How to judge insulator quality only by checking its appearance?

Though the main insulator quality can only by inspected by testing, however, when you don’t have facility, and still want to know how to know quality by appearance, how?

The main use of an insulator is support the electric wire and insulated the wire from tower. Therefore the main performance of an insulator should be mechanical strength and electrical ability. To inspect these, most of the inspection can only be done by testing machine in the lab.
 
However, when you only receive an insulator sample and don’t have the testing machine to inspect them, or when you visit a factory, and see the insulator product line, and it is not time to ask them to do test, how can you judge the insulator quality?
 

 
Porcelain insulator:
Glaze: the glaze should be shiny and dim, and good glaze should be smooth and continuous, without peeling. Though in IEC60383, small uncoated spots is allowed, big spots and more spots at least specify they are not good, especially in every piece.
 
If the glaze has small black points, which is obvious when the insulator glaze is grey, the black spot is easy to see. This comes from the production, one reason is that the glaze doesn’t do better when de-iron. If the iron removal is not good, and the glaze has iron spot, then it may cause bad insulation. Another reason is because it has some dust after drying and not clean before glaze.
 
If the glaze is not good, it means the factory is not in good control in production procedure, but if you only receive a sample, it is hardly to see the quality, since the sample may be good, but can’t representative all products.
 
Composite insulator:
Silicone rubber: the rubber should be shiny, and no defect of silicone rubber on the sheds, if it defects, means the injection is not enough.
 
Touch the silicone rubber: it should have flexibility. If it is too hard or too soft, means the silicone rubber is not good, tear the rubber, if it is easy to destroy, means not enough silicon in them.
 
Though only by check experience, it is hard to know the real quality of the insulator, but it can give us some guide of the product, if the insulator is bad from the appearance, then the quality can’t be good most of the time.
 

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