Once the control cabinet door is open, the cabinet contains live parts and moving parts at the same time. On the electrical side the issue is the three-phase 380V / 50Hz power circuit, and the total power of the ZYD-100 is 72 + 5 kW. On the moving side it is the motor, the pump and the heating circuit, which may still be turning or still be heating while the door stands open. Many site safety briefings put "cut the power before opening the door" into the instructions. The rule itself is not wrong, but a rule that depends on people remembering and people doing it is not an interlock.
Two kinds of hazard behind the door
The first is the electrical hazard. The control cabinet is where the main circuit enters the machine: the power side, the control side and the control devices of the heating circuit are all inside it. While a hand is reaching in, the cabinet may still carry voltage coming from the upstream distribution board even when the machine is stopped. A machine that has been stopped is not a cabinet without voltage, and these two are regularly confused on site.
The second is the hazard of movement and heat. The pump body, the motor and the heating elements remain hot after they have stopped, and the motor may start unexpectedly if the control signal is faulty or an interlock has failed. The cabinet groups these risks together, and once the door is open all of them are exposed at once.
- Electrical side: the power circuit is live and the cabinet holds exposed terminals and terminal blocks.
- Moving side: the motor and the pump shaft have rotating parts, and even after the power is cut they coast down.
- Heat side: the heating circuit and the pump body keep residual heat after a shutdown, and with no cooling inside the cabinet it takes longer to cool down.
What a prompt does and what an interlock does
A prompt tells a person; an interlock makes the machine unable to do it. The first depends on attention, and attention is affected by the light, the noise and the fatigue of continuous work on site. The second depends on the mechanism itself and not on the state of the person. A switch that can still be pressed with the door open is only a prompt; a door that cuts the main circuit when it is opened is an interlock.
| Approach | State of the machine when it triggers | Where it applies |
|---|---|---|
| A sound or light warning only | The machine keeps running and the door can still be opened | Where a hazard is signalled but brief approach is permitted |
| A door interlock that cuts the power circuit | The motor and the heating circuit are disconnected and the door cannot be opened while live | Where work is carried out inside the control cabinet with people present |
| A door interlock that also cuts the start circuit | While the door is open the machine can neither run nor be restarted | Where a mid-job re-energising or an unintended start has to be prevented |
The order that still has to be followed
An interlock answers the question of whether a person can put a hand inside; it does not replace isolating the power, proving it dead and locking it off. The correct order is: stop the machine, disconnect the main supply, prove it dead, lock off and tag, and only then open the door. The interlock is one additional layer of protection on top of that sequence, not a reason to leave out the steps before it.
In the same way, an interlock does not replace dealing with the moving parts. The pump coasts down after the power is cut, so wait for it to come to a complete stop before reaching in, and remember that the heating elements stay glowing after the heating circuit is disconnected, so that waiting time applies as well.
The interlock itself also has to be checked periodically: whether the switch travel is correct, whether the contacts are healthy and whether the reset works. These belong in the inspection checklist rather than being discovered on the day the door genuinely has to be opened.
A prompt hands the responsibility to the person, an interlock hands the outcome to the mechanism, and a safety measure can only be built reliably on the second of the two.
