How the machine continues after a protection has tripped is a question that is often answered loosely in electrical safety. Some protections let the site press a reset button and carry on, some cut the power and require a person to attend, and some restart by themselves once the condition clears. Sorting these three cases out is what stops an alarm from becoming an accident that happens twice.
Protections fall into two kinds: shutdown and tripping
The protections on the machine fall broadly into two kinds. The first kind is the protection of the operating conditions: the audible and visual overpressure alarm with shutdown, the motor overload trip and the phase failure and phase sequence protection. The reason for their operation is essentially one-off: the pressure falls back into the normal range, the load becomes normal again, the three phases are connected correctly, and once the condition is restored the machine can carry on working. The second kind is the interlock protection: the machine refuses to run with the cabinet door open, and the automatic defoaming action that follows an infrared level or foam detection. What those protect is the relationship between the person and the machine, not a permission to recover automatically.
Where automatic recovery is the right answer
Automatic restart once the condition clears suits cases where the cause is clear and the consequence is reversible. Filling the oil back up and letting the machine re-enter circulation after the level has dropped below the set point is one such case, and so is re-entering circulation after the automatic defoaming has dealt with a foam overflow. The value of these protections is the shorter stoppage, and that value depends on the reset condition being verifiable: the circulation only restarts when the level really is above the set point and the foam really has gone. Where the reset condition cannot be confirmed, automatic recovery turns into repeated restarting, and the cause of the fault has not been found at all.
There is one further premise that is easily missed: automatic recovery has to leave a record behind. How many times the machine restarted and how long it was between the restarts are data that only exist on the control panel when nobody is on site, and then nobody knows it happened. Keeping the number of operations and the times in the record is what gives the later investigation something to work on.
Why the protections that need a manual reset cannot do without one
Protections that concern personal safety and electrical safety must be reset by hand, and the reset has to be made by a person, because before pressing the button the operator has to confirm that the cause of the fault has been removed. The phase failure and phase sequence protection belongs to this group: after the three phases have been connected wrongly, resetting directly lets the motor run, but in the opposite direction, and the sequence of the internal actions is wrong with it. The cabinet door interlock is in the same group, because resetting with the door open hands an open energised machine over to the people on site.
What to confirm before a reset
| Type of protection | State after it operates | How it recovers | What must be confirmed before the reset |
|---|---|---|---|
| Overpressure alarm with shutdown | The pump stops and the heating is cut off, the system waits | Manual reset | The pressure has fallen back and the pipework is not blocked |
| Motor overload trip | The main circuit is opened | Manual reset | The load is removed and neither the motor nor the pump is damaged |
| Phase failure and phase sequence protection | Refuses to start | Manual reset | The wiring has been corrected and the phase sequence re-checked |
| Cabinet door interlock | Refuses to run | Manual reset | The door is closed and the energised parts are enclosed |
| Level and foam detection | Circulation stops, automatic defoaming takes over | Automatic return to circulation once the condition clears | The level and the foam condition really are normal |
- Before resetting, decide whether the cause of the alarm is still there or whether it has gone without the machine recovering on its own.
- Look at the alarm history rather than the present state, and confirm that no other protection operated at the same time.
- Any protection that involves the electrics has to be confirmed by a person with the right authorisation before it is reset.
- After the reset, run the machine unloaded and watch it before the load is put back on; do not go straight to full rate.
A protection is doing the judging on the machine's behalf. A manual reset is the step that hands that judgement back to the person on site.
