As shown in Figure 1, the grounding wire of the flexible cord of the
standard power adapter has a higher temperature near the screw terminal due to
the influence of the heating element.
Viewpoint 1: The temperature rise limit of the grounding wire insulation is
50K according to the power adapter cord.
Viewpoint 2: There is no temperature rise limit requirement for the
grounding wire insulation of the power adapter cord.
Which view is reasonable?
2. Standard terms
Standard GB4706.1-2005 "Safety of Household and Similar Electrical
Appliances Part 1: General Requirements"
11.1 In normal use, the appliance and its surrounding environment should
not reach an excessively high temperature.
...
11.311.3 Except for the temperature rise of the winding, the temperature
rise is determined by the thin-wire thermocouple, and its arrangement should be
such that it has the least impact on the temperature of the tested
component.
Note 1: Thin-wire thermocouple refers to a thermocouple with a wire
diameter of not more than 0.3mm.
The thermocouple used to determine the temperature rise of the test corner
wall, top plate and bottom plate surface should be attached to the back of a
small black disk made of copper or brass. The diameter of the small disk is 15mm
and the thickness is 1mm. The front surface of the small disc should be flush
with the surface of the plywood.
Place the appliance as far as possible so that the thermocouple detects the
highest temperature.
In addition to the temperature rise of winding insulation, the temperature
rise of other electrical insulation is determined on the surface of its
insulator, and its location is the location that can cause the following
faults.
——Short circuit;
——Contact between live parts and easily accessible metal parts;
-Jumper insulation
-Creepage distance or electrical clearance is reduced to below the value
specified in Chapter 29.
Note 2: If the appliance must be disassembled to arrange the thermocouple,
care should be taken to ensure that the appliance is reassembled correctly. If
in doubt, measure the input power again.
Note 3: The bifurcation point of each strand of the multi-core flexible
cord and the entry point of the insulated wire into the lamp holder are examples
of the placement of thermocouples.
The temperature rise of the winding is determined by the resistance method,
unless the winding is uneven or it is difficult to make the necessary
connections. In this case, use the thermocouple method to determine the
temperature rise. At the beginning of the test, the winding should be at room
temperature.
11.8 During the test, the temperature rise shall be continuously monitored,
and the value of the temperature rise shall not exceed the value shown in Table
3.
Three, case analysis
The author believes that viewpoint 1 is unreasonable, and viewpoint 2 is
more reasonable. The reasons are as follows:
The purpose of the standard setting temperature rise limit is to prevent
overheating and damage of the wire insulation, causing danger. Clause 11.3 of
GB4706.1-2005 standard enumerates some hazards, such as short circuit, jumper
insulation and so on.
In the case, the insulation of the grounding wire failed. As a bare copper
wire, the product can still meet the requirements of all chapters of the
GB4706.1-2005 standard, especially Chapter 19 Abnormal Operation, Chapter 27
Grounding Measures and Chapter 29 Electrical Clearance, Creepage distance and
solid insulation have no impact on safety.
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