When the maintenance in the switchgear room ends and the main breaker is pushed back up, the oil purifier may not really have stopped. Once the control circuit loses power, the relay contacts reset and the state of the power circuit inside the cabinet is no longer decided by a person. When the supply comes back and the control cabinet is energised again, whether the machine sits there quietly waiting for someone to press start or enters the running state by itself depends on whether its electrical logic recorded the power interruption.
At the moment of power loss the machine rests in three indeterminate states
A power cut is not a shutdown. What is interrupted at the instant of the cut is the running state, while the oil circuit, the vacuum and the heating are frozen where they are and continue from there once the power returns. The vacuum pump has stopped, and the negative pressure in the flash tank may still be sitting at some reading; the oil pump has stopped, so the pipework holds neither oil in motion nor released pressure; the heater has stopped and the oil temperature stays at the value it had when it was interrupted, but the temperature control circuit does not know under what conditions the previous heating ended.
Each of these states on its own is not abnormal; stacked together they are troublesome. A machine that has just had its supply restored and enters the operating cycle directly is facing a site where the vacuum has not been re-established, the oil circuit has not been re-connected, and the level reading has not been checked again.
- Vacuum state: residual pressure in the tank may still be there, but the vacuum pump is not running, so the working vacuum does not hold.
- Oil circuit state: the pumps are stopped and the pipework is static, with both the inlet and the outlet sections stationary rather than flowing.
- Heating state: the oil temperature stays at the value it had before the interruption, and the accumulated information of the temperature control is reset.
Three signs that can be read on the panel after the power comes back
In the period after the power is restored and before the machine is started, a few sets of information can usually be read at the operator station. The indicator lamps on the control cabinet panel light up, the vacuum gauge and the pressure gauge return to around zero, and the oil temperature display still shows the value from before the interruption. This combination of readings is meaningful: a reading on a gauge does not mean that the operating condition has been established.
The vacuum gauge returning to zero means the negative pressure in the tank has been released; the oil temperature holding the value from before the interruption means the heating circuit has not acted again. If at that moment the vacuum gauge pointer is already in the negative pressure region and the oil temperature is still where it was, the machine has most probably been running on its own rather than standing still waiting for an operator.
| What to read first after the power comes back | What it tells you | Action when the condition is not met |
|---|---|---|
| Vacuum gauge reading | Whether negative pressure has been built up on its own | A return to around zero is normal; a non-zero reading requires confirming whether it has run |
| Oil temperature display | Whether the heating circuit has acted on its own | The same value as before the cut means the heating has not started |
| Running indication and ammeter | Whether the power circuit is carrying load | A reading means the machine has entered operation with nobody operating it |
Break those few seconds into a pre-start checklist
Fixing the order is more reliable than going by judgement every time. After the supply is restored, make the static confirmations first and only then touch any button; the order is roughly as follows: confirm the emergency stop button is in the reset position, confirm the oil level in the tank is inside the working range, confirm the vacuum gauge and the pressure gauge have returned to around zero, and finally confirm the pipework valves are in their normal positions.
- Confirm the emergency stop button has been reset; while the button is not reset the machine should not accept a start command.
- Confirm the oil level in the tank is inside the working range; with no oil in the pipes and no level alarm, heating must not be started.
- Confirm the vacuum gauge and the pressure gauge have returned to around zero, which shows that the pressure of the previous run has been released.
- Confirm the inlet and outlet paths were not temporarily blanked off during the maintenance and that the valves are in their normal positions.
- Confirm the filter elements are fitted and that the position signals have reset; a missing stage in either the coarse or the fine filtration makes the start condition fail.
This checklist runs in the same direction as the interlock logic built into the machine. The oil draining system, the vacuum system and the heating system of the ZYD series are interlocked with each other, specifically to avoid heating without oil and dry burning; on overpressure there is an audible and visual alarm together with a shutdown, and phase loss and phase sequence protection are likewise inside the cabinet. These protections cover the anomalies that arise during operation, whereas a power interruption and restoration belong to the state before starting, which the interlocks may not cover. The state then has to be completed by the operator or by the control logic itself.
Let the control logic remember it for you
Writing the order into the operating procedure is the first step, and a more thorough approach is to let the machine remember it. Models with PLC smart control and a touch screen can set up a power-on self-test in the control logic: after power-on the machine first reads the emergency stop status, the level signal and the feedback from the vacuum and pressure switches, and only allows a start command once everything is confirmed normal; if the self-test fails it stays in the standby state. Some sites additionally fit a power-loss memory or an undervoltage protection device in the main circuit, so that after an abnormal power loss the machine has to go through a manual reset before it can be put back into service.
The real risk of a power cut and restoration does not lie in the machine itself, but in the possibility that it starts working again with nobody watching it.
