current. In fact, the manufacturers of these fuses recommend that for ordinary service. fuses of a smaller rating than 125 percent of the motor full-load current be used.
Even time-delay fuses may not be satisfactory unless they are shunted during the starting period because the 125 percent value cannot be exceeded.
The NEC® (table 430-37) indicates the number and location of overload protective devices, such as trip coils, relays, or thermal cutouts. These overload devices are usually part of a magnetic motor controller. Typical devices include thermal bimetallic heaters, resistance or induction heaters, and magnetic relays with adjustable interrupting and/or time-delay settings. Overload devices can have a manual or automatic reset.
Thermally protected motors are equipped with built-in overload protection, mounted directly inside the motor housing or in the junction box on the side. These devices are thermally operated and protected against dangerous overheating caused by overload, failure to start, and high temperatures. The built-in protector usually consists of a bimetallic element connected in series with the motor windings. When heated over a certain temperature, the contacts will open, thereby opening the motor circuit. On some types, the contacts automatically close when cooled, or a reset button must be operated manually to restart the motor.
The NEC® requires that live parts be protected in a manner judged adequate to the hazard involved. The following rules apply:
1. Exposed live parts of motors and controllers operating at 50 volts or more between terminals must be guarded against accidental contact by enclosure or by location as follows:
a. By installation in a room or enclosure acces- sible only to qualified persons
b. By installation on a suitable balcony, gal- lery. or platform so elevated and arranged as to exclude unqualified persons
c. By elevation 8 feet (2.4 meters) or more over the floor
2. If the live parts of motors or controllers, operating at more than 150 volts to ground, are guarded against accidental contact only by location, as specified in paragraph I, and if adjustment or other attendance may be necessary during the operation of the apparatus, suitable insulating mats or platforms must be provided so that the attendant cannot readily touch live parts without standing on the mats or platforms.
An equipment ground refers to connecting the noncurrent-carrying metal parts of the wiring system or equipment to ground. Grounding is done so that the metal parts a person might come into contact with are always at or near ground potential. With this condition, there is less danger that a person touching the equip- ment will receive a shock.
The word fixed, as applied to equipment requiring grounding, now applies to equipment fastened in place or connected by permanent wiring, as shown in figure 7-6. That usage is consistently followed in other Code sections also.
The Code requires that all exposed noncurrent- carrying metal parts, such as equipment enclosures, boxes, and cabinets, must be grounded. Equipment must be grounded where supplied by metallic wiring methods; in hazardous locations; where it comes into contact with metal building parts; in wet, nonisolated locations; within reach of a person who is in contact with a grounded surface; and where operated at over 150 volts.
Section 250-51 sets forth basic rules on the effec- tiveness of grounding. This rule defines the phrase effective grounding path and establishes mandatory requirements on the quality and quantity of conditions in any and every grounding circuit. The three required characteristics of grounding paths are very important for safety:
1. That every grounding path is permanent and continuous. The installer can ensure these conditions byContinue Reading