Arc Flash Step by Step #3 Data collection

Every analysis requires input data and there is no difference with arc flash. While for some studies we can be conservative for arc flash analysis it is better to be reasonably precise. Why? Because highest short circuit current doesn’t always lead to highest incident energies. This is especially seen when people tend to underestimate Low Voltage installation arc flash hazard addressing High Voltage as a real threat where reality is most of the time surprisingly different.

Key factors

It is important to be accurate when collecting data and data needs to be based on up to date documentation to reflect existing installation. Very often there is big discrepancy between what is on the drawings and what is in reality. While some facilities maintain documentation very well others not always. It is very good practice to verify documentation vs as-build reality. This means site visit for data verification and data collection. Key reason for such decision are:

• Lack of expertise on site
• Lack or limited documentation and related difficulties (system mapping)
• No available staff for arc flash data collection support
• Independent engineer opinion vs employee

What make difference?

Every installation is different and data collection also. It depends on :

• type of business sector e.g. chemical, oil&gas, industrial, automotive, BESS, PV&WIND, marine
• System size and voltage range (AC, DC, LV, HV, big or small system)
• available documentation and drawings
• local support (skilled electrician, engineers etc)

Who can do data collection?

Since we are talking about low and medium voltage systems it is required for such person to be certified or authorized electrician or engineer. Data collection can be elevated risk process since it requires access to electrical switch gears, cabinets and machines. Very often some covers need to be removed in order to retrieve right data. It is good to arrange:

• introduction meeting with technical team
• provide general plan and if needed arrange work permits
• follow site specific safety rules

What data needs to be collected?

It depends on specific facility profile and amount of documentation however some general list can be made. If documentation is good enough It is good practice to prepare a list of all (known) necessary information’s needed. What we can expect to identify and collect:

• System operation modes (normal mode, generator mode, emergency etc)
• Utility fault data, three-phase and single line-to-ground short circuit power, X/R ratios at the power delivery point
• Utility feeder(s) protection settings, relay type, CT ratio
• All available documentation for old equipment (generators, relays, breakers)
• Single line diagrams copy (if on paper only), even old are useful
• Switch gear, switchboards data (short circuit withstand rating, nominal current, bus bar gap, dimensions)
• Cable data, type, length, conductor material, amount per phase
• Generator data current decrement curves as per manufacturer tests, nameplate data 
• Transformer data, transformer type, winding connection, voltage ratio, rated power, short-circuit impedance (nameplate data)
• Motors data >37kW (50HP) nameplate and control system type (VFD, DOL, SS etc)
• Medium voltage protective relays type, manufacturer, settings and CT ratios.
• Low voltage circuit breakers data and trip unit type and settings
• LV and MV fuses size and type

 Specific data for AC and DC system analysis:

• type of system grounding (for protection and short circuit study)
• BESS: battery energy storage system type, designed power, batteries configuration, physical gaps between “+ and –“ electrodes, DC protection type and settings
• PV: inverter type, short circuit specification, configuration of modules connected, topology, DC protection type and settings
• WIND: generator data sheet and type, converter data and protection type, system topology and min max wind generators connected
• DC systems – verify electrode gap (min max), short circuit protective device, system voltage (model range check), DC source type for short circuit study
• UPS: configuration and working modes (external by-pass, electronic by-pass), power or battery package, UPS short circuit behavior (current and voltage)

Site visit and data collection activities need to be performed according site safety rules. It also requires site authorized person to accompany engineer during data collection and allow access to all necessary equipment, switch gears and switch rooms. Data should be collected and documented as it serves later as reference point for any assumptions and comments. Good practice is to transfer all the information’s later back to the customer for further use and upgrades. It can look like this example below

Figure 1. Example of data collection summary