The Arc Flash Analysis process Step by Step #2
What is Scope of Work?
In the initial phase before Arc Flash Analysis it is important to identify scope of work. Below I highlighted few most common things that will have the biggest impact:
- Customer specific targets (system retrofit, specific safety issues, modernization etc)
- Which part of installation? (complete system or part only)
- System voltage LV, MV or HV
- AC only or AC&DC
- Type of Facility (test lab, marine, data centre, industrial etc)
- Type of DC installation (BESS, UPS, UPSaaR, DRUPS, DC motors, DC generators etc)
- Operation scenarios, possible system configurations
- Related customers procedures (EHS, arc flash corporate rules)
- Any other relevant information’s
Where to find the information?
Our next step is to have a kick-off discussion between Power System Engineer and a customer. This is so far the easiest way to explain important details and identify scope of work for Arc Flash Analysis. To assess scope of work we need information which can be found in:
- Documentation (previous studies, load schedules, instructions)
- Single Line Diagrams with MV and LV network data
- Any excel load lists, motors lists
- EHS instructions or procedures
- Site specific requirements and documents
What are limitations for Arc Flash Analysis?
It is very good if installation falls into IEEE 1584 Model Range, which is:
- 208 V to 600 V: 500 A to 106 000 A (AC)
- 601 V to 15 000 V: 200 A to 65 000 A (AC)
Above 15kV we will follow EPRI or ArcPro (OSHA 1910.269) based guidance up to 800kV system range however supply voltage above 15kV for industrial application happens only in specific situations. Practical rules for Arc Flash evaluation limits looking from protective device perspective is to start from (or stop at):
- LV circuit breaker ≥63A 3P (MCB, MCCB)
- LV fuse ≥ 100A
- LV Motors≥ 50HP (37kW)
- MV system up to 15kV (*>15kV EPRI/ArcPro OSHA 1910.269)
- DC systems up to 1000VDC
What to do when there is no documentation or it is very limited?
We have two options:
- Wait for documentation update (longest)
- Arrange site visit for system estimation (fastest)
System estimation visit goal is to do a fast review of installation and collect information’s about:
- Size of the electrical system
- Condition of the installation
- Type of the equipment
- Amount of the equipment
Lack of documentation is an obstacle but for experienced engineer after such visit it should be possible to provide reasonable estimation on cost and general follow up plan. Missing documentation is less comfortable situation for installation owner thus it is good to have flexible approach and provide support.
What is system mapping?
If no or little documentation exists it is necessary to made it. Since Arc Flash Analysis requires some specific data to be collected it is good idea to arrange data collection with system mapping. So what is system mapping? It is simply creating single line diagram of the system (MV to LV feeders). This process and can take days or weeks depending on system size. It has an important additional value as it can be combined with data collection for arc flash analysis. Thus it will save time and money providing valuable system overview and all details necessary for further analysis.
(Figure 1. Example of Single Line Diagram)
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