Identifying Causes for Certain Types of Electrically Initiated Fires in Residential Circuits

Transcription

1 Identifying Causes for Certain Types of Electrically Initiated Fires in Residential Circuits John J. Shea Eaton Electrical 17 Industry Dr. Pittsburgh, PA

2 Some Sources of Potential Electrical Fire Sources overloaded wires - series arcing faults - parallel arcing faults - last strand - glowing connection - loose plug/outlet connection - loose wire nut - staple in NM-B - insulated overloaded extension cord

3 Outline Parallel and Series Arcing Circuit breaker response Extension cord abuse causing thermal overheating NM-B thermal overheating Glowing connections and PVC decomposition Series Arcing char and glow initiated Accelerated thermal aging of wire insulation Conclusions

4 Parallel and Series Circuits Current Arcing I available Parallel Fault I >> I nominal V ac - Wire Load Current Arcing Series Fault I= I nominal V ac - Wire Load

5 Breaker Response to Low Available Parallel Arcing Fault Currents Neutral I (A) Line I (A) Line Voltage (V) Time (s) Neutral I (A) Line Voltage (V) (A)2 Line I Time (s) Thermal-magnetic Breaker AFCI Breaker 125 A rms available 125 A rms available

6 Extension Cord Under Rug Conductor Self-Heating Ground I (A) Neutral I (A) Line I (A) Voltage (V) Time (ms) 125 A rms available Smoke Rug Burst Cord

7 Extension Cord Under Rug Temperature Results Wire Insulation Temperature ( o C) /3 AWG SPT-2 Extension Cord 8 ft total length (6 ft bundled with 5 wraps under 1% cotton rug 13 A rms (rated) 11% rated 1% rated 9% rated Time Current Flowing (h)

8 Overheating Abuse of Extension Cord Under Rug Conductor Self-Heating

9 Overheating Abuse of NM-B B Conductor Self-Heating Ground I(A) Neutral I (A) Line I (A) Voltage (V) Time (ms) 125 A rms available 18 A rms steady-state

10 NM-B B Insulator Temperature Results Cable Insulation Temperature ( o C) % rating 12% rating NM-B (14/2) Cable Insulation Limit Ambient } Air 2 Wraps 1 Wraps 5 Wraps Added Insulation Current (A) rms

11 Outlet Example of Glowing Contact Causing Overheating Glowing Contact Loose Connection Glow Connection overheats Insulation melts/burns

12 Glowing Can Form at Virtually Any Arcing Connection a). Initial Filament t= min b). Oxide Growth t= 2 min c). t= 8 min d). t= 28 min e). t= 31 min f). t= 64 min Glowing Contact (1.2 A rms Copper Wire)

13 Glow Persists and Lengthens Especially at Currents < 5 A rms Glowing Contact at 1.2 A rms Copper Wires

14 Glowing Filament Temperature TC Temperature ( o C) Current (A) Voltage Glow Voltage (V) Time (s)

15 Wattage of Glowing Contact up to 2 A rms (Cu Cu) Resistance (Ω) GlowVoltage Wattage Resistance Power = 53. e e I. 5 I +. 2 I j 5 4 Glowing Voltage, V glow, (V) rms V Resistance Glow = 876. e e I. 1I +. 5I j Power (W) Current, I, (A) rms

16 Low Current Glowing Contact Resistance 8 Resistance (Ω) Steady State R T Currrent (A) rms

17 Glowing Contact Dynamic Resistance 2.9 A rms 1 Resistance, R, (Ω) I R V Glow On Current (A) or Voltage (V) Time (s)

18 Unusual Remaining Glow Track Glow track CuO Bridge 1 mm Wire Glow can restart without motion

19 Series Arcing Can Lead to Glowing or Char Formation Make/break action under load leads to oxide formation and Char if near insulation PVC Insulation Decomposition 2 wire SPT-2 cord Glowing Contact Arcing ignites gases Arcing Over Char

