For the same sample, the company did the heating wire bending test and passed the test, but failed when applying for CCC certification. Why?

Date：2022/6/7　　Clicks：647

1. Case issues

An electric blanket enterprise has done 25,000 bending tests of the heating wire of the electric blanket, and the results are qualified. The test situation is shown in Figure 1. However, when it was sent to the laboratory to apply for the CCC certification of the electric blanket, the outer insulation of the heating wire of the electric blanket was broken after 23,681 times of bending, and it failed to withstand the test of 25,000 times of bending, which was regarded as unqualified.

Why do the same samples have different results for the same heating wire bending test?

Figure 1 Bending test of heating wire

(The picture comes from the real shot)

2. Case analysis

The author carefully observed the photos of the test equipment provided by the client, and found that the bending angle of the heating wire did not reach 180°, but was about 135°, as shown in Figure 2.

Figure 2 The bending angle does not reach 180°

(The picture comes from the real shot)

In other words, the bending effect of the company's own test is relatively "loose" for the heating wire, which is easier to pass. If it is bent according to the standard requirements, it is more "strict" and it is more likely to fail. The test results are acceptable, and one result is unqualified.

To sum up, in order to ensure the "consistency" of different laboratory test results, the test equipment that meets the standard requirements should be used.

3. Standard Terms

GB 4706.8-2008/IEC 60335-2-17:2006 Ed2.1 "Safety of Household and Similar Electrical Appliances Part 2: Special Requirements for Electric Blankets, Electric Heating Pads and Similar Flexible Heating Appliances"

21.111.1 Connect a sample of heating elements or internal wiring to a device as shown in Figure 111, which has a trolley with two wheels, each wheel having a groove of radius 4 mm, the diameter of the bottom of the groove 25mm, the wheels should be positioned so that the sample can pass horizontally between them.

Note 1: For samples without circular cross-section, the form of the groove on the wheel is also changed accordingly to suit.

The sample was unrolled on wheels with a load of 0.25 kg on each end. If necessary, the load at each end is increased in steps of 0.1 kg to ensure that the heating elements or wires coming out of the wheels are parallel to each other. The position of the restraint clamp should be such that the pulling force is always applied through the aforementioned mass and in the opposite direction of the motion of the trolley.

The trolley runs for 25 000 cycles over a length of 1 m at a constant speed of about 0.33 m/s by means of a chain arrangement.

Note 2: One cycle includes two actions, one for each direction.

During the test, the sample must not be damaged.

Note 3: During the test, a monitoring current of not more than 50 mA may flow through the sample.

For PTC heating elements, the input power was measured before and after the test. This measurement shall be made with the heating element suspended vertically in the atmosphere and supplied with the rated voltage of the appliance. Both measurements shall be made at the same ambient temperature and when the input power has stabilized. There should be no increase in the input power during the test.

The sample is then immersed in water containing about 1% NaCl and a DC voltage of about 500V is applied between the conductor and the salt solution.

Insulation resistance shall be measured after 1min immersion and shall be at least 1MΩ.

A: Wheels with a diameter greater than 50mm;
B: pulley;
C: Wheels with grooves;
D: limit fixture;
E: engaging pin;
F: A chain with a pitch of 12.7;
G: 20-tooth sprocket with 88.9 pitch diameter.