On site the emergency stop, the cabinet door interlock and the overpressure shutdown are often talked about as if they were the same thing, and the usual result is that the response is inverted: when the power should have been cut, only the reset is pressed, and when the machine should have stopped, the cabinet door is opened instead. All three actions end with the machine stopped, but the trigger, what is cut and the condition for restart differ in each case. Setting out the boundary between the three on a three-phase 380V / 50Hz supply is the one point in a safety briefing that is worth writing down properly.
Emergency stop: triggered by a person, cuts the power circuit
The emergency stop is a stop signal issued deliberately by the operator. Once the button is pressed the control circuit loses its power, the contactor in the power circuit releases, and the motor, the pump and the heating circuit stop with it. On this chain the heating system and the vacuum system sit on different supply branches, so after the emergency stop has acted the state of the two has to be confirmed separately: the heating elements being de-energised does not mean that they have cooled down, and the vacuum circuit no longer extracting does not mean that the pressure in the tank has equalised.
The emergency stop does not change the state of the upstream distribution board. The cabinet may still carry the voltage coming from the supply side. What it achieves is a machine that has stopped, not a cabinet without voltage. The reset is generally carried out by hand, and pressing it only returns the circuit to a state in which it can act again; it does not mean that the cause has been cleared.
Cabinet door interlock: triggered by the mechanism, and what it does is prevent the opening
The interlock is triggered by the mechanism on the cabinet door, and the condition is that the door has been opened. It also cuts the power circuit, so that the machine cannot continue to run while the door is open. The difference from the emergency stop lies in the purpose: the emergency stop has to bring an action that has already started to a stop, while the interlock has to block it before the action begins. The first comes from the judgement of a person, the second from the structure of the machine itself.
The interlock covers only the door path. It does not cover the tank, the pump body and the heating surfaces, which carry heat and pressure as well. After the interlock has acted, it cannot be taken as a conclusion that the whole machine is safe.
Overpressure shutdown: triggered by the machine itself, and what it terminates is the process
The overpressure shutdown with an audible and visual alarm is the third path, and what triggers it is not a person but the pressure signal itself. When the pressure goes beyond its limit and is not reset in time, the protective action stops the machine automatically and brings the rising pressure back down. What this path stops is the process step that is causing the pressure to keep rising, and that step sits on the oil inlet and heating side; it does not disconnect the whole machine.
This is the point that is most easily overlooked on site. After an overpressure shutdown the machine has stopped, but that stop is not the same stop as an emergency stop: it has kept the pressure from going any higher, but it has not carried out the power isolation, the voltage check and the lockout and tagout on behalf of the person. The running state of the vacuum pump, the residual heat in the heating elements and the residual pressure in the tank all have to be confirmed separately.
Side by side: the trigger, what it acts on, the condition for restart
Put the three paths side by side and the differences are easy to see. The only thing they share is the result, that the machine stops running. Who triggers it, how wide the scope of its action is and what has to be done before a restart are different in each case. Describing all three with the same word makes it easy for the response to be inverted.
| Stop method | Trigger | What it acts on | Condition for restart |
|---|---|---|---|
| Emergency stop | A person presses the button | The power circuit stops; heating and vacuum confirmed separately | Manual reset, with the cause already cleared |
| Cabinet door interlock | The cabinet door mechanism operates | The power circuit; the machine cannot run with the door open | The door is closed and latched |
| Overpressure shutdown | The pressure limit signal | The process steps on the oil inlet and heating side | Relieve the pressure and trace its source first |
Besides these three there are two further protections that do not rely on anyone pressing a button. Phase failure and phase sequence protection deal with the supply side: if a phase is missing or the sequence is wrong, the machine refuses to start, which keeps the motor and the pump from running in the wrong phase sequence. Infrared level and foam detection combined with automatic defoaming deal with the liquid and gas sides: the action is triggered when a rising foam level is detected, and it keeps the foam out of the vacuum chamber. Neither of these two is a power disconnection in itself, and mixing them into the three above only makes the explanation harder to follow.
- After the emergency stop has been pressed, confirm the stop state of the heating circuit and of the vacuum circuit separately, and only then decide when the cabinet door can be opened.
- The interlock acting means that the door path has been blocked; it does not mean that the tank, the pump body and the heating surfaces are free of heat and pressure.
- The first thing after an overpressure shutdown is to relieve the pressure and trace the cause, not to press the reset.
- When the machine stops, first establish which path acted: a button pressed by a person, a door opened, or a pressure signal of its own.
The emergency stop makes a person stop, the interlock makes the machine unable to carry on, and the overpressure shutdown holds the pressure down in place of the operator; the three must not be handled as one and the same stop.
