When you need to service a live line, how can you protect people from potentially touching the live line? One way is to use a Rubber Insulating blanket. Here we are talking about a rubber insulating blanket, not the blanket you have on your bed. This type of blanket, manufactured under ASTM D1048 Standard Specification for Rubber Insulating Blankets, and later tested under ASTM F479 Standard Specification for In-Service Care of Insulating Blankets, is designed to isolate you from any live line or circuit part. These blankets are typically either 24” x 24” or 36” x 36” and about ¼” thick. There are usually holes around the edge of the blanket such that it can be draped over or wrapped around something live and then ty-wrapped in place. These blankets come in two types: Type I is not resistant to ozone, and Type II is resistant to ozone. Additionally, there are 5 classes (designated as Class 0 through Class 4) to reflect different working voltages. The working voltages range from 1kV (Class 0) up to 36 kV (Class 4). These different class blankets need to be tested at a higher voltage than the working voltage to have a safety margin. This “test” voltage is about 3-4 kV higher than the maximum working voltage for the appropriate Class of blanket.
Rubber Insulating Blankets are tested by the manufacturer and annually thereafter. It the blanket has more than typical wear then it should be tested more often than annually to ensure the safety of personnel working in the area where the blanket is used.
Blankets need to be cleaned, checked for any mechanical defects, and then tested for electrical withstanding voltage. We wash all blankets in an industry approved detergent and then air dry them. Once this step is concluded we check for mechanical damage. This might include cracking due to age, corona damage due to the nearness of high voltage fields, cracking due to improper storage, and similar damage. Once a blanket has passed the above testing, it is taken to our automated electrical tester for the electrical withstanding voltage testing; first you need to know to what Class the blankets need to be tested. For example, if you are working on a 32 kV system you need Class 4 blankets. This class is tested to 40 kV. How do you get this voltage all around the blanket? The blanket is placed on a metal plate that has an appropriately sized insulation barrier around it. A second metal plate, again with an appropriate insulating barrier, is placed on top of the blanket. The test voltage is applied to the 2 metal plates (one hot, one ground). The system ramps up the voltage until it is at the required test voltage. This voltage level is held for at least 1 minute but not more than 3 minutes. During this time, the system is constantly checking the leakage current to verify that it never exceeds the spec (if it does the testing is stopped). If the leakage current is below the maximum limit, the blanket passes and is marked to indicate it passes for use at the appropriate class and the date of this test. We have an automated system that does most of this process without human intervention, so it is very consistent in the way this calibration is performed. The blankets are dried, given a quick visual check for any damage that may have occurred during the testing process, and then carefully packed for return to the customer.
Visit our laboratory today, to see our equipped facilities and trained experts to help prevent potentially touching the live line and help provide electrical safety in the place of work through proper rubber Insulating Blankets