History and background
- After the advent of atomic energy after WWII
- Found exposure of special thermoplastic formulation to high intensities of radiation led to cross linking.
- polymeric materials, which are cross linked either by irradiation, or by chemical means, such that when their temperature is raised beyond the normal base polymer crystalline melting point, they do not melt but merely become rubbery.
- if the material is then mechanically expanded and held in this position until the temperature falls below the crystalline melting point. It remains in the expanded state. Subsequent heating in an unconstraint state will then result in the material recovering or shrinking to its original extruded or moulded size and shape.
- Heat shrinkable products have perfect plastic memory and may be supplied in an expanded condition.
- Upon the application of a heat source (heat gun) the tubing will shrink and grasp tightly to the object over which it has been placed, such as a wire, lug, terminal or connector.
- Heat shrink tubing acquires this special plastic memory feature as a result of radiation cross linking.
Some of the materials and their properties
- Polyolefin (electrical and mechanical protection areas )
- FEP (Fluorinated Ethylene Propylene) (UV&normal rays)
- (Polytetrafluoroethylene- Perfluoromethylvinylether) (surface smoothness)
- ETFE (Ethylene Tetrafluoroethylene)(stren
- PFA (Perfluoroalkoxy) (PTFE & FEP)
- MFA gth & rigidity)
- PTFE (Polytetrafluoroethylene) (low friction, high life,mositure retention less)
HEAT SHRINK TUBE
Salient features while making HT tubes:
- Should provide a tight, protective covering to items that will be subjected to the extremes of heat, corrosion, shock, moisture, and other critical environmental conditions.
- Should extend the life of the underlying material.
- Should reduce its installation cost and increases its reliability.
- Should shrink both radially and axially.
USES OF HEAT SHRINK TUBES
- tubing is ideal for moisture-sealing applications, mechanical protection and strain relief.
- as component covering, water proofing, mechanical protection, strengthening,shock protection, abrasion protection, corrosion protection, encapsulation, insulation, dust proofing, sterilization, splicing, cable binding and tying, strain relief, marking and coding.
Heat Shrink Terminations
Salient Features to be kept in mind: while making of terminations
- Should provide electrical stress control
- Should be track and weather resistant
- Should provide good protection and sealing
- Should provide environmental seals at interfaces
- Should be light in weight and hence, can be installed at ground level, raised and connected to equipment or overhead line.
- Should confirm to class 1 as defined in IEEE-48
Electrical Stress Control
Without Heat Shrink terminations:
- operating on AC system the distribution of voltage along the surface of insulation from the conductor to screen terminating point is not uniform.
- results in degradation of insulation at this point,
Occurrence of high electrical stress is avoided by means of a heat shrinkable stress control tube having a closely controlled impedance characteristic per unit length built into the material.
Action of Tracking and weathering
Due to the flow of surface leakage current (under humid conditions)
Exposed insulation breaks down and burn
Leading to the deposit of carbon
Which is conducive to further arcing and leaking and flow of current.
Repeated arcing causes flashing and heating to about 5000 C
Action of the termination in protection from tracking
- termination are sealed against ingress of moisture
- Sealed against contamination by application of sealant to form environmental seals at interfaces.
Armour and screen bonding arrangement is designed
- to provide a permanent and low contactresistance between armour, screen and earth bond
- to limit the raise in temperature at bonding points even during the flow of high earth-fault currents.
Types of Terminations
- MV and LV terminations
On Core types
- Single core and three core (for MV)
Two, three, four core (for LV)
- Indoor (not subjected to rain and other environmental harshness)
Heat Shrink Joints
Salient features of Joints:
- Electrical field control – should have good defined impedance characteristics which smoothes electrical field over the connector.
- Insulation – should provide correct thickness of insulation
- Ensures flawless bond between joint insulation and screen
- Outer sealing – Heat would shrink the outer sleeve, causing precoated adhesive to melt and flow, resulting in protection from moisture and corrosion.
Types of Joints
On Voltage – 1
-MV and LV types
• On Core
– Single,three (for MV)
– Three, four (for LV)
Heat Shrink Boots/Gloves
- Should seal and waterproof cable terminations
- Should be resistant to decays, chemical and corrosion
- Should have high resistance to tearing and tensile strength
- Should have a strong bond with the cable
Types of Boots/Gloves
Two outlets – 2 fingers
Three outlets – 3 fingers
Four outlets – 4 fingers
Heat Shrink End Caps
- Used to seal cable ends during installation and storage
- Should be watertight and have unlimited storage life
- Should be resistant to UV rays, aggressive soils, chemicals and corrosion.
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