MOTORS FOR POTENTIALLY EXPLOSIVE ATMOSPHERE DESIGN SPECIFICATION GUIDE
What are the principles for designing explosion-safe electric motors?
The "non-sparking" version, Exn, according to IEC 79-15 (1987) is intended for use in Zone 2. IEC 79-15 has not yet been transferred to a national standard, but this is expected to happen after it has been converted into a European standard in CENELEC. British Standard BS 5000, Part 16 has a similar version.
The degree of protection of the terminal box must not be lower than IP 54 and any enclosure that contains only insulated parts must have at least IP 44. However, in the case of motors installed in clean areas and supervised by trained personnel, IP 23 is permitted for motors complying with explosion group I, i.e. for use in coal mines, and IP 20 for explosion group II, i.e. other areas. The requirements to be met by the terminal box are unchanged, however. The limitation of the field of application must be stated on the motor.
All connections between live parts must be secured so that they cannot work loose. Cable bushings and the cable branch in the terminal box are also subject to certain temperature limits. The motors must have both an
internal and an external earthing screw.
Pressurized enclosure, EEx p
In this version the motor must be under a given minimum positive pressure relative to the surroundings, so that the ambient atmosphere cannot penetrate the motor in service. The pressurization can take the form of positive pressure with compensation for leakage or positive pressure with continuous flushing.
A positive pressure of at least 0.5 mbar relative to the ambient pressure must be maintained in service. If the positive pressure is lost when the motor is in service, the motor must be automatically disconnected. The temperature of the outer surface of the motor must not exceed the stated figures for the temperature class in question. The motors must have both an internal and an external earthing screw.
Non-sparking design, Exn
This version to IEC 79-15 is a simpler version than EEx e. in general terms, a normal squirrel-cage motor may be approved, but, as for EEx e, there are certain minimum requirements for distances between moving and stationary parts, air gaps and creepage distances between winding leads and earth. The motor must be designed so that sparking cannot occur - "nonsparking" design. The outside temperature of the motor is determined in normal duty. The temperature rise on starting is not included if duty is continuous.
The degree of protection must be IP 54 for the terminal box and IP 44 for those parts of the housing that contain only insulated parts. The motors must have an internal earthing screw and, if requested, an external earthing screw as well. There is no mandatory requirement for a certificate from a testing station. There are national rules for the installation of version Exn motors in zone 2. Version Ex N to British Standard BS 5000 Part 16 differs in certain respects from Exn to IEC 79-15.
Following several years of international engagement by CENELEC, Comite European de Normalization Electrotechnique, common standards now apply regarding the design and testing of electrical equipment that is to be used in explosive atmospheres. These standards are based on the previously issued IEC Publication 79.
What are the principles for designing explosion-safe electric motors?
There are two main principles for explosion protection for electric motors. One is to design the motor so that no dangerous heat or spark occurs. This includes the increased safety version, EEx e. The other method is based on isolating any dangerous heat or spark inside the motor so as to prevent the ignition of any explosive mixture of gases outside the motor. This includes the version with flameproof enclosure, EEx d, and the version with pressurized enclosure, EEx p. These are the three internationally standardized versions that are suitable for motors to be installed in Zone 1.
The "non-sparking" version, Exn, according to IEC 79-15 (1987) is intended for use in Zone 2. IEC 79-15 has not yet been transferred to a national standard, but this is expected to happen after it has been converted into a European standard in CENELEC. British Standard BS 5000, Part 16 has a similar version.
Increased safety design, EEx e
The motor must not have any parts that, in normal service, produce arcs or sparks, or reach a dangerous temperature. Special steps must be taken in the design to prevent the risk of ignition by arcs or sparks or by excessively high temperature as a consequence of poor contact, overloading or the like. The temperature limitation applies to internal and external surfaces.
The degree of protection of the terminal box must not be lower than IP 54 and any enclosure that contains only insulated parts must have at least IP 44. However, in the case of motors installed in clean areas and supervised by trained personnel, IP 23 is permitted for motors complying with explosion group I, i.e. for use in coal mines, and IP 20 for explosion group II, i.e. other areas. The requirements to be met by the terminal box are unchanged, however. The limitation of the field of application must be stated on the motor.
