Heat Trace Flashcards
What are heat trace cables typically used for?
Prevention of freezing of liquids,
To melt snow and ice,
To maintain a specific temperature.
What are the basic design principles of heat trace cable?
Series fixed resistance (Constant wattage),
Parallel fixed resistance (Constant wattage),
Self regulating variable resistance (Variating wattage),
Skin effect.
What is used in MI cables as insulation?
A magnesium oxide powder or silicon dioxide powder.
Can you ever cut an MI heat trace cable to size?
No, as this will breach the seal. It is engineered for the job.
How far may polymer insulated series fixed resistance cables be run?
Up to 3500 m.
They are not practical for short runs.
What are the large wires in a parallel fixed resistance heat trace cable called?
Bus conductors.
What is a power limiting cable?
A self regulating (by use of metallic alloy which changes resistance with temperature) parallel variable resistance heat trace cable.
May be used for higher temperatures than polymer self regulating cables.
When the carbon polymer shrinks in a self regulating cable, what is the effect?
More electrical connections are made and the heat output increases due to the physical response to a cold environment.
What principle does skin effect heat tracing rely upon?
That current will only flow on the inside shell of a ferromagnetic tube.
How far may one run of skin effect heat tracing be used?
15 km.
If current is too high, how should a thermostat control a heat trace cable?
By use of a contactor.
Why is solid state switching advantageous?
The lack of moving parts allows for mush greater durability under constant switching.
Why is soft starting used in heat trace systems with self regulating cable?
Because often the cold resistance is low enough to cause bad nuissance tripping. a lower voltage is applied as the cable heats up so that it can heat and not trip the breaker.
What is average current control?
The pulsing of current through a self regulating cable to simulate a soft start, this is often mush easier to achieve than actually varying the voltage.
Must heat trace circuits be protected by class A GFCI?
No, not necessarily the same level as class A, check section 62-116 and APP B. for more info.
