MOTOR TERMINAL BOXES
The ultimate motor terminal box failure is one that explodes.
There are numerous examples of terminal box explosions due to faults in the
motor or terminal box.
The terminal box is often the weakest link of the motor structure. A rapid, almost
instantaneous, rise in pressure generated by the intense energy (heat) of an arc
can cause an explosion in the terminal box if an adequate pressure relief is not
provided. The possibility for explosion is greatest on enclosed motors where there
is a chance for a buildup in pressure.
During the repair of a motor, care must be taken not to seal a terminal box that
was intentionally vented by design in order to minimize the buildup of pressure.
An explosion proof terminal box must not be replaced with one that is not
explosion proof if the motor operates in an hazardous environment. The most
likely cause of an explosion, regardless of the motor enclosure, is a line-to-line or
a line-to-ground fault which builds up excessive heat and pressure that cannot be
The following is a broad and generalized list of various problems associated with
motor terminal boxes (main and auxiliary).
• Failure to replace a gasket that will not seal properly (except explosion-proof
terminal box which must not have gaskets).
• Failure to properly ground the motor terminal box to the motor frame.
• Improperly securing a line connector in the motor terminal box.
• The terminal box is too small for the number of leads. Sometimes an oversized
terminal box is necessary.
• Incorrect lug size on the motor line leads.
• Motor and line lugs are secured with improper torque.
• Improper removal of insulation from the motor and/or line leads.
• Improper crimping of lugs to the motor and/or line leads.
• Inadequate insulation at the motor-to-line connection.
• Damage of the motor or line leads by sharp edges on the motor frame or
• Accumulation of moisture in the terminal box.
• Inadequate drains to purge moisture or relieve excessive pressure.
• Improper mounting or spacing of accessories in the terminal box.
• Omission of lead positioning gasket.
• Failure to properly brace large terminal boxes to the motor frame.
Problems which evolve into a fault have the potential to cause severe damage or
injury and possibly lead to an explosion. If the original terminal box design is
altered without understanding the intent or purpose of a particular feature,
serious problems can result.
For motors rated above 600 volts, it is not permissible to locate accessory leads in
the same terminal box as line leads. Low-voltage accessories are to be located in
separate outlet boxes to prevent the possibility of inducing high voltage into low-
voltage devices thus creating safety risks.
If devices are commingled in the same terminal box, it is possible that high
potential can damage low-voltage devices.
MOTOR TERMINAL BOX EXPLOSIONS
Because of the safety issues associated with this subject, excerpts from a well-
known and accepted IEEE paper by E.I. DuPont engineers; K. S. Crawford, D. G.
Clark and R.
L. Doughty, has been included. The complete text can be obtained from IEEE by
referencing Motor Terminal Box Explosions Due to Faults PCIC-91-07.
The motor terminal box is the connection point that ties the motor to the power
system. Terminal boxes can explode due to the pressure generated by a high-
energy electric arc. Explosion of electrical equipment by ignition of flammable
mixtures is well documented but will not be covered.
It might seem surprising that an electric arc can cause a motor terminal box to
explode. Yet, on a typical 480 volt industrial distribution system, a high-energy
arcing fault can concentrate up to 15 megawatts of power inside a terminal box.
Since the terminal box is usually the weakest structure in the motor assembly, the
rapid pressure rise generated by the intense heat of the arc may result in the box
exploding. When a box does explode, the force is often strong enough to send
pieces flying more than 30 feet (10 m).
A fault inside a motor with an open drip proof, weather protected type 1, or
weather protected type 2 enclosure is unlikely to build up enough pressure to
cause an explosion. These motors have a natural relief vent because they allow
the free interchange of air between the windings and the outside air. A totally-
enclosed motor, on the other hand, is specifically designed not to allow any
exchange of air between the windings and the outside. Therefore, a fault inside a
totally-enclosed motor will allow the pressure to build up. If the opening between
the motor and the terminal box is not sealed, the pressure will rise in the terminal
box also. Since the terminal box is typically not as sturdy as the motor housing, it
may rupture and relieve the pressure.
