A safety interlock can show up in two ways, and the two have to be separated before anything is checked: either the machine does not act when the start button is pressed, and the panel gives a message or nothing happens at all; or the machine stops suddenly during operation, with an audible and visual alarm. Both are the interlock operating, but the direction of the investigation is completely different, and working them together only takes you in circles.
The order of investigation is: first establish whether the machine refused to start or stopped during operation → read the fault indication → check the trigger conditions one by one → reset and verify. Reversing this order most often ends in repeated resets and repeated stops, with the time wasted and the real cause possibly covered up.
Abnormal oil level: no acceptable oil level, no heating and no pumping
The machine uses infrared level detection combined with foam detection and automatic defoaming. The reason for the interlock is straightforward: if the level is too low, the heating element stands above the oil surface and runs dry, which is the most typical and also the hardest to trace equipment failure; if the level is too high and foam is present as well, the overflowing oil runs into the pump set and the electrical parts. To confirm it, compare the displayed value with the sight glass or the mechanical level gauge while the machine is stopped, then simulate the two conditions below the lower limit and above the upper limit and see whether the machine really refuses to start.
Overpressure: pressure above the set point has to be cut off
The overpressure protection gives an audible and visual alarm and stops the machine. It has to be interlocked because the pressure rating of the pipework and the filter elements is limited: a pressure that keeps rising means the element is blocked, a valve has closed by mistake or the downstream side is blocked, and continuing to run can push a joint open or burst the element. To confirm it, raise the pressure slowly or simulate the trigger following the test method in the machine manual, and confirm that both the sound and the light operate and that the pump and the heating circuit really are cut off. Looking only at the lamp is not enough.
Phase loss and phase sequence error: this one protects the motor
When one phase is missing from the three-phase supply the motor keeps heating under an undervoltage condition, and the overcurrent in the remaining phase burns the windings out quickly; a wrong phase sequence makes the pump run backwards, with both the thrust direction and the lubrication conditions wrong. To confirm it, check the phase sequence with the power off as described in the machine manual, and carry out a simulated phase-loss test at the distribution end. The test has to be carried out by qualified personnel under the electrical rules of the site. Do not carry it out with the machine energised, and never bridge a protection to save time.
Cabinet door not closed and foam overflow
The door interlock prevents the cabinet from being opened while it is live; it is the most basic electrical safety condition. Foam overflow is a problem because foam entering the pump set causes abnormal suction at the pump inlet and damages the pump in severe cases, so the level and the foam conditions have to be interlocked together, the level alone is not enough. The confirmation is the same in principle: simulate the conditions, open the cabinet door and see whether the machine refuses to start, create a foam condition artificially and see whether the alarm and the automatic defoaming operate.
Why a hard interlock is required rather than a message
A message-type protection depends on somebody noticing it every time and acting on it. During unattended night running, during continuous operation, or when the operator is busy, the rate at which such messages are actually complied with is low, and a single time it is not complied with the loss has already happened. A hard interlock turns the condition into a necessary condition in the electrical circuit: when the condition is not met the start circuit cannot close at all; when the condition arises during operation the cut-off circuit cuts the power directly. Neither human inattention nor the operating habits of the machine can change the result.
A hard interlock is not the only line of defence. Gradual pressure build-up, over-temperature protection and phase-sequence protection still have to be kept; the interlock governs the absolute ban on any action when the condition is not met. The two have different jobs.
How to reset after a false trigger
- Remove the cause of the trigger first, do not press reset first. For an abnormal level, bring the level back into the normal range and confirm that the probe has not been affected by an oil film; for an overpressure stop, establish whether the element was blocked, a valve closed by mistake or the downstream side was blocked; for phase loss and phase sequence, re-check the wiring as described in the manual.
- Put the site back in order: close the cabinet door and confirm that the latch is engaged, return the valves to their correct positions, and confirm that the alarm area is free of pooled oil and residual liquid.
- Reset the alarm as described in the machine manual and confirm that the fault indication goes out. Most machines require the fault to be cleared before the reset is accepted; the reset button itself is not a means of repair.
- Jog the machine once with no load and confirm the motor direction, the rotation of the pump and that the heating circuit is still under control before putting the machine back into service.
- Record this false trigger in the log: the trigger condition, the investigation and the final cause. When the same condition triggers repeatedly and no cause can be found, treat the interlock as failed, stop using the machine and carry out a complete check.
- Interlocks must not be bridged or bypassed. There is no exception to this.
| Symptom | Possible cause | How to confirm | Action |
|---|---|---|---|
| Nothing happens on start, the panel gives no message at all | The door is not closed or a connection in the start circuit has come loose | Check the door indication and the wiring of the start circuit | Close the door and confirm that the latch is engaged, re-check the wiring |
| It trips as soon as it is started | Phase loss or phase sequence error | Check the phase sequence with the power off as described in the manual, and carry out a simulated phase-loss test | Correct the wiring and re-test the operation of the protection |
| It stops suddenly during operation with an audible and visual alarm | Overpressure or foam overflow | Check whether the alarm indication refers to the pressure or to the foam | Stop the machine, release the pressure and disconnect the power, then check for blockage and the defoaming device |
| The heating does not operate and no alarm is given | The level signal is inaccurate, so heating is in fact inhibited | Compare the level at the sight glass with the displayed value | Clean and recalibrate the infrared level probe |
| The same condition triggers repeatedly and returns after the reset | The cause has not been removed, or the interlock itself has failed | Reproduce the condition item by item and record the result | Stop using the machine and carry out a complete check of the interlock circuit before putting it back into service |
When the interlock operates, find the cause before you reset. Resetting again and again is not investigation, it turns the safety device into a decoration.