TLDR: Shield excludes dielectric losses and evens the stress on inner dielectric.
Real EE stuff below:
Disagree with answers above (below) about safety aspect. No, it is not for safety. The dominating aspect in power distribution is losses. Having AC electric field contained in predictable space will exclude lossy dielectrics and conductors from participating in dissipation of energy (money).
If cable is not shielded, then for 3 of this in 3-phase line, the surrounding air, concrete, soil will be part of line, acting as lossy dielectric in 100 microfarad AC capacitor stretched for several kilometers and having massive dielectric losses.
In extreme case sharp conductive object next to cable will focus potential gradient lines and peirce dielectric. Shield removes this kind of stress completely. Same stress for field closest to central conductor is excluded by using semiconductor layer.
The mystery is why is it a copper. Possibly, if one do the math, the aluminum or iron will not be as efficient for the same (dielectric loss imunnity) aspect.
Digging firther: If shield is not conductive well enough, then ohmic voltage drop at shield at far point of the line (induced by zero-turn coax transformer + line as capacitor) can get to hundreds volts and cause other troubles. Here you have partly safety and losses covered better with copper than with aluminum.
And perhaps the shield is also have to be grounded and cross connected for 3 cables in few middle points of line for the same "loss reasons" to reduce induced current and shorten the shiled current path as 3 phase trigonometry give such advantage (advantage to create virtual floating ground midway on long line or just real ground).
Another observation: If it is russian customer in Moskow, then there probably is very limited space for power transformers in city, so such cable is economically reasonable, when there is need to deliver relatively low voltage with very high current from the parcels with less land cost to very expensive land parcels.
About zero-turn coax: One power station generator in Ukraine has 50KV/10KA outputs shielded with massive copper tube, opened on one end and grounded to generator's frame. At open end the voltage is about 500V. The AC current of the tube is unknown, but maybe be close to zero or few amperes. If not for this tube, then much higher current induced by open 3-phase capacitor could run through iron rods inside building walls, D/E losses will also heat the concrete walls and melt everything.
Each apartment is connected to one phase, different apartments are connected to different phases with interleaving so that phases are hopefully loaded equally.
Yes.
He calls a serviceman and the serviceman claims the substation transformer feeds such voltages and the only thing he can do is to disconnect the tenant from his phase and connect him to the phase with 215 volts.
Yes.
Is it unequal load on different phases or anything else? Also has the serviceman done the right thing or did he just put extra load on the higher voltage phase and induce risk of distribution failure?
Serviceman did the right thing.*
I think you've answered most of your own question!
The substation is presumably supposed to have some kind of voltage regulator (after all, the transformer has a nonzero output impedance, so voltages sag with load). I assume it's theoretically possible that this could be done on a per-phase basis, but my guess is that they only do it as a 3-phase set, in which case the imbalance is caused by unequal loading.
*caveat: I suppose technically someone should measure the currents out of the transformer, and make sure that the imbalance in currents matches the sag in the voltages. If the currents are about the same but the voltages are not, then it could be a higher-than-normal impedance somewhere in the distribution network.
Best Answer
Typically you want to completely isolate the AC voltage from the rest of the circuit. This is a safety issue, and it's done to make sure no harmful AC voltage leaks into the DC circuits. Most people would even recommend isolating the neutral line because there could be harmful current on that wire as well. This is usually done with a transformer. The delta-connected AC will go into a transformer. The output of the transformer goes into the rest of the supply circuit or whatever it is. You said this was a power supply, so this will most likely be a step-down transformer.
The DC lines should be isolated as well, depending on the use-case. If you are running DC power to many different devices, or chaining supplies together (which you mentioned), you will want to isolate the DC outputs to avoid ground current loops. Chaining boards together introduces the possibility of different ground references. When two ground levels have different voltages, a current will flow. This can cause noise in your DC circuits. Over short distances, this effect is minimized. You need to decide if you need to isolate DC outputs. Usually this is done using a transformer with multiple taps, which gives you multiple isolated AC outputs, which are fed into separate power supply circuits. If you don't care about noise or ground currents, you don't need to isolate the DC outputs.
To answer your original question, I would not connect the shields together. Typically with power supply circuits you want to completely isolate AC power sources for safety reasons, and isolate DC power sources for noise reasons.