20 Ignitable Gases from Pyrolized PVC Insulation Gas Chromatograph of SPT-2 PVC Insulation Pyrolyzed in Air SPT3white Oct. 17, 22 J.J. Shea 3 CO 2 Dilution Factor not known Balance He N 2 Column Temp: 15 o C HTS Temp: 6 o C He 3 ml/min N 2 5 ml/min EG&G AGC 4 Voltage (mv) 2 1 Hydrogen H 2 Ethylene C 2 H 4 Ethane C 2 H 6 O 2 Methane CH 4 CO C 2 H 2 Acetylene Time (minutes)

21 High-Speed Video of Series Arc at 3 ArmsA Series Arc Energy 2 J/cycle at 6 A 6 fps Chemical Energy 36 to 44 J Heat of reaction from gas (6 cm diameter) Arc 6. A rms Arc Overheated PVC insulation can produce flammable gasses that can start fires

22 Series Arcing Initiated by Glowing Contact at 5 A rms (Cu Cu) Overheating Gas Ignition Burnout Current (A) Current (A) Current Glowing a). Overheating 3.6 s b). Gas Ignition s c). Burnout s Overheating Phase Glowing (V) Resistive Joint Time (ms) Current Voltage Ignition Phase Time (ms) Current Burnout Phase Voltage Time (ms) Time (ms) Time (ms) Time (ms) Gap Voltage (V) Gap Voltage (V)

23 Series Arcing by Over-Surface Char at 1.67 A rms (Cu Cu) Overheating Gas Ignition Burnout Insulation Smoke Flame Burning a). Overheating Arcing Over Char b). Ignition c). Burnout Current (A) Current (A) Arcing Over Char Voltage Ignition Phase Voltage Voltage Burnout Gap Voltage (V) Gap Voltage (V) Time (ms) Time (ms) Time (ms) Time (ms) Time (ms) Time (ms)

24 Gas Ignition at Outlet 15 A rms

25 Dielectric and DC Resistance on Accelerated Aged Wires 1 2 Impedance (MΩ) o C 13 o C NM-B (14/2) Line-Neutral 1 o C 13 o C Z 15 V rms R dc Time in Oven (h) dc Resistance (TΩ)

26 Capacitance and Tan(δ) ) on Accelerated Age Testing NM-B (14/2) 25 Line - Neutral.16 Capacitance (pf) o C 13 o C 1 o C 13 o C C Tan(δ).11.6 Tan (δ) Time in Oven (h).1

27 Hardness Indicates Insulator Life Shore D Hardness o C SPT-2 Insulation = Insulation cracked Time (h) 11 o C SPT-2/2 Brown SPT-2/2 White SPT-2/3 White SPT22White

28 Conclusions AFCI type breakers may reduce the likely-hood of starting a fire by reducing the effective magnetic trip level and recognizing sputtering arc patterns Bundled extension cords under rugs is not a good idea! Bundled NM-B wire at the load center has the potential for overheating, especially if added thermal insulation covers the wires. Glowing connections alone can cause overheating of electrical wire insulation at currents as low as.9 A rms. Glowing can initiate continuous arcing. Glowing can lead to series or parallel arcing

29 Conclusions (continued) Intermittent series arcing can produce char on electrical insulation that can lead to continuous series arcing and ignition of insulation Series arcing can initiate fires at currents of 1.7 A rms and potentially lower. Both series or parallel arcing can cause PVC insulation to decompose and produce ignitable gases. Residential electrical wiring insulation becomes brittle with accelerated thermal aging. Arcing faults can result from cracking and breakage of the brittle wire insulation even though electrical properties, including withstanding 15V rms are maintained in aged wiring

30 Conclusions (continued) Hardness may be a useful measure of wire end-of-life. Data showed time to cracking for various wire types with brown SPT-2 cords having the shortest life. Unintentional series or parallel arcing or glowing has the potential to produce fires in aged as well as new residential wiring. Continuous improvements in connection methods, insulation materials, standards, and arc fault detection/protection and insulation monitoring systems can be used to reduce residential fires.