Protecting AC motors in hazardous environments is critical to ensuring safety and compliance. Different methods are utilized to prevent electrical equipment from becoming sources of ignition. This guide provides an in-depth explanation of the three primary protection types—Flameproof (EExd), Increased Safety (EExe), and Non-Sparking (ExnA)—used in explosive atmospheres, their construction, characteristics, and applications.
The following table summarizes the concepts and standards applied to electric motors and other electrical equipment in environments with gases, vapors, and mists:
| Type of Protection | Code | Standards | Use in ATEX Category/Zone | Principle | Application |
|---|---|---|---|---|---|
| Flameproof | d | 50018 | Category 2, Zone 1 | Enclosure housing prevents ignition of surrounding atmosphere, even during internal explosions | AC motors, control panels, light fittings |
| Increased Safety | e | 50019 | Category 2, Zone 1 | Methods eliminate arcs, sparks, and hot surfaces capable of igniting flammable atmospheres | AC motors, terminal/connection boxes, light fittings, squirrel cage motors |
| Non-Sparking | nA | 50021 | Category 3, Zone 2 | Prevents arcs and sparks during normal operation | AC motors, terminal boxes, light fittings |
Flameproof
Motors – Protection Type EExd
Construction
of Flameproof Motors
Flameproof EExd motors are designed as Category 2G equipment for Zone 1 environments. Their construction ensures that internal components capable of igniting explosive atmospheres are enclosed within a robust stator housing and flanges. These enclosures withstand the pressure of internal explosions and prevent the propagation of flames to the external atmosphere by cooling the explosion through flame paths. The flame path dimensions are specified in the EN 50018 standard. External surfaces of flameproof motors are classified for temperature to ensure safety.
Characteristics
of Flameproof Motors
Key features
include:
- Flame paths for explosion cooling.
- Reinforced frame, terminal box, and end shields.
- Enhanced contact surfaces between components.
- Reduced clearance between the motor shaft and
bearing cap to avoid spark transmission.
- Pressure testing of all components (e.g., frames,
shields, terminal boxes).
- Certification by authorities like DEMKO, PTB, KEMA,
or BASEEFA.
- Use of Ex-approved cable entries.
Applications
of Flameproof Motors
Flameproof
motors are widely used in:
- Pumps, fans, blowers, crushers, conveyor systems,
and cranes.
- Environments requiring explosion-proof motors.
Some motors feature dual protection types, such as "de," where the stator housing uses flameproof protection (d) and the terminal box uses increased safety protection (e). The EExde configuration adds a safety terminal block in the terminal box to prevent sparks or excessive heat.
Increased
Safety Motors – Protection Type EExe
Construction of Increased Safety Motors
EExe motors are designed for Category 2G environments (Zone 1) and focus on minimizing the risk of excessive temperatures, sparks, or arcs during normal operation or predictable faults. Unlike flameproof motors, EExe motors are not built to withstand internal explosions. Their construction emphasizes maintaining lower temperature limits for both internal and external surfaces, which are carefully monitored to prevent ignition.
Characteristics
of Increased Safety Motors
Notable
features include:
- Reduced power output relative to frame size.
- Precision in air gap concentricity and clearance of
rotating parts.
- Impact-tested components.
- Temperature rise limits 10K below insulation class
maximum (e.g., ΔT = 70°C for Class B).
- PTC thermistors rated for 110°C (compared to 155°C
in standard motors).
- Time tE compliance: Ensures stator windings stay
within safe temperature limits during locked rotor or starting current
conditions.
- Terminal board ensuring proper creepage and
clearance.
- IP55 terminal box enclosure and mandatory
grounding.
- Use of drip covers for vertical applications.
- Certification from DEMKO, PTB, KEMA, or BASEEFA.
Time tE in EExe Motors
Time tE
represents the period required for motor windings to reach temperature limits
under locked rotor conditions. It ensures the motor stops before exceeding safe
temperature levels. Manufacturers provide tE values on motor nameplates and in
data booklets.
The interval OA in the illustration above represents the maximum ambient temperature, and the interval OB represents the maximum temperature the stator windings reach under normal operation. If for example the rotor locks and the temperature consequently increases, the protective device turns off the motor. This scenario is illustrated in interval 2 of the chart as shown below:
The interval OA in the illustration above represents the maximum ambient temperature, and the interval OB represents the maximum temperature the stator windings reach under normal operation. If for example the rotor locks and the temperature consequently increases, the protective device turns off the motor. This scenario is illustrated in interval 2 of the chart.
In interval 2, the motor temperature increases quickly, and it has to be lower than its maximum surface temperature classification. In order to avoid an explosion, it is therefore important that the motor is put to a stop before it reaches its maximum surface temperature. If you need to know the value tE of an EEx e motor you can find it in the manufacturer’s data booklet or on the motor nameplate.
Applications
of Increased Safety Motors
EExe motors are suited for Zone 1 and Zone 2 areas with frequent explosive atmospheres caused by gases such as ammonia, butane, methane, ether, and hydrogen.
Non-Sparking
Motors – Protection Type ExnA
Construction
of Non-Sparking Motors
ExnA motors are
designed for Category 3G environments (Zone 2), where explosive atmospheres are
unlikely under normal operation. The construction emphasizes safety by ensuring
no ignition occurs during standard operations.
Characteristics
of Non-Sparking Motors
Distinctive
features include:
- Precision in air gap concentricity and rotating
part clearance.
- Impact-tested components.
- Temperature classification for internal/external
surfaces (T3, T2, T1).
- IP54 or higher protection.
- Manufacturer self-declaration of compliance without
third-party certification.
Unlike EExe motors, ExnA motors do not require derating or time tE monitoring. These motors are ideal for environments with limited exposure to flammable gases like ammonia, butane, and methane.
Applications
of Non-Sparking Motors
ExnA motors are used in Zone 2 environments, where the risk of an explosive atmosphere is minimal. They are commonly deployed in areas with gases like ammonia, ether, and hydrogen, providing reliable safety under normal conditions.
By
understanding the EExd, EExe, and ExnA protection methods, engineers can select
the right AC motors to ensure safety in hazardous environments. Each method has
specific advantages and applications, ensuring compliance with ATEX and other
safety standards. Whether it's the robust design of flameproof motors, the
precision of increased safety motors, or the simplicity of non-sparking motors,
proper selection is critical for safe operations in explosive atmospheres.
