Power Systems – click the questions below to go the answers

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Q. Why should we go to all of the expense of monitoring our facility’s power?

Q. When performing a short-circuit study, can you explain the difference between selective coordination vs. series rated?

Q. What are the advantages of three-phase system supply over single- phase supply?

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Q. Why should we go to all of the expense of monitoring our facility’s power?

The Pro Says: The answer is simple: Power disturbances within a plant can interrupt production lines, cause damage to products and equipment, result in lost orders or transactions, corrupt data communication and storage, and cause an overall decrease in productivity in today’s global economy. Estimates put power-quality-related losses at $50 billion to $150 billion annually in the U.S.

Power monitoring can address these issues in a number of ways:

• Evaluation of incoming electric supply and distribution throughout the facility to determine if power quality disturbances or variations are impacting, or have the potential to impact, facility operations and/or manufacturing processes.
• Identification of power quality trends to provide a baseline for establishing predictive maintenance activities and avoiding interruptions of critical business activities.
• Optimization of power mitigation equipment using a reliability or condition-based monitoring approach. Power parameters can be correlated with process performance and output to locate production defects caused by poor power quality.
• Reduction of energy expenses. In some industries, such as textiles or pulp and paper, electricity consumption of electric motors alone accounts for 90 percent of the total energy bill.
• Assessment of energy and electricity issues related to capital investments and new equipment. There are many examples of multi-million-dollar equipment that performed flawlessly at the vendor’s test site, but did not operate as specified at the customer location due to poor power quality.

Lastly, and more importantly, a power system study will uncover any hazardous situations that can harm personnel and property.
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Q. When performing a short-circuit study, can you explain the difference between selective coordination vs. series rated?

The Pro Says: Each circuit protection method must be able to safely interrupt the expected short circuit current through it, but this is accomplished in different ways.

In a selective coordination system, the breaker closest to the fault should trip to interrupt the fault, thereby limiting the outage to the smallest portion of the system as possible. The main goal of selective coordination is to isolate an electrical fault to the closest overcurrent protection device upstream while maintain power to the remainder of the electrcial system. The NEC requires selective coordination in situations where an orderly shutdown is necessary to minimize the hazards to personnel and equipment, such as an elevator system, emergency system or legally required standby system.

By definition, a series rated combination is a combination of two devices that have been tested under specific test conditions that work together to clear a fault. The allowed combinations are limited to those that have been selected by the circuit breaker manufacturer for testing. It can consist of circuit breakers or fuses and circuit breakers that can be applied at available short-circuit levels above the interrupting rating of the load side (protected) circuit breaker, but not above the interrupting rating of the main or line-side device. This method has a lower initial cost as it avoids buying more expensive breakers with high interrupting ratings. A disadvantage is that, by definition, for fault conditions the line-side (protecting) device must open at the same time, and in conjunction with, the load-side (protected) circuit breaker. This means that the entire panel loses power because the device feeding the panel must open under medium to high level short-circuit conditions.

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Q. What are the advantages of three-phase system supply over single-phase supply?