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The Challenge: PTC and Self-regulating Cable

PTC Explained

A polymeric PTC(positive temperature coefficient) material comprises a non-conductive polymer matrix that is loaded with carbon black particles to make it conductive.

While cool, the polymer is in crystalline state, with the carbon latched into the regions between crystals, forming many conductive chains.  If electric power is applied, current will pass through the polymer and generate heat, which causes the polymer to expand.  The expansion separates the carbon particles and gradually breaks some of the conductive chains, causing the resistance of the device to increase.  

PTC curveAs the polymer heats up to certain temperature, it changes from crystalline state into amorphous state and breaks most of the conductive chains.  The dramatic increase in resistance substantially reduces the electric current.  A small current still flows through the polymer and is sufficient to maintain the temperature at a level which will keep it in the high resistance state.  When power is removed, the heating will stop.  As the polymer cools, it regains its original crystalline structure and returns to a low resistance state.

Self-regulating Heating Cable

SR structure

Self-temperature regulating cable consists of two parallel metal bus wires encased in PTC polymer matrix forming a heating unit with a polyolefin insulation and metallic braided shield.  For corrosive and some hazardous environments, additional fluoroplastics overjackets will be applied.

SR outputAs the electric current flows from one of the copper wires, passes through the conductive polymer and arrives in the other copper wire, a close circuit is established.  The electric power causes PTC polymer to heat up and gradually raises the resistance value.  As the temperature of the conductive core increases, so does the electrical resistance.  The result is a diminishing output for each temperature increment.  In other word, self regulating heating cables adjust their power output in response to the surrounding temperature, in comparison with constant wattage heating cables that give out constant output to the heated objects.  

Once the PTC polymer reaches the transition temperature, its resistance value takes a step increase, which is high enough to almost cut off the current and keep the temperature from going any higher, and the purpose of self-temperature regulating is served.

Epoly manufactures all PTC compounds in-house.  We believe strongly that this is the only way to maintain the highest quality finished self-regulating cable products. 

The Advantages and Disadvantages of Self-regulation Heating Cable

Advantages

  • Ensure the temperature of object heated constantly stays within an ideal range.
  • Can be overlapped without the risks of overheating or burnout.
  • High electro-thermal conversion rate. Even heat distribution. Low energy consumption.
  • Self temperature regulating and self heat output regulating.
  • Can be cut to any length. Systems are easy to design and install.
  • Its tendency to reach a limiting temperature means safety.

Disadvantages

Self-regulating heating cables have a specific maximum exposure temperature based on the type of polymer which is used to make the heating core (PTC) which means that if they are subject to high temperatures then the cable can be damaged beyond repair.  Also self-regulating cables are subject to high inrush currents on starting up similar to 'induction' motor so a higher rated circuit breaker is required.