Читать книгу Arc Flash Hazard Analysis and Mitigation - J. C. Das - Страница 35

The internal arcing fault can be caused by various factors, such as:

1 Insulation defects due to quality deterioration of components

2 Overvoltages of atmospheric origin or generated within the equipment due to switching transients

3 Incorrect operation due to not adhering to the procedures

4 Inadequate training of the persons involved with installation, maintenance, and operation

5 Breakage or tempering of safety interlocks

6 Highly polluted atmospheric conditions, high humidity, temperatures, corrosive gases, and salt-laden winds

7 Overheating of the contact area due to presence of corrosive agents, loose connections.

8 Faulty assemblies

9 Lack of proper protection and inadequate relaying. Though protective relaying by itself will not prevent an arcing fault, it will limit the arc fault energy and consequently equipment damage.

10 Insufficient maintenance and testing procedures, like insulation resistance records, infrared testing, and partial discharge measurements which can indicate a deteriorating insulation situation.

11 Incorrect operation of disconnects, switches, and grounding switches because of lack of interlocks

12 Problems with cable terminations, like inadequate design, faulty installation, and insulation failures

13 Ferroresonance voltages of instrument transformers

14 Inadequate protection for ground faults and improper selection of system grounding

15 Aging under electrical stress

16 Entrance of dust and rodents, corrosion at contact surfaces producing heat, loose contacts creating sparks. Snapping of the connections and wiring due to inadvertent force, human errors, and incorrect operation.

17 Closing on to an existing fault, without prior rectification and analyses of the faulty condition.

18 Sparking produced due to racking of circuit breakers, operation of fuses, and excessive current flows through loose contacts.

19 Human errors, that is, some parts or tools left or dropped inside the equipment during maintenance

This list may not be exhaustive and is indicative only. In the first place, if the electrical installations are inadequately designed and do not meet the requirements of national standards and safety codes, these will be more prone to higher incidents of arcing faults. Design and operational measures can be adopted for enhancing safety (see Chapter 2). The list also implies that improvements in arc flash hazard reduction can be achieved by avoiding the listed items and taking remedial measures. As an example, the incipient breakdowns can be predetermined by proper testing with infra red scans or partial discharge measurements (see Chapter 14).