groundingmainssafety
Testing the Schaffner FN1394-10-05-11 power entry module with a multimeter shows continuity between the ground pin and the body of the module. Consequently, when the module is fitted into an aluminum panel (into a non-anodized, raw metal, opening), the chassis is connected to earth.
In this situation, is a separate safety earth setup necessary, as per this diagram?
When a fault occurs in an un-earthed metal cased device the case becomes live. When you touch it you form a circuit. Now you'd think that current couldn't flow through solid wood, etc. Well, that would be the case for DC, but not for AC. You see, you form part of a capacitor. You're basically one plate of a capacitor, the ground being the other. The floor, your boots, etc, form the dielectric insulator between the two.
The capacitor would block any DC current, but it allows the AC to flow quite happily. The equivalent circuit would be:
simulate this circuit – Schematic created using CircuitLab
The resistance of your body, and the resistance of the ground both limit the amount of current, but it only takes a tiny amount to kill you.
Now with the case grounded, and typically this involves a wire connecting the ground direct back to the ground point, which is also connected to neutral, and a fault occurs, the current will flow straight down the earth wire back to the neutral. This is a much much lower resistance connection than any human could provide, so considerably more current flows. This results in the fuse blowing isolating the circuit.
Also, if the fuse were not to blow for whatever reason (too high a rating?) the low resistance path would effectively short out the human reducing the current available to shock you to an absolutely minuscule amount.
The UPS includes that connection when required internally. That's why there is a "UPS Neutral Out"
Most consumer products labeled "UPS" are actually SPS systems. UPS (Uninterruptable Power Supply) means that the nominal power path includes the battery output. That way power comes from the UPS battery power path regardless of whether there is any AC input or not... hence output is uninterruptable.
Consumer units tend to be Standby Power Supplies where the output is switched (quickly) to the battery path when mainline AC becomes unavailable.
Commercial systems are designed to achieve complete isolation from the power grid when it faults. Consumer systems tend to lack the relevant circuits (ostensibly for cost reduction reasons) and rely on GFI, breakers, fuses, and/or other safety devices for protection. In the event that those protective devices are activated, the "UPS" will no longer function, but no fire or shock will occur.
Commercial systems are designed to go further and continue to operate in the presence of grid fault and to contain the grid fault to the external side of the power supply.
If you have an SPS, the internal connection is already present inside the device and shorting it externally could interfere with safety devices.
If you have a UPS, there are intentional switches to isolate the neutral and bypassing them will disable this protection.
It's not going to help you in the best case; it will hurt you in the worst.
Best Answer
the problem is that raw aluminum quickly forms an aluminum-oxide coating which is insulating. This is the reason for the recommended ground connection with a toothed washer to cut through the oxide coating and contact the metal.
The module appears to have a plastic front through which the screws go so the mounting screws will not provide a ground path, even with a toothed washer. The incidental contact of the module body to the edges of the opening will not prevent formation or break through the oxide layer so there will not be a good ground connection.
To ground the aluminum chassis, I highly recommend following the diagram.