EHT High Temperature Alarm

Just off the top of my head I'd say you should alarm it if you want to know if it reaches a certain temperature and that temperature probably depends on whatever it is that you're heating and whatever external factors may influence it. How about some more info about your process?

Thank you for your quick comments. What you say certainly make sense to me.

I found hi-temp alarms are set up for some EHT circuits, but not for the others. I am just trying to understand the generic reasons behind it.

A high temperature alarm is not needed for self-limiting trace heating tape as, by definition, the tape is self-limiting on temperature.

Mineral insulated (MI) cable can provide quite a bit of heat (as opposed to the plastic self regulating cable).

Some centralized heat trace controllers use solid state relays for the control outputs, whose failure mode is the ON state.

Combine MI cable with a failed controller output and the process medium which is supposed to be protected from freezing could conceiveably overheat.

The unintended removal of pipe insulation calling for low alarming is a more typical situation to protect against, but depending on what's in the pipe, some stuff take well to overheating.

Canterburian,

The other replies to date all help. There are four ways in which heat trace cable can be built:

1. The cable is a single conductor with a fixed resistance per unit of length, connected to power at the two ends; its heat production depends on the resistance, length, and applied voltage. This one is fairly uncommon.
2. The cable has two parallel conductors with a fixed resistance per unit of length, joined together at one end and powered from the other. These are often sold pre-assembled with a designated power output per unit of length at a specified voltage, and a fixed length.
3. The cable has two parallel conductors of negligible resistance, with an overwind of small-gauge resistance wire that is bonded alternately to each conductor at fixed intervals along their length. They are cut to length as needed and give a fixed heat output along each section where the overwound wire is a complete circuit between the two conductors.
4. The cable has two parallel conductors of negligible resistance, separated by a carbon-embedded plastic membrane that acts as a variable resistor. As it heats up the plastic expands, making the conductive carbon particles separate, so the resistance rises (and heat out falls); as it cools off, the opposite happens. This is called a self-regulating heating cable, because the heat output will self-adjust everywhere along the length according to the degree of local heat loss or cable spacing, etc.

Where/when do you want/need a high-temperature alarm? Anywhere that the type of cable used, the local materials being heated, or the environment can become dangerous or damaged if the cable overheats. This includes places or pipes that can or do contain materials that can degrade or explode if heated above a given temperature. This includes installations where failure of a controller can heat the cable above its safe maximum operating temperature. This includes installations where people can be injured if they contact it when a controller has failed.

Note that type-4 cables listed above are the only ones that can be applied where they cross over themselves; and are frequently the only ones approved for plastic pipes.

Other types of controls you may want or need include an indicator to show when heating power is applied. (On the #3 and #4 cable types above this should be at the far end so they show continuity of the cable as well as applied power.)

I suggest you look at manufacturer's web-sites and catalog data for a lot of information. Heat tracing can be for freeze protection, maintaining ready hot water in buildings, process piping, snow and ice melting, and many other applications. The design operating temperature, type of cable, control method, protective alarms or monitors, and many other details are all chosen to meet the needs of each application. I've done so to keep pipes from freezing and to keep flue gas sampling lines above 300°C (two very different types of applications).

When is an alarm necessary? Whenever the manufacturer says it is needed to comply with its tested performance or warranty. In the USA (and other countries that use it), the National Electric Code says that you MUST comply with all instructions given or included with the product in accordance with its listing (its being independently tested and certified to comply with a recognized and published standard).

Keep asking and learning--that's how I did it. --JMM