Is it possible to connect the minus of the output of a pulsed power supply unit and the zero of the network?

Question

Microcontrollers

Good day to all! There is a main three-phase network, and a backup single-phase network.
It is necessary to:
1. Measure the voltage on 3 main and one backup phase
2. Additionally determine the presence of voltage at 5 points of the shield, into which the initial 3 + 1 phases are connected.
I want to:
1. Install a three-kopeck PIC with an ADC, to which connect 4 measured phases. I connect through a divider and 2 limiting diodes, one pulls up to + 5V, the other to the ground, in order to at least somehow protect the ADC input.
2. Measure the RMS voltage relative to zero
3. Transmit the measured voltage values ​​to the main controller via UART. We separate 2 parts of the circuit through an optocoupler in order to get rid of the dangerous galvanically non-isolated circuit
4. Power the hot part from a pulsed PSU that I picked out from a Chinese phone charger for 50r) In this case, you will have to connect the zero of the network (namely zero, not the ground) and the minus of the PSU output.
5. Make the hot device as cheap as possible and arrange it in the form of a removable module, so that in which case, pull out and put a new module, and burnt out in a bucket, like a fuse)
p.1, p.2, made, more precisely modeled, to a piece of iron it hasn't arrived yet.
There are no questions with paragraph 3, but with paragraph 4 there is a question. Is it possible to connect minus and zero?
With item 5, everything is also not so simple .. There are special mikruhi for measuring RMS, there are space ... I didn’t find any small-sized and cheap trances through which to measure the voltage either ..
Thank you!

Answer 1

If you like fireworks you can. Well, or if you have a direct current in your receptacles suddenly started flowing, then you can also.

Answer 2

4 point - you can.
It is necessary:
​​1 point - to replace with a three-phase meter with the possibility of taking readings.
2 point - PLC.
- Mikruhi are expensive for such measurements, but they justify their existence.
- Power supplies from Chinese chargers do not justify their existence in the industrial automation device.