The air gap between rotor and stator is subject to certain minimum dimensions. There are also minimum dimensions for creepage distance and the air gap between winding leads and earth.
All connections between live parts must be secured so that they cannot work loose. Cable bushings and the cable branch in the terminal box are also subject to certain temperature limits. The motors must have both an
internal and an external earthing screw.
Flameproof enclosure, EEx d
The housing of the motor must be so designed that ignition and combustion of any explosive mixture inside the housing cannot be propagated to a similar mixture outside the housing and that the housing can withstand without damage the explosion pressure thus caused.
The housing of the motor must be so designed that ignition and combustion of any explosive mixture inside the housing cannot be propagated to a similar mixture outside the housing and that the housing can withstand without damage the explosion pressure thus caused.
The motor need not be hermetically sealed; gas may therefore penetrate the motor. The permitted temperature inside the motor is limited only by the insulation class of the motor. The rated output depends on how hot the outer surface of the motor is permitted to be with regard to the relevant temperature class. Slip-ring motors, commutator motors and brake motors can all be made in flameproof versions. No external parts may cause sparks. The motors must have both an internal and an external earthing screw.
Pressurized enclosure, EEx p
In this version the motor must be under a given minimum positive pressure relative to the surroundings, so that the ambient atmosphere cannot penetrate the motor in service. The pressurization can take the form of positive pressure with compensation for leakage or positive pressure with continuous flushing.
Monitoring of the winding temperature is recommended in cases where the pressurization system is also responsible for cooling the motor. Before the motor is started, it and its associated ducting for supply and exhaust air must be flushed through with fresh air or a protective gas for long enough to ensure that any explosive gas mixture has been reliably removed. The amount of fresh air or protective gas flushed through must in any case be equivalent to at least five times the total free volume.
A positive pressure of at least 0.5 mbar relative to the ambient pressure must be maintained in service. If the positive pressure is lost when the motor is in service, the motor must be automatically disconnected. The temperature of the outer surface of the motor must not exceed the stated figures for the temperature class in question. The motors must have both an internal and an external earthing screw.
Special requirements to be met by motors in increased safety version. EEx e.
Non-sparking design, Exn
This version to IEC 79-15 is a simpler version than EEx e. in general terms, a normal squirrel-cage motor may be approved, but, as for EEx e, there are certain minimum requirements for distances between moving and stationary parts, air gaps and creepage distances between winding leads and earth. The motor must be designed so that sparking cannot occur - "nonsparking" design. The outside temperature of the motor is determined in normal duty. The temperature rise on starting is not included if duty is continuous.
The degree of protection must be IP 54 for the terminal box and IP 44 for those parts of the housing that contain only insulated parts. The motors must have an internal earthing screw and, if requested, an external earthing screw as well. There is no mandatory requirement for a certificate from a testing station. There are national rules for the installation of version Exn motors in zone 2. Version Ex N to British Standard BS 5000 Part 16 differs in certain respects from Exn to IEC 79-15.
Following several years of international engagement by CENELEC, Comite European de Normalization Electrotechnique, common standards now apply regarding the design and testing of electrical equipment that is to be used in explosive atmospheres. These standards are based on the previously issued IEC Publication 79.
Only intrinsically-safe circuits of category EEx ia may be used in Zone 0. Motors are thus excluded. Motors of category EEx d, EEx e and EEx p may be used in Zone 1.
In Zone 2, equipment permitted in zones 0 and 1 may of course be used. Under certain conditions the equipment, motors for instance, need not be of explosion-protected design. An example is shown in Fig. 1.4B. These conditions are as follows; they must all be met:
- The degree of protection of the motors must not be below IP 54.
- In service they must not produce a temperature of more than 200 °C. This requirement applies to both internal and external parts. In duties S1 (continuous) and S6 (continuous with intermittent load), the temperature may briefly exceed 200 °C in conjunction with starting.
- The motors must not produce sparks or arcs in service.
- They must be placed in a hazard area that has been assigned temperature class T1 to T3.
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