ECN Forum Forums Technical Discussion Electrical Theory and Applications 208V single phase

Ethernet rejects induced hum from external wiring due to the twisted pair nature. Hum as a point to point potential between the interconnected devices is not so well rejected. That's the mode I always see when I do run into hum problems with it. It's a problem that can't be 100% eliminated short of running everything on DC. But the higher the voltage between equipment, the greater it can be. A pair of ungrounded 208V systems could have the effect of a 416V system, depending on the phase angles and other aspects at the time. Grounding the 208V systems on one end would prevent it being like 416V. Grounding on the center tap would be even better. There are other ways to mitigate the problem, too, but grounding the power system is one of them.

There's also the risk that the ungrounded system can exhibit higher differential voltage in a surge than an ungrounded system. This can be mitigated by additional surge protection to help maintain the voltage equality (e.g. clamping the 208V system into ground, during the transient).

Here are two different UPSes. The first appears to be a single ungrounded-output inverter. The second (more costly) appears to be grounded.

http://www.apcc.com/products/resource/include/techspec_index.cfm?base_sku=SURTD5000RMXLT3U

http://www.apcc.com/products/resource/include/techspec_index.cfm?base_sku=SURTD5000RMXLP3U

A simpler case, but much the same. I want to bring a 240V 20A circuit to my computer room at home. Output will be a dedicated 6-20R or L6-20R. As you know, this is a split-phase system, and both line wires will be 120V relative to ground and no neutral is present or needed. So I want to have a UPS provide exactly the same thing for a group of computers, without bothering with a transformer. What are the issues?

A 240V system has the 2 separate 120V waveforms at 180 degrees between each other. The only way to get 208V out of 2 120V circuits is for them to be at an angle of 120 degrees.
For that you either need 2 legs of a 3-phase circuit, or you need to play with the phase angles using a transformer.

I agree if the GROUNDS between the systems have a shift it can show up in the ethernet but in switch mode power supplies the DC voltages are very well isolated from the line voltage.
We have had to go as far as to bond the frames of remote machines together to avoid transient problems.

gfretwell ... I think that's what I'll do, since it looks like I'll probably be using the UPS that has a single 208V output only. So now I need to find a good ground strap. Fortunately all the machines will be in a room, metallically isolated from anything else (network connection via fiber), with an SPD on the subpanel that feeds them (and nothing else).

Where we have seen ground shifts were in places where there were separate grounding systems in separated buildings but the buildings were connected with LAN cables. We bonded the machine frames to prove the problem but in the end the right fix was probably bonding the ground electrode systems together.
At that time I was in the data end of the business and bonding machine frames was the fix we often used. If there was a scary amount of current on that wire we got the electricians involved. If the voltage was much more than 35 volts or so we wouldn't touch it until the grounding system was inspected but 20-30 volts was not unusual.

In the same room, I'd bond the machine/cabinet/rack frames together. At greater distances, these days, the way to go is fiber for data. Then just power each "zone" separately, with its own surge protective device. All communications in/out of any "zone" has to be restricted to fiber only unless it can be protected with the SPD for the power (not something you are likely do be doing in an I-Line, for example).

This has turned out to be quite a detailed thread!!!

I have some comments regarding the items covered so far.

Per the UPS equipment specified by the OP "pdh":

After reviewing the Products' Documentation, I am unsure if either of the Two Models will have a Grounded Output - while the Output is running Isolated via the Inverter / UPS Transformer (in Normal Mode or in Bypass Mode).
OP indicates that, after contacting the Tech. staff for verification of a Grounded system, it appears that the single output voltage system does not have a Grounded System Conductor.
This would be the UPS with an output of 208V 1 PH. 2 Wire (with EGC).

The above UPS _MAY_ have a Center Tap on the Secondary, and _MAY_ be Grounded via that Center Tap.
Results would indicate 104 VAC L-G from either of the 208 VAC lines, and would deliver a Grounded system without a Grounded System Conductor.

It could be possible that either of these UPS units do not have the Secondary Winding bonded to Ground. Only a Schematic or Field Test could verify.

When the UPS is set in Bypass Mode, the Output is directly connected to the Circuit driving the UPS Input - and the Isolated System is not connected.

As to using an Ungrounded System (setting up an Ungrounded System)...

Let us first off designate what will be considered an Ungrounded System for this scenario.

My description of an Ungrounded System is:

*** A Power System of "X" Volts AC, which does not have a DIRECT connection to a Grounding Electrode System, yet will have Metallic Enclosures Bonded to a Grounding Electrode System, along with a Measurable Voltage-To-Ground across a given Impedance ***

The Voltage-To-Ground on the above described Ungrounded System will vary, according to the Impedance between a given System Conductor and the Grounding Electrode System.

The higher an Impedance value is, the higher the Voltage between the Two points will be.

My description of a Grounded AC System, which does not use a System Grounded Conductor would be:

*** An AC System, which has a DIRECT connection to a Grounding Electrode System, yet does not utilize a System Grounded Conductor with the normal "Output" Circuitry from the Secondary Winding(s) of the Transformer, but does utilize Equipment Grounding Conductors which are Bonded to both the System and the Grounding Electrode System ***

In the case of the above System, the Center Tap of the Secondary Winding (or one Secondary Winding), will be Bonded to the Grounding Electrode System, without deriving a Grounded System Conductor from that Tap point.
All EGCs will be bonded to that Center Tap, as well as the GES.

The 60/120V 1 Phase 2 Wire System is one example.

The issues involved with high L-G Voltages experienced on an Ungrounded System may rapidly damage SPDs, as well as line Filters at the input of a SMPS.
The typical MOV would be subject to high L-G Voltages, corresponding to the Impedance of the device.

To reduce the issues connected to using an Ungrounded System - in addition to using an AC Circuit without a Grounded Conductor, the "Center Tapped" System above would be a good choice.

I agree with the Fiber Backbone idea between two or more Buildings, and/or for long distances.

Even with all Electrodes connected together, there will still be a difference in potential between the two Electrodes.
Circulating Currents are likely to flow on Conductors bonded to the GES.

Scott

Most of the places where we were having problems with remote terminals did not have enough on the far end to justify running fiber. At least not in the customer's mind.
It was usually just one point of sale terminal out in a tiki bar, restaurant or golf cart barn/starter shack.
They only got blown up when we had a thunder storm but that was every afternoon in the summer.

I protect my home computers by having the internet connection done via wireless, with a pre-configured pre-tested backup router ready to replace the one in use. Electric comes in underground (underground MV to pad transformer and underground LV to home) and goes through point-of-use protection, then UPS, then a couple IsoBar filters (not cascaded). I'm intending to add further surge protection to the panel some day.

I'm still trying to figure out a good way to switch this to 240V L-L.