
The step from X5 to X6 is a nozzle twice the diameter and eight times the flow. A detector rated X5 survives being splashed. The same unit under a washdown gun is being asked to do something it was not tested for.
Neither grade covers immersion
A frequent and expensive assumption is that IP66 includes IP67. It does not. The jets in the X6 test are aimed at the enclosure from outside, at 3 metres, for three minutes. Water that arrives from below, through a flooded cable trench or a pit that fills during a storm, is a different load case and is not tested.
Where a sump, a wet well or an underground valve chamber can flood, the enclosure is the wrong defence. The answer is to mount the transmitter above the flood line and use a remote sensor head, or to specify an immersion-rated assembly. A gas detector that has been underwater is not repairable in the field.
The rating applies to the assembly, not the box
This is where most installations lose the protection they paid for. The IP test is performed on a complete, correctly assembled enclosure. Once it is on site, four things break the seal and none of them are the enclosure.
Cable glands. An unused entry with the plug left out is a hole. A gland tightened onto a cable whose jacket is the wrong diameter is a hole with a cable through it. Glands are part of the rating and are usually supplied separately, with their own IP number.
The cover. Screws tightened in sequence to the specified torque, with the gasket seated and not pinched. A gasket that has been opened three times for wiring and re-closed by hand has been deformed.
The sensor inlet. A detector that measures by diffusion needs an opening to the atmosphere. That opening is a sintered disc or a membrane, and it is the one place where gas must pass and water should not. It is also the place a washdown jet should never be pointed at. Fit a splash hood or a deflector where the installation faces a hose.
The breather. A completely sealed enclosure cannot measure anything, and it also cannot equalise pressure. Some enclosures use a breathable membrane for this. If that membrane is painted over, blocked with dust, or replaced with a blind plug during maintenance, the reading will drift as barometric pressure moves.
What the rating does not cover
Three failure modes on a plant are outside the IP code entirely.
Corrosion. IP is a mechanical ingress test, not a materials test. A stainless enclosure and a painted mild steel enclosure can both be IP66 and have very different lives above a wet well. Chlorides, H2S and ammonia attack the metal, the gasket and the terminals, and none of that appears in a two-digit code.
Condensation. Water that forms inside a sealed enclosure from daily temperature cycling is inside the rating. A well-sealed box in a humid plant with a large day-night swing will collect water on the inside of the window and eventually on the electronics. Desiccant packs and breather membranes address this; the IP number does not.
Heat. The same sealed enclosure that keeps water out keeps heat in. A transmitter in direct summer sun can run far above ambient, which shortens the life of the electrolytic cells inside it. A sunshade is worth more than one grade of IP.
Specifying it in practice
Write the mounting location into the specification alongside the IP grade, because the grade only has meaning against a location:
IP66, wall mounted under a roof, 2.5 m above floor, splash hood fitted, glands supplied and torqued to datasheet, no direct jet.
That sentence tells an installer what to do. IP66 on its own tells them nothing they can check, and the first thing they will do is remove the gland plug and leave it out.
Ingress ratings and enclosure materials vary by product and by configuration. Confirm the grade and the required accessories against the datasheet for the specific model before design freeze.