Heat Tracing Questions

XX  Frost Protection Heat Tape
Prevents water from freezing in metal and plastic pipes


The PW’s built-in thermostat means that no additional control device is required.


The PW Frost Protection Heat Tape is designed for use

for the positive protection of hot and cold water pipes that may be subject to freezing conditions.


This can be used in flats, houses, mobile homes, farms, sports pavilions,

both for water supply and fire protection sprinkler systems.


The simple self contained nature of the XX lends itself to use for the

protection of single pipes such as in domestic lofts or gardens, for use with condensing boilers and widely used on mobile homes.


It is also ideal where many identical pipes require to be protected ie. In blocks of flats or sprinkler networks.


Z.

1. In most conductors, the resistance goes up with temperature.  So the current is greatest when cold, and drops as the cable warms up.

2. Because of 1.  The cable is naturally self-limiting - as it gets hotter, the current drops, which prevents any thermal runaway.

3. Who knows?  Ask the manufacturer.  Roughly speaking, the current is proportional to the cable length.  But the breaker values there only vaguely approximate to that.

The safety thing is the absence of  external wiring to a thermostat with possibly arcing contacts - there are no moving parts, so no worry about vapours and gas ignition.


The wire alloys and insulation between them are chosen so that there is a sharp rise in resistance with temperature - far more than with a normal metal, where resistance rises with absolute temperature (so freezing is 273 degrees, and boiling is 373 -  a change in resistance of a mere 35% over 100degree change is not enough.)


The kind where both L and E go in at the same end are easier to deal with, as  it is more like a set of festoon lights, with lots of little variable resistance heaters strung like the rungs of a ladder between low resistance wires with supply between them.  These can be made to length more or less as required.


The kind with one wire core and L and N at opposite ends are fixed resistance must not be shortened without adjusting the supply voltage so the number of volts per metre is constant.


However in either case there are now a couple of considerations when sizing the breakers.

1 temperature matters - no good if the current is so high the breaker fires on a cold morning. (!!!)

2 the wire has resistance - but if the cable is cut and shorted out near the far end, you need enough extra current to flow, (even on a hot high resistance sort of day), that the breaker will trip correctly.

The practical upshot is that there is both a max length and  breaker rating limit for the breaker to work if there is a  fault, and another breaking limit for a given length for it not to fire on cold days. the table tries to indicate a range of sensible cable lengths for a given breaker rating.

In general short runs of trace cable are easy, and long ones are better sub-divided or fed from the centre outwards.

Mike

Many modern types of heating tape do not use traditional resistance wire heating elements.

Instead they use Positive Temperature Coeficient materials as the heat source.

PTC elements have a lower resistance when cold and a higher resistance when hot, this change in resistance is very, very much greater than for metalic resistance wire.

A traditional resistance heating element might use 10 amps at freezing point and 9.5 amps at boiling point.

A PTC heater might use 10 amps at freezing point and almost nothing at boiling point. At sub zero temperatures it might use a lot more than 10 amps.

PTC heating tapes are much safer than the old resistance sort, since they cant overheat, any increase in temperature reduces the heat output. The operating temperature is fixed in manufacture, many types are available.


The MINIMUM size of circuit breaker is determined by the need to avoid tripping when first turned on in cold conditions. The actual operating current may exceed the MCB rating, but only briefly and should remain under the thermal trip curve.


The MAXIMUM circuit breaker size is determined by two factors. Firstly, if the heating tape suffers internal short circuit or external damage then an over-large circuit breaker makes "a bigger bang" with more risk of fire or other damage.

Also the PTC heating tape consists of two copper wires that run the length of the tape, with the PTC material between them. Like any other conductor, these copper wires have a finite current carrying capacity and that limits the size of OCPD that can be used. They are however pushed much harder than copper house wiring cables.


For a very large installation of PTC heat tape, circuits may need some form of sequential start to avoid overloading the DNO cut out fuses or any sub mains.


Operating temperatures vary from about 10 degrees to protect water pipes from freezing, up to about 70 degrees, to avoid domestic hot water cooling in the pipes.


Another advantage of PTC heaters in tape form or otherwise, is that they are not sensitive to voltage variations within reason. At a lower line voltage they will draw more current and maintain the same temperature.


ELV PTC heating mats are available to preserve the lives of premature infants, to warm pets, to warm fermentation vessels for home wine making, and for many other purposes. They are built into heated car seats.

"Another advantage of PTC heaters in tape form or otherwise, is that they are not sensitive to voltage variations within reason. At a lower line voltage they will draw more current and maintain the same temperature."

Surely the current will reduce at lower line voltage as per Ohms Law.

"No, Ohms law is not applicable in this case as the resistance is not fixed but varies substantially. Suppose that a given installation requires say 1,000 watts to maintain the required temperature. At 250 volts that will be 4 amps, and at 200 volts it will be 5 amps.

 This is a considerable advantage and simplifies the design, and may save energy."


Hi Broadgage. I'm still trying to get my head round this. The implication from above is that varying the line voltage to a fixed length of PTC tape varies the resistance of the tape to allow the current to adjust to provide a specified power. This requires the PTC ceramic to change resistance with applied voltage. I thought the resistance was temperature dependent but your saying it's also voltage dependant. Ohm's law will of course still apply, i.e. V=IxR irrespective of the material characteristics surely? Am I still missing something?

Cheers Andy

The resistance of the PTC tape is not DIRECTLY voltage dependent, but it is very strongly temperature dependent. At a lower line voltage, the input wattage would drop slightly, and this would reduce the temperature. This SLIGHTLY lower temperature will significantly reduce the resistance, and significantly increase the current.

The PTC tape will therefore be APPROXIMATLY a constant wattage load at any reasonable voltage. An ALMOST constant wattage requires more current at a lower line voltage.


Experiment and observation beats argument, buy a small PTC heating element and take measurements at different voltages and at different temperatures.

Most PTC tapes are based on carbon loaded polyethylene. This is extruded around the two conductors and then crosslinked for thermal stability. Additional layers of insulation and protection will be added according to the end use. As the material heats up it expands reducing the contact between the carbon particles hence increasing the resistance. The tape will find its own balance point reaching  a resistance where the heat generated (V²/R) matches the heat flow out. The art of these is selecting (and manufacturing) a suitable carbon loading. Some self resetting fuses use the same technique.