Need help troubleshooting after short-circuit, with heaters and thermocouples

Question

There was a short circuit in one of the furnace that I designed.

Description of the furnace and its components: The furnace is a copper tube, with 1/2 inch thickness, and 10 equally spaced 1/4 inch bores for inserting the heaters and thermocouples.

• Heaters
• Thermocouples

I'm controlling the heating of the furnace with the following controllers:

• AC solid state relay
• 16 output Relay
• VINT hub

The working of the heaters are controlled by the solid state relays and they are in turn controlled by the 16-output relay. The VINT hub is used to connect the 16-output relay to the computer so I can control them with LabVIEW. There are 10 heaters in total and each heater current consumption is 3A. I used two separate outlets to power 5 heaters each as the power outlet rating is only 20A in total.

Circuit diagram: I apologize for the clumsy circuit diagram, it has been quite some time since I did circuit diagrams and as of now my diagrams quite clumsy.

Description: The DC source is for controlling the solid state relays and goes through the 16-output relay. The AC source is for the heaters,not shown in diagram, the connector HTC2F is the connector which connects to all the heaters. All HTXL's are the solid state relays.

I have made a LabVIEW program where I monitor the reading from the thermocouples and control the 16-output relay for switching the heaters on and off through the solid state relays.

The incident: I was running the LabVIEW program to heat the furnace, but I accidentally ran a previous version I wrote which does not switch off the heaters when the desired temperature is reached. I initially plugged in one outlet and 5 heaters started heating up, and when I switched on the other outlet after a few seconds I heard a pop sound from the furnace and there was short circuit. The phidgets (VINT and the 16-ouput relay) burned. It also burned my computer mother board.

Need help with: Now I'm not sure how to test the system to check which heaters and thermocouples failed and replace them. Quite honestly I'm afraid to control them with LabVIEW as I'm afraid of the short circuit ruining my computer and the phidgets again. Any troubleshooting tips are appreciated. If there are any design flaws please let me know and I'll work on them.

Please let me know if any additional information is needed, I'll update the post with additional information.

Answer 1

It’s not clear to me how you are reading the thermocouples, but I suspect the cause of your disaster was a short between the thermocouple and the “hot” mains line causing current to flow through all the non-isolated analog bits back to the (grounded) computer motherboard. Insulation breakdown was caused by the heater/thermocouple assembly melting.

That would not happen if you used thermocouple signal conditioners with isolation (cost would, of course, be higher). Solid grounding on the metal parts and fusing on the mains lines may or may not prevent such damage, but is required for safety. If your computer was improperly grounded it might have become energized at mains potential. The thermocouple wires must be insulated to the same standard as any nearby mains wires (for example with fiberglass sleeving)- melting that insulation could otherwise have a similar effect to a short in the heater/thermocouple assembly.

It’s also quite normal to have an independent over temperature controller, usually a very simple reliable device. In your case it could be something like thermal fuses or stand-alone limit controllers.