Arc Flash Analysis and Protection

Tom Short, tshort@epri.com

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Larger Panel Test Results

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Larger Panel Test Results

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Padmounted Transformers

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PG&E Padmounted Transformer Tests

2.2 cycles

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Arc Energy Increases in Proportion to:

•Fault current

•Duration •Arc length

P = I × V Voltage per unit length stays about the same.

~ 35 V/inch or 420 V/ft

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Filtered View

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Filtered View

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Arc Elongation

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Quiz time:

If the available fault current is twice as high, what happens to incident energy? Assume a 0.2-sec clearing time.

1)Stays the same

2)About doubles

3)Almost quadruples (I2t)

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Quiz time:

Comparing a 25-kV system to a 12.5-kV system. Which is true (assuming the same fault current).

1)25-kV energy is about 2 times the 12.5-kV energy

2)25-kV energy is about equal to the 12.5-kV energy

3)None of the above

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Protection Options

• Better flame-retardant clothing – Shirts, pants – Face protection – Flash suits

• Better work practices

• Faster relaying

• Lower fault currents

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Analysis Approaches

• NESC tables

• NFPA tables

• Study

– IEEE 1584 – ARCPRO

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2007 NESC

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2012 NESC

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Quiz time:

Which scenario has the worst arc flash hazard?

1)480-V transformer secondary compartment 20 kA available fault current

2)480-V self-contained meter 10 kA available fault current

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PMH Switch Testing at PG&E

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Fault Initiation

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Test Results

Heat rate = 19.5 cal/cm2/sec working distance = 48” Iarc = 6.0 kA V = 21 kV

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Comparisons of Tests and Predictions

Test: 48” working distance 20 cycles duration 6.3 kA Results: 6 cal/cm2

ARCPRO 4” arc Results: 0.38 cal/cm2

IEEE 1584 Grounded; switchgear Results: 0.7 cal/cm2

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More information

• Video summary of EPRI research

– http://youtu.be/fZP47mlELSc

• Papers – Short, T.A.; Eblen, M.L., "Medium-Voltage Arc Flash in Open Air

and Padmounted Equipment," Industry Applications, IEEE Transactions on , vol.48, no.1, pp.245,253, Jan.-Feb. 2012.

– Eblen, M.L.; Short, T.A., "Arc-Flash Testing of Typical 480-V Utility Equipment," Industry Applications, IEEE Transactions on , vol.48, no.2, pp.581,592, March-April 2012.

– Reports on www.epri.com