Some RCDs open their contacts when the supplying net shuts off and do not automatically reconnect when the power returns. A brief black-out (say, some 10 ms) may be enough.
Your substation may have experienced such a black-out.
To be sure, you could try to find a data sheet of your particular model or do a test if you are able to safely turn off the net supplying your RCD.
If refrigerators or alarm clocks are connected to such RCDs, such a behavior would be a major annoyance. If machines, stoves or tools are connected, they may remain switched on while unpowered and you may even need this feature to avoid unintentional turn-on once the power to your home returns.
The printer draws large amounts (20+ Amps) of current at 12V and these supply exactly that - nothing more, nothing less!
The power supply will certainly deliver less current if the load demands less. If the 3D printer draws 1A, the supply will supply 1A. Your statement that the power supply delivers a certain power level - no more, no less - is incorrect.
I'm looking for a more stable power supply for my 3d printer.
You haven't mentioned stability in your posting. You did say that at high loads, the output voltage sags. The stiffness of a power supply is related to how well the voltage regulates at high load. The stability of a power supply is related to how well the voltage regulates when the output is subjected to a rapidly-changing dynamic load.
There could be two things going on causing the voltage sag:
1) The voltage sensing point is close to the power supply; it is regulating the voltage at that point and the loss you see at the load is due to resistive losses between the sense point and your measurement point
2) The power supply is entering a protection state and is limiting the voltage to limit the output power and keep the power supply thermally safe.
I just have a few concerns about the safety of using one of these devices.
A 'safe' power supply, in industry parlance, means it has been evaluated by a regulatory authority and found to comply with certain national / international safety standards for the application in which it was intended to be used. A single abnormal should not cause a safety hazard (shock / fire / shrapnel). The unit should bear one or more well-recognized safety marks (UL, CSA, TUV, etc.)
My main concern is that I noticed there is nowhere to simply plug a mains cable as an input to the power supply.
As Olin pointed out, this is a unit meant to be permanently installed into some other piece of equipment, not something that a user would be expected to swap-in or swap-out often.
However, isn't there a fair likelihood that someone may just pick it up by putting their fingers and short the live and neutral together? The exposed screw contacts look awfully prone to accidental contact with not just fingers, but nearby metallic objects. A built-in fuse won't exactly help here would it?
Notice in the photo that there's a clear insulating shield over the terminal block. That shield is part of the inherent safety of the unit and should prevent against mains shock from casual contact with the unit. If someone wants to hot-screw a powered mains cord onto this UUT, well, they deserve what they get. Not trying to be facetious, but these sorts of power supplies are meant to be installed by 'qualified' personnel who have some basic knowledge.
It's quite common for power supplies like this (meant for use inside other equipment) to get fed from a feed that has a fuse or breaker in it; there's the possibility that the internal wiring may make contact with the equipment itself. That doesn't mean there isn't a fuse in the power supply though (there should be!)
The other concern I have is whether it will be extremely dangerous if I accidentally reverse the live and neutral wires?
Yes, but only if there's a fault. The power supply will 'work' with reversed L and N. However, the power supply internal fuse is in series with the terminal marked L (line). Blowing the fuse means the neutral is now floating, which is a big no-no (netural must never be interrupted) and a big risk that your chassis can become a shock hazard.
At the moment, I am using a typical 320W PSU for a desktop computer to handle the power workload.
PC power supplies have minimum load requirements on various rails and are rarely a good choice for industrial applications like a 3D printer. Spend the money and get a single-output 12V supply that can deliver the power plus has remote sensing capabilities so that you get the best regulation possible.
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This is more or less correct. In fact, you can extend your tree analogy down to the branch circuits off of your electrical panel and the devices you have plugged into them.
At the most practicable location. This can be an electronic remote switch in modern systems, and is often simply disconnected by a power linesman, either at the nearest utility box, or by physically pulling out your meter head.
Controlling the flow of electricity from the power station is actually what is accomplished by disconnecting the power by a switch or other means. The most important thing to note about the means of disconnection is that it is accessible by the power company, but not by the end user or building maintenance, etc.
In many areas in order to theoretically increase market competition governments force the company that owns and maintains the grid to allow others to subcontract the sale of electricity.
The power companies don't need to track individual electrons because power use by each customer is measured, and the total output of each generator on the system is known. From this data, power companies can figure out transmission losses.
The companies that don't own the system but sell electricity are acting as a middleman and the only thing of real significance to note as a result is usually that it may actually be cheaper for you to utilize a middleman, which is somewhat counterintuitive.