OVER TEMPERATURE SAFETY CIRCUIT
OVER TEMPERATURE SAFETY CIRCUIT
(OP)
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
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





RE: OVER TEMPERATURE SAFETY CIRCUIT
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.
RE: OVER TEMPERATURE SAFETY CIRCUIT
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
RE: OVER TEMPERATURE SAFETY CIRCUIT
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).