The use of heat shrink tubing

For the use of heat shrink tubing, foreign customers accept it as an insulation medium. According to the requirements of RAYCHEM 24kV heat shrink tubing, for 95kV impulse withstand voltage, rectangular copper bars with a clear distance between phases greater than 85mm can be used. 75kV only needs to be greater than 65mm.

Why do some domestic users not use heat shrink tubing as an insulation medium? Worries about aging, high temperatures leading to the rupture of heat shrink tubing, and concerns about local damage caused by bumps during production and assembly can all affect insulation performance. For copper bars with complex shapes, heat shrink tubing often cannot tightly adhere to the surface of the copper bar, resulting in gaps and high electric field strength of partial discharge, leading to breakdown discharge.

Firstly, high-quality heat shrink tubing can have a service life of over 20 years under suitable usage conditions. The reasonable and high conductivity copper bar design ensures that the equipment does not experience high temperature rise during long-term use. Short term overcurrent and instantaneous short circuit will not cause the heat shrink tubing to rupture. Similarly, the uniform electric field design, combined with the composite insulation of the heat shrink tubing, can achieve miniaturization.

For IEC 36kV air insulated switchgear, the air clearance required for the 170kV lightning impulse withstand voltage specified in IEC60071 standard is 320mm, while for GB40.5kV switchgear requiring a lightning impulse withstand voltage of 185kV, a minimum air gap of 360mm is required.

By using heat shrink tubing, the electrical clearance can be greatly reduced, but there is a risk of high-temperature aging/rupture. For example, the RAYCHEM model BBIT sleeve has a clear distance of 140mm between rectangular copper bars and 190mm relative to each other. At the same time, it is very reasonable to provide that the required spacing for circular busbars should be smaller than that for rectangular copper bars.

The IEC standard specifies a minimum air clearance of 160mm for 95kV lightning impulse withstand voltage;

Therefore, for a 600mm wide switchgear, the phase to phase distance is 150mm, which cannot guarantee a net distance of 160mm. However, composite insulation such as heat shrink tubing, epoxy resin vulcanization coating, and insulation boards can be used to reduce the distance. By using heat shrink tubing from companies such as Ruikan as composite insulation, the net distance can be significantly reduced. 1250A uses two 50 × 8 copper bars per phase, wrapped with Raychem 25kV thermoplastic sleeve BPTM. The phase to phase clearance can be greater than 95mm, which meets Raychem's technical specifications for a minimum of 85mm between phases. The minimum clearance between copper bars relative to ground is 115mm, which is also greater than the required 105mm. The requirements for net distance of Ruikan BPTM heat shrink tubing are shown in the table.

Requirements for static distance of Ruikan BPTM heat shrink tubing

The maximum working temperature range of Ruikan heat shrink tubing is up to 125 degrees Celsius. Prolonged use below this temperature has been proven by Ruikan accelerated aging tests, which have shown that the risk of heat shrink tubing rupture and insulation failure is very low.

Through optimized design, 2 50 × 8 copper bars are used for each phase, and the temperature rise under 60Hz 1250A current is only about 60k, which is lower than the maximum heat-resistant temperature limit and will not cause cracking problems. The reason for the occurrence of heat shrink tubing cracking and insulation failure is that some companies use substandard materials and low product quality in their copper bars, resulting in excessively high temperatures in the copper bars and low heat-resistant temperatures in the thermoplastic sleeves.

Can RAYCHEM heat shrink tubing be used for North American IEEE C37.20.2 switchgear? Firstly, RAYCHEM heat shrink tubing is UL certified. Secondly, whether it can be used specifically requires passing the specified combustion test in C37.20.2 before it can be used.

The general busbar of American standard medium voltage switchgear is treated with epoxy fluidization, which must make sense.