A fault inside a terminal box can result in an explosion, no matter what type of
motor it is connected to. If the opening between the terminal box and the motor
is sealed or partially restricted, the pressure will rise in the box. If the opening is
unobstructed, then the fault products will pass into the volume of the motor and
will cool. A terminal box explosion may occur depending upon the fault energy
and location, the terminal box design, the area of the opening between the motor
housing and the terminal box and the type and size of the motor.
EXAMPLE OF A TERMINAL BOX EXPLOSION
Near New Orleans, Louisiana, in May 1990, on a solidly-grounded 480 volt system,
a 200 hp TEFC
pump motor in a Class 1, Division 2, Group D area had a winding failure that
caused a Class L current limiting fuse in the 480 volt switchgear to blow. The fuse
was replaced without locating the fault and an attempt was made to restart the
motor. When the start button was pressed the motor terminal box exploded. The
18” x 18” sheet metal cover for the box, which was held in place by 12 screws,
was propelled about 30 feet (10 m).
The heat generated by the winding fault resulted in a rapid pressure rise in both
the motor and the terminal box. The terminal box exploded because it was not as
sturdy as the motor.
Motor terminal boxes don’t explode every time a totally-enclosed motor or
terminal box has a fault because not all faults are high-energy arcing faults.
The fault that causes the most current to flow, the bolted fault, involves no arcing
and dissipates fault energy throughout the distribution system resistive elements.
However, an arcing fault releases large amounts of energy at the point of the
Since arc energy is proportional to the duration of the arc, protection systems
which offer high-speed fault clearing are most effective in reducing arcing fault
energy in terminal boxes. The arcing fault energy developed during ground faults
can be significantly reduced by using resistance grounded systems. As soon as the
fault escalates to more than one phase, however, the resistor is no longer
effective in reducing fault energy.
INTERNAL PRESSURE RISE DUE TO FAULTS
An arc which is confined to a closed terminal box or motor housing generates a
pressure rise due to the heating of the air surrounding the arc, and the heating
and vaporizing of conductors and other metal components.
The following general observations may be made in regard to pressure rise due to
• Pressure rise increases as the motor terminal box volume decreases.
• Pressure rise increases as arc duration increases.
The use of current limiting fuses to interrupt current in 1/4 cycle is beneficial in
reducing the released fault energy in the terminal box and the resulting pressure
• For extended fault duration, it is difficult to construct a terminal box with
sufficient mechanical strength to contain the pressure generated by a fault.
TERMINAL BOX BURSTING PRESSURE
A structural analysis was completed on typical motor terminal box designs to
determine the bursting pressure.
Motor terminal boxes are generally of two designs.
• Rectangular enclosures with bolt-on covers that are fabricated from aluminum
or steel plate.
• Cast iron enclosures, typically with a diagonally split cover. This type enclosure
is commonly supplied by manufacturers of TEFC motors in NEMA-frame sizes.
On NEMA frame TEFC motors, 100 HP and larger, it is common practice in some
companies to replace the terminal boxes supplied by the manufacturer. The
replacement is a field fabricated rectangular type, and is significantly larger to
facilitate termination of cables.
The additional space allows increased cable bending radius and phase-to-ground
Possible modes of failure for the rectangular enclosure are:
• Shear failure of the female screw threads in the enclosure wall which engage
the enclosure cover screws.
• Shear failure of the enclosure cover screw male threads.
• Tensile failure of the enclosure cover screws.
• Tensile rupture of the enclosure sides.
The most commonly observed failure mode for rectangular terminal boxes is
shear failure of the female screw threads in the enclosure wall. Analysis of
terminal boxes bursting strength for the above modes of failure also verified that
shear failure of the female screw threads is the weakest link.
Since many explosions have occurred during motor starting, this is not a safe
location for the pushbutton.
The alternative is to locate the button near the end of the motor at right angles to
the plane of the terminal box front cover, and away from the motor ventilation
openings (if applicable). Another possibility is to start the motor from a remote
location with an operator observing from a safe vantage point. shown that taped
connections will not support an arc. Therefore, a minor fault is unlikely to develop
into a major one if all connections are insulated.