Читать книгу Arc Flash Hazard Analysis and Mitigation - J. C. Das - Страница 36

We can summarize the calculation procedures as follows:

1 Calculate bolted three-phase symmetrical short-circuit currents throughout the system where arc flash calculations are to be carried out. The data collection, single line diagrams, and switching conditions to be studied are all akin to the short-circuit calculations; see Chapters 5 and 6.

It will be necessary to consider the various operating conditions, for example:

 Normal operation

 A tie circuit breaker is closed

 Alternate operating situations that is transformers run in parallel, a generator is brought in or taken out of service

 Dual feeds.

It will be erroneous to conclude that if the arc flash calculations are based upon the maximum short-circuit currents in any operating conditions, the results will give maximum hazard levels, and other scenarios need not be considered. This is so because at reduced short-circuit currents, the protective relaying times may increase, giving rise to even greater arc flash hazard.

1 The arcing currents can be calculated based upon the system voltage and the bolted three-phase currents and gap length. Calculate second arcing current at 85% Ia.

2 The working distance and gap lengths in all the examples and discussions in this book are according to IEEE standard. There impact on arc flash calculation results is, however, documented in Chapter 3.

3 Input the correct equipment type, switchgear, MCC or panels, or arc in the open air.

4 Conduct a rigorous relay coordination study throughout the distribution system. This is an important step, where alternate protective relay types, relay characteristics, and settings, protection strategies, current limiting devices, equipment selection, and alternate settings during maintenance modes become important. The protection and relaying is important, and for arc flash hazard reduction, it becomes an iterative calculation. With all other parameters welldefined, relaying can be manipulated to reduce HRC.

5 Calculate the incident energy, arc flash boundary, and the PPE level.