JAPANESE REPORTS ON ELECTRICAL FIRE CAUSES

15. Protection Failure of Short-circuit by Arcing along the Insulating Material between Two Conductors of Wiring or Wiring Devices

Y. Hagimoto

NRIPS, Japan

Journal of Tokyo Electric Management Engineering Association, January 1992.

Abstract written by translator

Fires from electrical cords or from wiring devices are increasing in these years. The reason is supposed to be the increase of electrical power dissipation for domestic use.

A circuit breaker is used to protect the circuit from overcurrent or short-circuit. If the conductors do not touch directly and they are shorten by an arcing along the carbonised insulating material, the current flows intermittently.

This is the reason why a circuit breaker does not cut off the current more than 100 Amps and fire hazardous sparking continues.

Peak value of the short-circuit current is limited by the resistance of the circuit. In typical conditions the circuit breaker does not cut off the intermittent current under about 200 Amps in peak value. This condition is easily made when an extending cord of about 10 meters long is used.

Translation begins here

1 Introduction

Japan has approximately 60000 fires a year. The number of fires related to electrical cause or electrical appliances are shown in table 1. Although insulating materials have been increased their quality, the number of electrical fires have not been decreased.

Table 1 Statistics of electrical fires in Japan

Year

1980

1981

1982

1983

1984

1985

Total number of fires

59,885

59,865

63,272

58,833

59,674

55,763

Electric heater for cooking

 

192

193

155

176

190

Electric room heater

363

457

539

511

637

503

Cable for lighting or telephone

800

881

930

944

972

978

Electrical equipments

502

445

471

447

453

489

Electrical appliances

       

819

713

Wiring devices

398

459

468

450

428

507

Light including neon light

358

321

331

310

   

Electric Iron

130

128

94

98

   

Others

1,659

1,564

1,582

1,590

   

note: Blank is due to the change of classification.

2 Electrical fire causes

Electrical fires are caused by heat of poor connection, glowing connection, arc tracking, short circuit, disconnection, overcurrent and so on.

3 Arcing along the carbonized insulating material

Electrical failures above mentioned sometimes result in an arc tracking along the insulating material between two conductors of wiring or wiring devices.

A circuit breaker is used for protecting the circuits from short circuit and overcurrent. If the conductors of wiring circuit contacts directly, the circuit is cut off by a circuit breaker immediately.

But If the conductors of wiring do not touch directly and the circuit is shorten by an arc current along the carbonized insulating material between the conductors, the short circuit current flows intermittently and the circuit can not be cut off quickly. This current usually exceeds a few hundred Amps because no load except an arc resistance exists in the circuit. A circuit breaker does not operate until the total energy (integrated square of current) through it reaches its own safety level. This intermittent current often continues for over several minutes.

This phenomena is very likely to cause a fire because of its large and continuous (but intermittent) current. We call this phenomena 'short circuit arcing' for the sake of convenience.

4 Mechanism of intermittent arcing

Figure 1 shows a stranded cord insulated by chloroprene rubber used for the experiments in this study. The cord before the experiment is shown in the upside and the cord after the experiment is shown in the downside. This figure shows the extreme consumption of the cord by short circuit arcing.

Intermittent arcing process is shown in figure 2. (Translation is in the figure.)

a) Arc tracking by a small current

b) Short circuit by a large arc current

c) Explosion of the arc by an extremely large current

Repetition of these process, a), b) and c), makes the current intermittent. Then the larger the current is, the longer the interval of the repetition is.

5 Influence of kind of metals

Influence of the kind of metals on the current was examined by the experimental arrangement shown in figure 3. A pair of metal conductor (2 mm in diameter, 10 mm in length) was put on a synthetic rubber (10 mm x 20 mm x 3 mm in thickness). Copper, brass and iron was used for the conductor. A negligible small resistance (0.01 ) was used for measuring the current.

At first, an arc tracking is made on the surface of the rubber by hand. The current of this arc tracking is limited by 10 of resistance in series of the tracking. When the switch paralleled with the resistance is turned on, this circuit is shorten by an arc current along the surface of the rubber and the current reaches over a few hundred Amps.

Brass conductor makes the current more intermittent than other metals as shown in figure 4 (a: copper, b: brass, c: iron). This is considered to be due to the melting point of brass which is lower than those of other metals. The longer the interval of the intermittent current is, the longer the time needed to operate the circuit breaker becomes.

6 Protection characteristics by circuit breakers

The short circuit current is varied owing to the resistance of the power source, wiring, arcing itself, and so on. The current was controlled by a variable resistance (0 - 1.15 ). Stranded cords (30 wires of 0.18 mm in diameter) covered with chloroprene rubber as is shown in figure 1 were used for this experiment because of the high stability and high possibility of arcing.

The interval of intermittent current varies according to the resistance of the circuit as shown in figure 5.

The operating time (from the time of beginning the short circuit to the time of cutting off the current) were measured for various circuit resistance and various types of circuit breakers as shown in table 2. This table shows that the short circuit by arcing, the current of which is below 200 - 300 Amps (peak value), are failed to be protected by the circuit breakers (rated current: 10 A, 15 A).

Table 2

Note 1: Circuit breakers of type A have both electromagnetic mechanism and thermal mechanism for protecting operation. Type B have only thermal mechanism.

Note 2: Each data are average of 5 measurements.

Note 3: '*' means that arcing ended before the operation of the breaker

Note 4: 'infinite' means that the breaker did not operate and arcing continued.

Note 5: The values in ( ) is the maximum peak value of each short circuit current.

(Table 2 unavailable)

Operating characteristics (I -t characteristics) of type A (5A) and type B (10A) are shown in figure 6 and 7 respectively. The operating characteristics for the short circuit arcing are shown by the marks, 'o', and the characteristics for the normal short circuit (a direct contact of the conductors) are shown by the mark, 'x' in those figures. These figures show that a short circuit arcing needs longer time to be cut off than a normal short circuit.

7 Influence of the length of wiring

Short circuit arc current was calculated for typical domestic wiring conditions. Internal resistance of power supply and a circuit breaker used in this calculation were 0.12 and 0.01 respectively. Figure 8 and 9 shows the relation between the short circuit arc current (vertical axis) and the length of the cord (horizontal axis) for a stranded cord (50 wires of 0.18 mm in diameter) and a VVF cable (single wire of 1.6 mm in diameter) respectively. Rc is the total resistance of every connections in the circuit.

Figure 8 indicates that if a stranded cord is extended to 10 m long, arc current of a short circuit is decreased to about 200 Amps. It was already mentioned in paragraph 6 that a short circuit by this arc current has a high possibility to be failed in protection.

Figure 9 shows that if the connection resistance, Rc, increased up to 0.2 , arc current of a short circuit is decreased to about 200 Amps. This current also means a high possibility to be failed in protection.

8 Example of short circuit arcing

A pair of metal parts of a plug melted experimentally by a short circuit arcing is shown in figure 10. Similar plugs are sometimes found in actual fire scenes.

A short circuit arcing which occurred in an electrical switch is shown in figure 11. High temperature sparkle and gas blowing from the switch is very likely to ignite combustibles nearby.

Figure 12 shows a stranded cord which caused a disconnection owing to its long use. This cord was found just after ignition and the fire was prevented.

9 Postscript(?) (This is not a conclusion)

Short circuit arcing was made intentionally in this study. Although insulating materials have been increased their quality, fires are still caused by these failures. It is necessary to doubt about their safety and to make a study on the mechanism of the beginning of short circuit arcing in order to prevent these fires.

Next | Previous | Index