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Investigating Ways to Prevent Electrical Arc Flash

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Summary

• Introduction

• Background

• Experimental configuration

• Experimental results

• Conclusions

• Future research

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Introduction

• Intense heat• High temperature (>3000o C)• Pressure waves (203 kPa at 0.6 m)• Copper vapour• Hot air-rapid expansion• Intense light (IR & UV radiation)• Sound waves • Molten and vaporized metal• Shrapnel/debris (~1000 km/h )• Flames• Smoke• Toxic gases

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Arc Flash Severity

• Arc current and voltage (arc power)• Arc duration • Conductors gap• Working distance• X/R electrical system ratio• Installation type/layout• Earthing/grounding system• Other factors such as:

– Gas nature/state – Conductors material– Equipment age/condition– Environment

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Comprehensive System to Manage Arc Flash Hazards

Arc flash hazard analysis:

– Hazard identification

– Risk reduction measures

– Safety labeling

– Personnel training

– PPE

Mitigation methods to reduce:

– Arc incident energy

– Arc incident impact

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Background: From Arc to Arc Flash

Electrical Discharges

• Extremely dangerous when they appear unexpectedly – flashovers and arcing faults

• Measureable electrical parameters – V, I and V-I characteristic

Arc Discharge• Continuous plasma discharge• Series or parallel• Non-axisymmetric/free burning or

axisymmetric/stabilized• Time dependent arc impedance –

resistive, inductive and capacitive parts

• Initiationi. Insulation

breakdown/flashover (mainly gas dielectrics)

ii. Conducting object introduction

Vs.• Spark – short living transient

plasma discharge

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Arc Flash Initiation

Focus on arc onset to increase our understanding about how an arc evolves into an arc flash event

Experimental investigation on short time scale arcs (sparks)

Evaluation of the electrical characteristics of the transient plasma discharge

Possible mitigation means of the risk of arc initiation under a given set of conditions

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Experimental Configuration

Charging resistor 25 MΩ

Discharging resistor 25 MΩ

Capacitor1.9 nF or 1.0 nF

Limiting resistor 1 Ω

Current viewingresistor 25 mΩ

22 pF

1 nF

HV probe(1000:1)

Spark gap0.1 – 1 mm

Attenuation 40 dB(50 Ω coaxial cable)

Oscilloscope

Normal laboratory conditions: Temperature (20 oC) & Pressure (1 atm) Atmospheric Air Density (1.204 kg/m3)

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1 Ω//22 pF

25 mΩ//1 nF

Earth plane

50 Ω cable output

HV busbar

Spark gap

Grounded pin

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• Focus on the initiation: Electrical properties of spark/arc may help to mitigate arc flash in a wider context

• Accurate measurements with challenging conditions:– High level of the current and very fast rise time– Small experimental rig dimensions in order to limit total inductance

• Custom designed current shunt method: directly electrically inserted in the circuit – Very fast rise time response and measuring accuracy

• V and I measurements• Vspark, Ispark, Rspark and Espark accurate calculations

• Inductance of the current viewing resistor according to the manufacturer datasheet ~ 0.7 nH

• Theoretical calculated inductance of the circuit branch that includes the spark gap ~ 90 nH

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Experimental Results

Typical current & voltage acquired waveforms 0.5 mm gap length & 1.9 nF capacitor

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An approach which allows separation of the resistive and inductive voltage drop was developed in order to make precise calculations of the energy delivered in the spark.

0.7 nH 25 mΩ

V

dt

dILRI(t)V(t)

0.1 mm & 1.91 nF

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Typical current, voltage and energy calculated waveforms 0.5 mm gap length & 1.9 nF capacitor

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Typical current, voltage and energy calculated waveforms 0.5 mm gap length & 1.0 nF capacitor

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Temporal development of spark energy 1.9 nF capacitor

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Temporal development of spark energy 1.0 nF capacitor

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Typical dynamic spark resistance and equivalent spark resistance 0.5 mm gap length & 1.9 nF capacitor

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Typical dynamic spark resistance and equivalent spark resistance 0.5 mm gap length & 1.0 nF capacitor

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Breakdown voltage and spark current for both capacitors1.9 and 1.0 nF capacitors

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Spark energy for 1.9 and 1.0 nF capacitors

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Conclusions

• Spark (short living transient plasma discharge): an early time process in the arc

• Gap length increase: spark current increases and dissipation energy intensifies

• Electrical energy stored in the system increase: energy dissipation and spark current increase

• Geometrical characteristics of the electrical system & electrical energy stored in the circuit physically close to the arc: This affects arc initiation and should be taken into consideration during the design phase of the electrical installation.

• Future investigations – Arc power determination during the transition from transient to

steady-state phases and a comparison between them– Energy partition of the electrical energy delivered in the

spark/arc

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