Richard Lu

Power strips are essential supplies for every home and office. Many families may even have more than one power strip. Because of quality problems or inappropriate use of power strips , fires occurs quite often.

A paper issued by the China Fire Defense reports that thousands of fire accidents a year are attributed to tacking or overheating of power strip. Under such a circumstance, it is strongly requested to establish an experimental method in order to discover the nature of tacking or overheating and then find fire reason.

WHY I CHOSE THIS TOPIC

What reasons can a strip caused spontaneous combustion?

Insert inferior plug with electrical appliances into the outletCircle round and strap the the power cord of stripPower strip used in dusty and wet circumstance for an

extended period

Stuff inferior plug with electrical appliances into the outlet

iron material plug

Thermocouple connectionwith plug

Plug heating and melting

Temperature records

Time （ min ） 5 10 15 20 24 28 32 37 45 50 55

Inferior strip N

128 157 164 160 171 171 171 182 191 218 235

Inferior strip L118 131 134 138 142 141 145 152 176 203 230

The temperature of the inferior outlet rises quickly at the beginning, which after 5 minute rise to 128 degrees; after 24 minutes rise to 171 degrees; after 55 minutes rise to 235 degrees and then begun to burn.

Circle round and strap the power cord of strip

The power cord is not loose

Form a larger eddy current

Temperature records

Time （ min ） 10 20 30 33 40 50 60 70 75

The exit of strip

sheath line 28 29 28 53 62 69 67 67 70

Among the coil 93 136156 162 170 187

Temperature is measured with the thermocouples among the coil. Short circuit happened when the temperature up to 187 degrees.

Power strip used in greasy dirt and wet circumstance for long-term, for example in a kitchen.

Dust and moisture accumulate on a gap between a cord plug and an outlet.

Contain a trace of salt in the uncleanness, and after absorbing water be affected with damp be affected with damp has certain conductive ability.

The experimental principle

When the plug is between the two insert micro conductive impurity conduction of moisture, although not short circuit, but there are still weak current flow. Carbide point conductive ability is stronger, with years of discharge process, the carbonation increases slowly. When the points reach a certain degree, the accumulation of carbide discharge no longer rely on be affected with damp be affected with damp, discharge.

The experimental method

It is preferable to use duct collected in houses for the experiment from a practical point of view. But the ingredients of dust are not always the same, leading to difficulties in analysis of experimental results. Thus, an accelerated test using electrolyte (0.1 or 0.2 % NH4Cl solution) and napkin was carried out, taking into account domestic and international standards for tests of materials against tracking. The Napkin was inserted in the gap between a cord plug and an outlet and then NH4Cl solution was supplied to the napkin with a wetting agents automatic drop device . The volume of the solution supplied was dropped every 20 seconds. Surface current was monitored continuously with a data logger. The sampling time was 100 us.

1.wetting agents automatic drop device

2. high frequency CT (current transformer)

3. oscilloscope

4. infrared thermometer

Insert after corrosion

NH4Cl solution

Current of 200-900 milliampere was observed just after injection of solution.This current source from the surface discharge of outlet.

Temperature is measured with an infrared thermometer among at the outlet . Tracking between the two inserts connected to the outlet occured when the experiment began after about 40 seconds, lasted until the end. The max temperature can reach nearly 600 degrees in the process of the discharge which can lighting strip shell or near combustible.