My question is regarding a 3-phase delta maint bypass circuit seemingly feeding a 3-phase wye panel in Italy. In the US, it would be illegal to do anything but bring the bypass neutral directly from the switchboard- does IEC (or any other european code) allow deriving the neutral at the branch panel or an arbitrary mid-point of the circuit? (No transformers, it's straight-through.)
Could it possibly actually be wired up this way, or do I need to beat up my eyes and ears for giving my bunk information, and try to get my boss to pay me to fly out to Europe again?
In older European installations, you will certainly find that the neutral and grounding conductor have been derived from the combined grounding/neutral conductor in the last subpanel. It is still legal to some extent, but hardly recommended and may conflict with other parts of the regulations.
German code at one time allowed you to split the conductors at the receptacle. (Sic!)
[This message has been edited by C-H (edited 08-08-2006).]
I'm not quite certain I grasp the precise wiring configuration here.
Is there just some sort of balancing arrangement between the phases to derive a neutral, or is there a grounding conductor to the wye panel and then the neutral is "borrowed" from the panel frame/ground as C-H is suggesting?
The latter arrangement would certainly not be to "code" in the U.K. where neutrals and protective grounding conductors must be kept strictly separate beyond the service entrance. Italy? No idea......
This is a maint bypass circuit for a UPS system- the UPS operate at a different voltage than the switchboards and thus has step up/step down transformers. This bypass circuit is also coming from a different switchboard than the primary UPS circuits.
The existing system had a UPS working at switchboard voltage, and all feeders (UPSs 1 & 2, static switch & maint bypass) are 3-phase delta bus duct. UPSs being UPSs, they all derive the neutral on the output, no problem there, and the (new) static switch will run through the transformers, but the maint bypass... when in bypass, the UPSs might as well not exist- I can't very well have delta input to the switch and wye output! At least not in the US. Unless it's legal in some codes to just sink a cable to ground anywhere along the run and call it a neutral. Sounds like that MAY be the case? As I stated in my OP, we always like to take the stricter of the two codes, which means I potentially have a "problem" here if the panel *is* wye with a deta feeder.
Unfortunately, I'm in Virginia and this site is in Italy, so I can't easily verify... I'm trying to get someone local to double-check for me, but having trouble finding someone qualified/availible to do so. And kicking myself for not looking closer when I popped open that critical panel in february to look at the bus tap to the other critical panel. Such a fundamental question, but so easy to miss...
[This message has been edited by SteveFehr (edited 08-09-2006).]
To my knowledge, you just can't technically have hot-neutral loads in a delta system! So if there's a neutral popping up somewhere, the only possibility is a wye without a neutral provided, which is legal in most European countries (no need to bond the neutral if it is a TT system, so if there are only symmetric loads you don't need a neutral, can omit it right from the transformer). I don't think adding a neutral from some other circuit to such a system would be legal.
On the other hand, in some countries it's legal to split up the yellow-green PEN far up in the system, so what looks like a ground might actually be a combined ground and neutral.
I really think we have problems with technical terms different on both sides of the pond.
An "IEC-neutral" is connected to the star point of a transformer or genny. Almost always this constellation provides two different voltages A(V)= 1,73 *B(V) with three phases at 120°, although sometimes only on e voltage is used. Any conductor between this star point and a load is practically called "neutral" <>AE:~ grounded conductor.
So if your switchbord is running on two voltages you can't use a delta supply.
Let's assume your installation is running on just one voltage, then no neutral is necessary. Maybe it is like that in your case.
Next thing is protection when faulty. For this different systems/philosophies exist.
These will be described by the TN, TT or IT letter combinations.
Let's assume you need one voltage, but the philosophy ist TN. Then you will have to connect the gr/ye PE conductor to the star point AND to the ground. No star point ->no TN. If the short circuit current is sufficient, no RCD (~GFI) is necessary, otherwise yes.
Let's assume you need only one voltage, but the philosophy ist TT. Once more: no star point -> no TT. The star point and PE conductor are grounded separately. RCD is mandatory. In a local system like yours possible, but doesn't make sense from a German point of view.
Let's say you're philosophy is IT. Then no star point is necessary. Protection becomes a bit more complicated.
At this point I'd like to know whether it makes sense to continue? Did I get your point?
BTW: all other configurations like open deltas and corner grounded deltas or other weird stuff are extremely uncommon.
The switchboard is a single voltage- 208V L-L delta / 120V L-G wye. The Neutral (center of the star, at 0V reference to ground) is grounded in the switchboard for sure. There are no transformers in series with the maint bypass I'm talking about- it's straight through to the maint bypass switch, and from there to the critical panel.
The UPS comes off of another switchboard (also 208/120V) but the voltage has to be stepped up to 400V because nobody makes a 208V 50Hz UPS in this size.
Could the ground bus from the switchboard be a combined ground/neutral? Does European code allow using a combined ground/neutral?
UPS inverters are fed from switchboard A, and static bypass and maint bypass are from SWBD B. The switchboards can be bus tied together, but normally aren't.
If it is a Wye, then you need a delta-wye transformer for the maintenance switch.
If it is a center tapped delta, then you need a delta-wye transformer for the maintenance switch.
How does the output of the UPS’s output transformer get reference the earth? Since there are transformers on the input and output of the UPS’s, is there an issue with phase shifts? How does the maintenance switch get used? Is it thrown and the UPS’s are brute force paralleled with switchboard B?
Questions, questions, questions.
Edit for content.
[This message has been edited by LarryC (edited 08-10-2006).]