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Pittsburgh, United States

Lovorn K.L.,Lovorn Engineering Assocs.
Consulting-Specifying Engineer | Year: 2012

The recent 2011 National Electrical Code (NEC) edition has undergone some important changes. Electrical service equipment must now be field-labeled with the maximum available fault current at the incoming terminals of the equipment and the date that the fault current calculation was made. In section 230.44, cable trays containing service conductors shall be identified with permanent affixed labels with the wording 'Service-Entrance Conductors'. Now, ground-fault circuit interruption (GFCI) receptacles are required to be installed in a readily accessible location. When independent electrical equipment support wires are installed within dropped-ceiling areas, they shall be distinguished by color, tagging, or other permanent effective means. Conductors smaller than 1/0 are no longer permitted to be paralleled for increased ampacity. The temperature correction factors that were previously located as small print, below the conductor ampacity Table 310.16, have now been relocated to their own table. Source

Lovorn K.,Lovorn Engineering Assocs.
Consulting-Specifying Engineer | Year: 2010

The selective coordination of breakers in hospitals is required to ensure the effective working of emergency power systems when required. The accidents in the hospital caused due to the failure of electrical service can be prevented if the substation ground fault protection is properly coordinated, or if the ground fault elements are not included in the emergency distribution system. The NEC states that emergency systems overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices. The codes mentions the life safety loads, which includes illumination of means of egress, exit signs, alarm and alerting systems such as fire alarm and medical gases, or communication systems. The critical branch loads include critical task illumination in anesthetizing gas locations, selected receptacles associated with anesthetizing gases, ward lighting and nurse station illumination and selected receptacles. Source

Article 430 of the National Electrical Code (NEC), Motors, Motor Circuits, and Controllers, provides guidelines for electrical design of HVAC systems. The article specifically focuses on electrical design of single-speed, nonreversing, single- and three-phase induction motors in fractional and integral horsepower sizes. Engineers need to determine the ampacity of the fan or pump motor or motors being circuited. NEC article 430.22 states that need to 125% for better electrical design and performance. The requirements of NEC 430.24 state that the largest motor ampacity is multiplied by 1.25 and the full load ampacity of each of the smaller motors are added to this value for multiple pump or fan applications. The nice part of any electrical design is that the engineer has to perform very few calculations, as most of the work has been already done for him. Source

Lovorn K.L.,Lovorn Engineering Assocs.
Consulting-Specifying Engineer | Year: 2011

The cause, different result, and different method of mitigation of transients for mission critical facilities (MCF), are discussed. The most common transient conditions are voltage spikes and dips, which are short-term increases and decreases in the voltage of the distribution system. One common cause of high-voltage transients is the result of the opening of medium-voltage (1000 to 35,000 V) vacuum breakers, vacuum contactors, and current-limiting fuses. Frequency transients in UPS output frequencies below 58 or above 62 Hertz will cause the UPS system to go offline and connect the load through the static bypass switch, directly to the generator, if the utility source is not available. Repetitive short circuits cannot be avoided, so mitigation measures must be employed to prevent damage due to this transient source. The best line of defense for voltage spikes is the surge suppressor, the sacrificial elements where each of the metal oxide varistor (MOV). Source

Lovorn K.L.,Lovorn Engineering Assocs. | Divine T.,Smith Seckman Reid Inc.
Consulting-Specifying Engineer | Year: 2010

Droop control system, isochronous control system, reactive droop compensation system and cross-current compensation system are some of the systems that are used to achieve load-sharing. The simplest system is the droop control system, in which the voltage and/or frequency drops as the load increases. In contrast to droop control, the isochronous system maintains a constant engine speed with either a constant or varying load. A reactive load sharing control system will have a current transformer that allows the control system to proportion the load provided to the system by each generator. With cross-current compensation system, changes in the load do not affect the steady state voltage, which dramatically improves the system voltage regulation. Selection of the right load-sharing control system depends on the sizes of the generators, their governors and voltage regulators, the sensitivity of the loads to voltage and frequency, and desired system reliability. Source

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