Arc Flash Potential and Accident Investigation

Arc Flash Potential and Accident Investigation

Arc Flash Potential and Accident Investigation

Trevor Brown Inspector of Mines (Electrical)

Worker Limits

•2 nd degree burn -79°C for 0.1 seconds

•3 rd degree burn -93 °C for 0.1 seconds

•Lung damage > 83 kPa

Just A Small Arc Flash

•Feeder protected by a 640A noncurrent limiting over-current protective device

P1 >103 kPa

Sound 141.5dB @ 600mm

T2 >225°C

T1 >225 °C

T3 50°C

•1 cal/cm2 equals the exposure on the tip of a finger by a cigarette lighter in one second

•An exposure energy of only one or two cal/cm2 will cause a 2nd degree burn on human skin

•Up to and exceed 20,000°C 4 times Sun temperature

Thermoacoustic Shock Wave

Shrapnel and Blinding Light Shrapnel and Blinding Light

Toxic Smoke Toxic Smoke

Contact with Energized Components

Florida US in 2000A ground fault on a capacitor bank on the

A Big Arc Flash low voltage side of this substation creates an arcing fault.

Protection hardware either:

fails to open on the HV side, or was unable to detect the arcing ground fault.

Excessive current eventually causes the substation's power transformer to overheat.

The transformer's pressure release valves finally vented which subsequently ignites and explodes in a ball of flame.

During the forceful explosion a phase to phase fault occurred causing an upstream circuit breaker to open.

Recent Arc Flash Accident Recent Arc Flash Accident

At approximately 1:47 hours on Thursday 15 June 2006 a Field Service Technician-Electrical, received severe burns when an 10 volt arc flash explosion occurred inside a main incomer circuit breaker cubical.

11kV switchroomwhere accident occurred

Rings in locked position for safety lock attach me nt

3. Mechanism for placing shutter in locked position note it is pulled out while lower one is in unlocked position

Technician’s clothing Technician’s clothing

Auxiliary switchgear adjacent to accident

Main incomer switchgear

The Switchgear The Switchgear

5. Accident scene as found on 18 June 2006

The technician in this area at time of accident

Auxiliary switchgear similar to main incomer

Testing Testing

Trouser legs showing tears and melted reflective tap

Reflective stripes

Shirt showing tears and burn damage

Reflective stripes

34Polymer layer 16 Transparent microspheres 18reflective layer, elemental metal capable of reflecting light.

Aluminium, silver, chromium, nickel, magnesium etc.. 5-150 nanometres thick 20Binding fabric

Extracts from M3 Patent No 5976669

Findings from SIMTARS on reflective tape

•The reflective tape material, when exposed to a flame for a short duration (5–10 seconds), will self-extinguish upon removal of the flame.

•The cotton material when exposed to a flame for a short duration(5-10 seconds), will continue to burn until completely consumed.

•The reflective tape will limit the heat transfer through to the cotton material underneath.

•The reflective tape material on trousers does not offer same level of shielding to that of the shirt.

•The reflective tape material on the trousers does shrink and shrivel to form a molten mass, which can stick to the skin in the molten state.

Findings

–Electrical supply abnormalities

–V ermin

–May not have closed after withdrawal

•The shutters were incorrectly marked

•Electrician had received training

•Risk assessment had been carried out

•Correct PPE was not worn

•Procedures were not followed

•Lack of contractor management

•Unable to determine what initiated the fault

Recommendations

–Switching, access and working permits are developed prior to working on high voltage equipment.

•Procedures/Standards include arc fault protective clothing and equipment.

Very Similar Arc Flash Very Similar Arc Flash

Two electricians and an instrumentation mechanic were dispatched to investigate a problem.

The three workers determined that the switching problem was isolated to the outside power distribution switchgear.

The electricians opened the outside switchgear door that housed a three-phase, in-feed power supply of 13.2 kilovolts and a 120-volt, four-ampere switchgear motor.

During the troubleshooting process to transfer power from the emergency generator to utility power, a ground fault or phaseto-phase fault occurred causing an arc flash and arc blast to occur.

The electrical supervisor, lead electrician, and instrumentation mechanic were engulfed in flames.

Questions Trevor.Brown@dme.qld.gov.au

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