Cyndi Nyberg
Former EASA Technical Support Specialist
Motors that will be used in hazardous locations must meet a variety of special requirements. In these cases, if the motor is not properly specified and/or repaired there can be serious safety consequences.
Hazardous locations are defined as environments where explosive or ignitable vapors or dusts are present, or may become present. Explosion-proof motors are required to ensure that the external temperatures of the motor will not ignite the vapor or dust and also to ensure that any internal faults will be contained within the motor enclosure under all operating conditions.
Requirements for electrical installations in hazardous locations are covered in NFPA 70, the National Electrical Code (NEC).
If there is no specific classification rule for a particular location in the code, it is up to the facility owner, their insurance company, or the electrical inspector or fire department in the locality to determine whether an explosion-proof motor should be used in that application. The NEC does not specify that an Underwriters
Laboratory (UL) listed motor be in place, but only that the motor is “approved” to be in that location. However, in practice, this means that the inspector will be looking for a listed motor, if the environment warrants it.
Explosion-proof motor classifications
Before looking at some specific requirements for the repair and rebuilding of an explosion-proof motor, a quick summary of the classifications is necessary. A motor is broken down by UL into a Class, Group and Division.
A Class I motor will be in an environment where explosive gases or vapors are present. The Class I motor is designed to withstand and confine the effects of an internal motor explosion, and must also meet surface temperature requirements under all normal operating and fault conditions. A Class II motor specifically covers dust in amounts sufficient to create explosive atmospheres, and dusts that are electrically conductive. A Class II motor is designed to exclude dust and maintain the motor’s surface temperature such that the dust will not be heated above the minimum ignition temperature. The motor can be rated for both Class I and Class II environments (dual rated). However, not all Class I units are suitable for a Class II operation and not all Class II units are suit- able for a Class I operation.
In a Class I environment, special seals are used to confine an explosion inside the motor so the flame will not escape. In a Class II environment, the seals are there for a different reason. The dusts in the environment will not cause an internal explosion, but they will harden the bearing grease if mixed. The seals, therefore, keep any dust out of the bearings to prevent them from overheating, which can increase the surface temperature of the motor.
Motors assigned code letters
Within Class I and Class II, group designations are assigned to different combustible substances by code letters. The letters A through D are assigned to Class I, and letters E, F and G are assigned to Class II. The Class I group designation letters categorize gases and vapors by degree of combustibility. Within Class I, Group A has the highest combustible substance (acetylene) followed by B, C and D. The designs for Group A motors have the longest flame paths and tightest fits.
All explosion-proof motors manufactured after February 1975 must carry an operating temperature code designation identifying the maximum motor surface temperature that will be developed under operating conditions, including overload up to motor burnout.
Class II, Group G (grain dust) has the lowest external surface temperature of 165º C (329º F); E (metal dust) and F (carbon dust) have external surface temperatures of 200º C (392º F). The lowest ignition temperature of any material within the group determines the surface temperature limit for each group. Very few motor manufacturers produce a hazardous location motor suitable for a Class I,Group A and B atmosphere.
Table 1 summarizes the materials within each Group for Class I and Class II locations.
Hazardous locations are further broken down into Division 1 and Division 2. The distinctions are defined in detail in Article 500 of the NEC. A Division I location is one in which ignitable substances are likely to be present continuously or intermittently in the course of normal operation. In a Division 2 location, ignitable materials are handled or stored so that the flammable material will only be present under abnormal conditions. Class I and Class II motors for hazardous locations have no division designation on the UL label. All Class I and Class II motors are designed to meet Division 1 requirements and are, therefore, suitable for use in both Division 1 and Division 2 locations. Standard TEFC and sometimes even ODP motors are used in Division 2 locations.
Maintaining explosion-proof listing
The UL listing mark on a new motor means that it complies with and has been built to UL’s requirements. For a rebuilt or repaired motor, it is imperative that the motor be restored to its original specifications to maintain the explosion-proof listing. It is the responsibility of the owner of the motor to ensure that the motor is repaired so that it maintains its explosion-proof listing.
requirements. There are a number of requirements placed on the motors in order to retain UL certification. First, the motor to be repaired must have been listed by UL when it was originally manufactured. The motor can be rebuilt for any Class and Group in which it was originally designed, but it cannot be rebuilt and listed in a Class or Groups for which it was NOT originally designed.
Service center responsibility
All electrical work must be done by a UL approved service center for the motor to maintain its listing. Mechanical or electrical work can be outsourced, but it is the responsibility of the original service center to ensure that any work doesn’t alter the motor so that it would not retain its ability to contain an explosion.
Winding over-temperature protection (thermostats) is required for every rebuilt motor. No modifications are permitted that could change the motor’s surface temperature or its ability to contain an internal explosion. However, UL does allow a service center to change the voltage of a motor as long as the original winding data is available. Potting of the motor leads is required for explosion-proof motors, but will also be beneficial in other repairs. In an environment that is very dirty or wet, potted leads will help to seal out some of the contaminants.
Variable frequency drives
For explosion-proof motors used with variable frequency drives, it is especially important to have from the manufacturer the speed range where the motor will still meet the temperature requirements of the listing. A motor that was not originally listed for inverter duty cannot be used on variable-frequency power unless the motor manufacturer has tested the motor and has shown that the motor surface temperature will not exceed the required maximum throughout the frequency range for the application. For non-sinewave power, it is assumed that the motor will run 20°C (36°F) hotter than it will on sinewave power.
Maintenance of hazardous location motors is essentially the same as for all totally enclosed fan-cooled motors. It should not be assumed that since an explosion-proof motor has such stringent requirements, it requires less maintenance. In fact, a regular maintenance schedule should include lubrication at proper intervals, inspection for damage or evidence of contamination and other debris accumulation on the frame and fan cover, checking thermostats, and checking vibration levels. These procedures will ensure that safety will not be compromised in a hazardous environment.
ANSI/EASA AR100
More information on this topic can be found in ANSI/EASA AR100
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