Heat Tracing Questions

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

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