Electric heater safety

[I14R10]

So, I built electric heater and I put it into a stainless steel enclosure. There is an opening on one side that has a fan that blows air through the heater. I already have two thermal switches, one on 120C and other at 100C. They are located on the outside wall of the enclosure. This is low power heater, up to 200W. I have, of course, fuse right after the transformer (220V-12V). Fuse breaks at 20A. And there are circuit brakers on main electrical panel.

What precaution can I take to further improve safety of the heater?

 

[FvM]

Reliable protective earth connection of enclosure and heater body.

 

[schmitt trigger]

120C can burn the skin.

You didn't tell us the purpose of the heater or it's location.

Can it be touched by children?
Are any flammable materials nearby?
Can a liquid be spilled into it?
Could a small animal be trapped inside?

 

[I14R10]

Heater is for the doghouse. It will be outside, not inside. Warm air will be transferred inside by a short hose and a fan. Basic configuration is like this - Heater is inside metal can. The thermal switches are on that can. Hose is also attached to it. And entire thing is inside larger aluminium casing. 220V is only going to the transformer. Everything after that is 12V.

There is flammable material - wood, but nothing could get inside the outer aluminium casing, let alone the can where the heater is and which I expect to be under 100 degrees C. No children and no liquid is present.

 

[Warpspeed]

I am wondering how your dog will like 100 degree Celsius air ?

I would have thought that more air volume and a much lower cut out temperature.
The sensors that you have will be fine to protect the heater, but what about protecting the dog ?

A bimetallic adjustable room thermostat in the discharge air might be worth thinking about.

 

[I14R10]

OK, I forgot to mention the rest of the thing. I was asking about the safety measures so I only wrote them. I control the heater with triac and arduino. Arduino sends signals to the gate of the triac. In fact, I only plan to have temperatures in the doghouse at 5C maximum. I just don't want that it goes down below that. Of course I have temperature sensor in the house that tells the arduino what temperature is inside.

Thermal switches are there just to turn off the power if something goes wrong.

By the way, to simplify the circuit, I am not using zero crossing detection. For example, I planned to turn the triac on for 1/10 of the second and then leave it off for the 9/10th of the second for the lowest setting. Will there be any problem with this kind of control? Heater is 12V AC of course.

 

[Warpspeed]

Many types of industrial heater control systems work that way, for example a gas fired pizza oven. Goes on for a few seconds, then off for a few seconds.
It is also done with triac and SCR temperature control too, with electric heating elements.
It will work fine.

 

[tinystorm]

i see a few "nodes" of interest
1) fan draws air into heater, if fan fails, is there code for failsafe? ie : check fan is working?
2) if thermostat fails, is there another monitor to know the heat is not right?

@ about 16++A heater current, i have some experience that high current heater may have chance of the contact sticking or self welding together during surges and the thermostat then fails to function. so cycling life of thermostat is in question and what kind of surge it can hold? cold heater resistance vs hot heater resistance?

so reliability of fan and thermostat perhaps?

 

[I14R10]

i see a few "nodes" of interest
1) fan draws air into heater, if fan fails, is there code for failsafe? ie : check fan is working?

Not failsafe code, but bimetallic thermal switch. Two in fact. One opens at 100C and the other to 120C.

2) if thermostat fails, is there another monitor to know the heat is not right?

@ about 16++A heater current, i have some experience that high current heater may have chance of the contact sticking or self welding together during surges and the thermostat then fails to function. so cycling life of thermostat is in question and what kind of surge it can hold? cold heater resistance vs hot heater resistance?

so reliability of fan and thermostat perhaps?

That's a good point. I might put a piece of code in there when the temperature monitor fails that it shuts the heater. The thermometer that I have is I2C thermometer connected to arduino. I guess if it fails it will not be possible to open I2C communication to it?