OVER TEMPERATURE SAFETY CIRCUIT 1

[PNA]

There has been a concerned raised regarding the placement of a snap disc temperature control switch should be.

We are using a standard temp controller
on one side of a n/o set of contacts is a timer and the other side is an SSR connected to a heating element

main power feeding the controller comes from a simple on/off switch and in line with that is a temperature limiting snap disc

there is a thermocouple connected to the controller as well

Is it intrinsically safer for the snap disc to be on the contact line or the main power line in case of an over temp condition


Regards

Paul

 

[logbook]

Hopefully you have wired the thermocouple circuit such when the thermocouple is open-circuit the the controller detects over-temperature and shuts off the power. This is a standard technique.

Can you confirm that the "snap disc" is actually rated for the main power line? (Are you running 120V or 240V?)

Your question is not totally clear. Are you proposing having the thermal cutout as either
a) in series with the load only
b) in series with the load plus the controller
as your two options to choose between?

A simplified schematic would greatly aid the quality of possible answers.

 

[PNA]

Running on 120VAC

currently it is in series with the line that powers the controller, the controller would shut down completely if the thermal cutoff is exceeded

proposed version is to move the thermal cutoff to the line side (not load) of a relay contact only on the controller and the load side of that contact is connected to the SSR

My drawing in text would be atrocious!

Paul

 

[bogeyman]

Greatest safety will be given by cutting all power under fault conditions. Self reseting thermal swiches should be avoided if you want to be as safe as possible.
Solid state relays can often fail short circuit the over temperature protection must include this possibility.
Domestic tumble driers, microwaves and the like have a snap disk (often several thermal cut outs are used) in the supply line which cuts ALL power on over temp, I would strongly recomend a simlar scheme i.e. make absolutly sure that all power is cut by a manually reset safety switch in the line side of the supply.
Some industrial kilns use a fusible link (a loop of wire that melts at the limit temp) connected in series with the elements inside the chamber to achive a similar result (with a one shot action).