MIT and LIT in ATEX practice – two temperatures, two completely different hazards

MIT looks safe. The number is high. 400 °C, 500 °C, sometimes more.

And that is precisely why it can be so dangerous to interpret.

In explosion hazard zones, many design decisions are still based solely on the ignition temperature of dust clouds, even though actual ignitions very often start with a thin layer of sediment on a hot surface. This is where LIT comes in – a parameter that can lower the permissible temperature of equipment by up to several hundred degrees.

If you want to choose ATEX devices wisely, you need to understand the difference between MIT and LIT, rather than treating them as interchangeable.

MIT and LIT are two different physical phenomena, not two ways of recording the same temperature.

MIT describes the ignition of a dispersed cloud of dust in the air, while LIT describes the ignition of a layer of dust lying on a hot surface. This is not a terminological nuance. It is a fundamental difference in the ignition mechanism.

MIT (Minimum Ignition Temperature):

• refers to a cloud of dust suspended in the air,
  
• ignition occurs in milliseconds,
  
• each particle is cooled by air,
  
• The test is carried out in a Godbert-Greenwald furnace.
  

LIT (Layer Ignition Temperature):

• refers to a layer of dust 5 mm thick on a hot surface,
  
• ignition may occur after several tens of minutes or hours,
  
• the layer acts as thermal insulation,
  
• The test is carried out on a hot plate.
  

The result? The same dust can have MIT = 520 °C and LIT = 160 °C. The difference is not due to measurement error, but to physics.

Why does the layer of dust ignite faster than the cloud, despite its lower temperature?

The dust layer heats up differently than particles suspended in the air. And it is this difference that accounts for the dramatically lower LIT values.

In the layer:

• the lower particles have direct contact with the hot surface,
  
• heat is not dissipated by air flow,
  
• the effect of a “thermal blanket” is created,
  
• the temperature at the point of contact with the metal is higher than on the surface of the layer.
  

The finer the dust:

• the greater the reaction surface area,
  
• the faster the oxidation,
  
• the lower the ignition temperature.
  

Changing the granulation can reduce the MIT or LIT by as much as 200–220 °C. Therefore, assuming that “the dust is the same” very often leads to incorrect temperature classes.

If you want to take a broader look at how different ignition mechanisms work in practice, it is worth comparing this topic with an analysis of ignition sources in explosion hazard zones, as these are what initiate both scenarios.

How do MIT and LIT affect the selection of the temperature class of ATEX equipment?

The temperature class of the device must always be based on the more restrictive parameter – MIT or LIT. Not the higher one. The lower one.

Two rules apply:

• from MIT → maximum surface temperature = 2/3 × MIT,
  
• from LIT → max. surface temperature = LIT – 75 °C.
  

Example:

• MIT = 400 °C → 2/3 × 400 = 267 °C,
  
• LIT = 250 °C → 250 – 75 = 175 °C.
  

The decisive factor is 175 °C, not 267 °C. This means T4 or T5, not T3.

In practice:

• high MIT does not protect,
  
• low LIT eliminates entire groups of devices,
  
• Incorrect T class is one of the most common audit errors in ATEX.
  

This topic often comes up when analysing documentation, especially when checking what the DZPW should contain and what is missing from it.

Dust layer thickness – a parameter that is almost always overlooked

The standards specify the LIT for a 5 mm layer, but actual conditions are rarely standard.

• 10–20 mm may ignite at a lower temperature than the measured LIT.
  
• 5 mm requires additional risk assessment,
  
• The constantly present layer poses a real risk of spontaneous combustion.
  

Therefore, ATEX recommends:

• avoiding permanent deposits,
  
• active cleaning programmes,
  
• surface temperature monitoring,
  
• update DZPW after each process change.

This is where there is often a link with other hazards, such as a secondary explosion of dust that has been detached from the surface.

The most common MIT and LIT interpretation errors in industrial plants. What to watch out for?

Errors in the interpretation of MIT and LIT do not result from a lack of data, but from their simplification.

The most common ones:

• MIT always higher than LIT – not true, e.g. flour,
  
• T1 is safer than T6 – conversely,
  
• high MIT is sufficient – LIT often decides,
  
• laboratory results = hall conditions – prolonged heating changes everything.
  

The end result:

• devices operating at excessive temperatures,
  
• no monitoring,
  
• false sense of security.

FAQ

Do MIT and LIT always occur together?

Yes. If dust forms a cloud, it can also form a layer – both parameters should be analysed in parallel.

Can I select a device based solely on MIT?

No. LIT is very often a limiting parameter, even with high MIT.

Why is LIT subtracted by 75 °C?

This is a safety buffer that takes into account prolonged heating, surface irregularities and local overheating.

Is a thicker layer of dust safer?

No. The thicker the layer, the greater the insulating effect and the higher the risk of ignition.

When do you need to return to the DZPW at MIT/LIT?

Always after changing the dust, granulation, ambient temperature or technology.