UPS Input Connection Without Neutral

What does it say in the Manual that came with it?

What did the UPS manufacturer have to say during the telephone call to its technical helpline?

Is the output galvanically isolated from the input and what does the downstream wiring do in terms of earthing and the provision of an IT earth system?

If the UPS's case is electrically connected to the designated neutral input, hooking it up to your 220/3 supply will energize the case to ~127V, which is dangerous. Discuss with the mfr how this unit is grounded/earthed, and what modifications may be needed.

Whilst I am not particularly au fait with US wiring rules, here in Australia it is expressely forbidden (and highly dangerous) to have any connection between the neutral and appliance casings at any point, or the neutral and earthing conductors at any point other than the main distribution board.

I would have thought that this would also be the situation over there, and as such there would be no appliances with a neutral connected to their cases on purpose.

I also question the assertion that the case would be energised even if the neutral were to be connected to the case, do you not have functioning earth connections that would immediately cause the CB/RCD to operate and so disconnect the supply if this were to occur? I assume that if the case is metallic (and the appliance not of class 2 configuration), then an earth connection to it would be a legal pre-requisite

I would have expected that a 220v single phase appliance such as a UPS would perform OK across two phases of the same RMS voltage (a motorised appliance would present a different situation), again, I am not up to scratch with your electrical system, so a bit of clarification would be appreciated.

The case ground connects to ground at the the distribution panel. The neutral connects to a neutral bar, to which ground wiring is also connected. Thus, indirectly at least, the neutral is connected to the appliance casing. If, in the OP's situation, an ungrounded L2 phase is connected to the appliance neutral, there will be a ground fault. If there are protection devices, the appliance case may be energized only momentarily. But still....

I would expect that if the OP were to connect the 2 phases to the UPS rather than 1 phase and neutral, then a neutral connection back to the neutral bar from said UPS would not be made. How then can the case become energised at all?

Why not? If there is a case ground back to the distribution panel, and if there is a neutral also grounded there....

My main point was to be sure of what grounding provisions may (or may not) be on the UPS, and to be sure there are no conflicts.

Only the OP knows whether the casing is metal or non-conductive. Only the OP knows whether a metal case is grounded or whether it is a double-insulated piece of equipment that doesn't need a ground. Only the OP can answer the questions raised so far.

The neutral at the input or output of the UPS?

Surely the source neutral is not bonded to the output. That would defeat the power factor correction, harmonic balancing, and isolation features for the UPS.

And would be a violation, to boot. There shouldn't even be a place to bring the neutral into the system and bond it to the output.

That should never, ever be true......but buying stuff from China means that it might be a good idea to check.

But even if you don't, it should drop off the breaker for that circuit....

...and what about the neutral connection from the N inlet terminal to the N outlet terminal? If they are connected through the UPS, then the outlet N terminal becomes live with no downstream circuit protection!

The forum is still awaiting information from the original poster, and to suggest the L1 and L2 connections in its absence is premature, at best.

I gotta say that I have never seen a UPS with through connections of the mains input to output and would be interested to see a circuit diagram of one such beast.

They normally incorporate an isolating step down transformer to feed the electronics for battery charging and switching and then feed that DC to an inverter for either backup or continuous supply depending on type. Even the switchmode UPSs have double pole isolation.

Keeping the input totally isolated from the output and using electronics to clean up the waveform is one of the primary functions of a UPS in order to prevent line distortions from affecting the load, this is especially the case with line interactive and online types.

Wonderful. So what equipment disconnects a phase-to-earth fault downstream of the unit?

Most smaller UPSs are dedicated to one piece of equipment and their outputs are normally isolated supplies in that both active conductors are above earth potential. Therefore a phase to earth fault is no danger.

Larger UPSs designed to serve multiple appliances usually incorporate their own overload device, and most often these days, a core balance unit as well where one active of the output is tied to earth to form a neutral and to allow the RCD to function on earth fault.

Some larger units are routed via their own distribution board which has all of the necessary safety functions fitted there.

There may be through connected setups out there but it seems to defeat the purpose somewhat, and I suspect they would only be used for very specific situations (which I can't envisage). Would still love to see a circuit for one.

GA.

Surprisingly I was the first....you should get more.

It should not be connected. The neutral and body casing will have coupling capacitor which will not be rated to do duty with this kind of voltage applied to the neutral reference bus. Best way to go ahead is to provide 2 winding step up isolation transformer and tie the secondary neutral to ground and generate 220 volts phase to neutral voltage.

Another point. What about distribution downstream of UPS. The neutral point will be live. If only SP MCB is provided for isolating during maintenance of down stream circuits it will be a huge hazard. If not anything else, the idea itself is bad engineering.

Quite.

Is the input neutral terminal of the UPS connected to the casing (body)?

Sorry, but where do you get equipments whose neutral terminal and body casing are directly connected to each other?

That has not been suggested; instead, an indirect connection has been mentioned.

It's not uncommon to have a three wire feed into a four wire appliance like an oven. In that case, where the house wiring is hot-hot-neutral and the oven is hot-hot-neutral-grd, the neutral and ground are tied together in the box to the house. However, the box must have a good ground as the conduit and the flex-conduit from the oven must be connected securely to both the oven case and the box.

"can expert give us some comments."

hold up people!! In answer to your question as quoted above.. NO!

Why?

Because whatever we say is pure conjecture on our part, as 1. we can't see what you'd got or done, 2. we are not experts in that type of unit, and 3. Why didn't you try the manufacturer first.

anyone answering this question cannot be correct and give up the ultimate answer to your problem. Only the manufacturer can!

GA

...and the forum is still awaiting the original poster's answers to a number of questions with this flavour.

Dear Mr.mohamedihaiah

NO. You should not connect L1 and L2 and strictly the input voltage is to be perfectly matched.

DHAYANANDHAN.S