We should find out why the fuse is blowing.
But, in a fixed resistive load like you have, eliminating the fuse should work just fine.
The overall system is protected by the breaker connected to the thermostat. That breaker should be appropriate for the load on that circuit. That is easy if this is a dedicated circuit.
The fuse on the SCR controller is only for fault protection of the SCR. It is not an overall safety issue. A length of resistive wire is unlikely to have a fault mode that fries the SCRs.
One point - since you are not varying the voltage applied to the floor wire (you removed the potentiometer on the SCR controller), how about a transformer instead of the SCR thing?
A note for the lawyers out there, advice is not guaranteed. It can be confusing to people who don't do this sort of thing.
Yet another point. For this use a properly sized light dimmer would work fine.
A 1000 Watt dimmer would work ...
I would put a stop on it, and probably a fuse, because putting 120 Volts on a heating circuit designed for 70 Volts would nearly triple the heat output.
Sometimes you have to just sit back for a bit and think about what you were trying to do, rather than how to fix what has been done.
Do not assume the thermostat will protect the wire under all conditions.
Even if the thermostat sensor was tightly coupled to the actual heating wire, the minimum cycle of the thermostat is 15 seconds ...
From what I read about the thermostat, when you are referring to the % power it is talking the amount of on time vs off time, with the minimum interval of 15 seconds and max of 15 minutes.
If you are controlling floor temp with an embedded sensor it stands a better chance of helping some. If you are measuring air temperature it will have no useful impact on controlling the wire temp.
AF [Air/Floor] mode controls the ambient air temperature and maintains the floor temperature within desired limits using an external temperature sensor.
I went back and read the documentation of the thermostat. The only place I could find that they talk about the output is where they talk about percentages. The words are ambiguous ...
If the thermostat switches complete half cycles rather than PWM, it could work. The fact that you got a stable 70 VAC leads me to lean toward it not being a PWM output from the thermostat. Anyway. I would suggest waiting until I get a response from Aubetech. Some of the things I am talking about probably don't make a whole lot of sense to you. But I think they are correct and it would be safer for you to be sure.
This is wearing my brain out, but I live to confuse ...
Have you thought of a kerosene heater
Circuit diagrams don't always show all of the connections because the connections are implied to people who understand the complete circuit ...
What are the yellow and magenta wires shown in the diagram of the trigger of the SCR connected to?
Can you draw a diagram of the complete circuit, especially any connection of those "unconnected points" to the return line through other components.
I continue to believe that the fuse blowing is related to a failure of the controller.
Can you measure the resistance of both pieces of wire again. Am I right that the extra piece in the floor plus the extra one together would be a single heating mat for 240 V (I know you said 208, but that is kind of unusual for a residence)?
The different results with and without thermostat may be a function of a programmed delay in the thermostat. Do you know if the SCR powered up immediately with the thermostat?
One thought about why this might be happening. You have to think about how an SCR works ...
What happens if the the voltage output is 100% of line input ... peak voltage/current would be the same as if you connected the load to the voltage controlled source turned full on.
In any modern power device at this power level I don't think that sort of fuse is used. It is just too fast. If the manufacturer insists it will damage the controller, you may wind up trying without the fuse and/or try a dimmer ...
Incidentally, your volt and ammeters are unlikely to show you this happening. A scope will.
Can you draw a diagram of the complete circuit, especially any connection of those "unconnected points" to the return line through other components.
You should be shooting for about 56 V to keep the wire within spec.
If you were to put the floor and the 24 ohm piece in series, the fuse would probably be fine. Probably even with the 9 ohm in series it might work. There is probably a minimum resistive load before the fuse reacts to the current of a switching SCR. It is less than 24 ohms and greater than 14 ohms.
With what you know now, perhaps you should try calling the controller manufacturer tech support. You have troubleshooting data now. Not just calling and saying my fuse breaks. They should know if what I have suggested can happen. Or give a better idea of what may be wrong.
This is awkward, but...
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