MIE (without inductance) simulates a purely capacitive electrostatic discharge such as from isolated conductors in an industrial situation. MIE (with inductance) simulates longer duration discharges as the introduction of the inductor into the circuit, delays the spark discharge to earth, hence this corresponds to MECHANICAL SPARK SENSITIVITY.

1. Health, Safety and Environment
Frequently Asked Questions from our Dust Explosion Seminars

2. What is the significance of MIE (with inductance)
and MIE (without inductance). Which one of them
is lower?
• MIE (without inductance) simulates a purely capacitive electrostatic discharge such as
from isolated conductors in an industrial situation. MIE (with inductance) simulates
longer duration discharges as the introduction of the inductor into the circuit, delays the
spark discharge to earth, hence this corresponds to MECHANICAL SPARK SENSITIVITY.
• As the spark duration is shorter for “pure” static electric discharges so the probability of
ignition is less than for the MIE (with inductance) and therefore, in many cases the MIE
(with inductance) produces a lower value of MIE. A review of your facility will determine
whether electrostatic spark discharges or mechanical sparks need to be assessed and the
corresponding MIE test performed.

3. How do you establish the maximum allowable
temperature for equipment using powder
properties?
• The maximum surface temperature of enclosure for equipment located in hazardous
zones is based on Minimum Ignition Temperature (MIT of a dust cloud) and Layer Ignition
Temperature (LIT of powder layer). After establishing these values, then the lowest value
of the two is selected to give the maximum surface temperature after applying a factor of
safety as follows:
• For MIT data, Max. Allowable Surface Temperature = 2/3 * (MIT)
• For LIT data, Max. Allowable Surface Temperature = LIT – 75o
• Methods of protection along with Ingress Protection are equally important along with the
maximum allowable temperature of equipment.

4. What is the significance of particle size for dust
explosions. What is the smallest particle size where
a dust explosion could not occur?
As the particle size distribution decreases so the explosion properties of the powder
changes, sometimes dramatically. Explosion severity will increase, for example aluminium
with a particle size of 150µm will have a Pmax of 8 barg and a Kst value of 150 bar.m.s-1
whereas at < 10µm the Pmax rises to 17 barg and the Kst to > 650 bar.m.s-1. The same applies
to explosion sensitivity where the material becomes more sensitive to ignition especially
with MIE. Using the same example of aluminium then the MIE value drops from 100 mJ to
<10 mJ as particle size distribution decreases. Usually it is accepted that when the particle
size distribution is > 500µm then ignition of a dust cloud cannot occur. However, even where
a powder has a large particle size distribution, attrition can occur which can result in fines
being present. Due to their nature these fines will remain in suspension in air for a
considerable period of time and should be taken into consideration when performing a risk
assessment.

5. What is the difference between St1, 2, 3 and St0 H,
1H, 2H?
Explosion severity is quantified in terms of the Maximum Explosion Pressure and the Max. Rate
of Pressure Rise (which in-turn relates to the Kst using Cubic Law, i.e. (dP/dt)max * V1/3= Dust
Constant or Kst). As can be seen Kst is a measurement that is used as it is a constant and does not
change with vessel volume. The St Class is based on Kst value, i.e. St 1 (Kst<200 bar.m.s-1), St 2
(200bar.m.s-1 < Kst< 300 bar.m.s-1) and St 3 (Kst > 300 bar.m.s-1). The complete explosion severity
test is normally carried out in a 20 Litre Sphere, although the 1m3 vessel can also be used.
Sometimes, for screening purposes (St 0H, 1H, 2H determination), you can use the Modified
Hartmann tube apparatus (1.2 L), which can indicate Explosibility and dust explosion class. The
MIE tube is fitted with a hinged lid and a constant arc ignition source. When an ignition occurs,
the lid opens to various angles depending upon the severity of the explosion. Therefore, the
results are not accurate but do provide an indication of explosion violence.
• St 0H – No explosion – St 0
• St 1H – indicates an explosion which indicates a St 1 powder.
• St 2H – indicates an explosion which indicates a St 2 powder.
• Note: there is no ST 3 indication and design of Explosion Protection Measures cannot be
defined based on Modified Hartmann tube apparatus results. This can only be achieved with
full 20L Sphere testing.

6. What explosion protection measures should be selected,
such as Explosion Pressure Relief Venting, Explosion
Suppression and Explosion Containment?
All Explosion Protection Measures serve the same purpose and can be used depending on
feasibility with the process. Explosion Venting is the most common example that is being
used in industry. The important aspect is that all three measures must be designed using
Explosion Severity data of the material being handled. Another important aspect is that
explosion isolation needs to be provided along with explosion protection measures to
prevent explosion propagation to unprotected systems. Also, it is necessary to assess the
specific requirements for any operation. Explosion venting needs to be designed and
installed correctly and it may not be suitable for materials of a toxic nature, explosion
suppression is expensive to install and maintain but can be used in places where venting is
not an option. Explosion containment needs to be maintained over the longevity of the
process and maintenance engineers must understand the complexities of the system. As
stated previously, all three methods of protection will require explosion severity data (20
litre sphere) to ensure safe design.

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