UPS bonding

[chris_carlson]

I am trying to find out proper information on bonding of a ups system.

The normal feed has a 4 wire (3 phase, ground) input to the 160 kVA UPS. The AC is converted to DC, and back to AC through the inverters, while charging the battery circuit (trickle charge). The output has a 5 wire (3 phase, neutral, ground). The UPS has a bypass feed input which has 5 wires (3 phases, a neutral {bonded to ground at the input transformer approximately 75 to 100 feet away}, and ground) input. That same neutral is directly connected to the load neutral and UPS output neutral. While in normal operating conditions the normal feed has a balanced current draw on the 3 input phases of about 80 amps each. The bypass 3 phases inputs have no current but the neutral bypass current is about 14 amps. The two feeds have grounding straps connected the conduit to the ground. The normal feed conduit straps have less than 1 amp of current. The Bypass conduit straps have about 3 amps of current.

Some of the specific questions I have are:

1) With a 3 phase input and a 3 phase, neutral output, is the UPS a separately derived service?

2) If so then I am assuming that the UPS output neutral should be grounded at the UPS output?

3) And If so how do I separate the bypass neutral from the output of the UPS and the load neutral?

4) Would an isolation transformer or a neutral overlapping 4 pole switch be better to isolate the neutrals?

 

[charlie b]

This question is not related to the changes being considered for the 2008 NEC. I am moving it to the Electrical Calculations/Engineering topic area.

 

[dereckbc]

I design a lot of these systems up to 750 KW and am a little confused about your configuration on the bypass. Let me make a stab at your questions first.

1. Yes
2. Correct
3 & 4 Lets discuss.

All of the dual conversion units (AC-DC-AC) I have designed are 3-phase delta input as you described 3-P +G, and the bypass is 3-P +G. The output of the UPS has a built-in isolation transformer and the bypass is on the primary side, so no need for a N.

Even on the output side of the UPS I never use the neutral for distribution. The output is either 208/120 or 480/277. We take the output to PDU's for branch circuit distribution. A PDU is a isolation transformer with built-in branch circuit breaker panel, filter, monitor circuit, alarms, bells, and whistles.

This is pretty standard for large scale UPS, so I am puzzled by your configuration.

 

[dave_asdf]

I've never wired a UPS but i think i've read a manual on just about every double conversion ups bigger than 150kva made. Check the user's operation/installation manual. most of them will give specific instructions, if not call up the manufacturer. dereck's answer sounds right but i know some UPSs need to be wired a certain way. some are separately derived some aren't.

 

[dereckbc]

some are separately derived some aren't.

Dave, any dual conversion UPS (AC_DC_AC) has to be SDS.

However I do agree with the rest of your assesment. After thinking about this for a while I suspect there are optional bonding straps that will allow him to provision in the manner I described.

 

[dave_asdf]

" The ----- 480/480V UPS is not a separately-derived source. The ----- 208/208V UPS is a separately-derived source. The neutral must be grounded on a separately derived source (like the ----- 208/208V UPS)."

i took that from a UPS tech manual, i'm not sure how the UPS isn't a separately derived source but i would assume it has something to do with where the static switch connects to the input/output and where the feedback contactors that disconnect the rectifier/inverters from the input/output are located.

 

[ron]

Dave,
It depends on whether the output of the UPS contains a neutral. The 480V model may not have a neutral in the output, since it will eventually serve a transformer downstream. However some 480V UPS outputs do have a neutral anyway, even if they will serve soley 3 phase loads, if they will be paralleled with another module, the neutral is needed for syncronization and control purposes.

 

[dereckbc]

the neutral is needed for syncronization and control purposes.

Ron you have me confused. Sync control is done at the input and the output is monitored. Maybe I am just looking at it differently. What does the neutral have to do with sync, that the EGC cannot supply on a 4 wire delta input?

 

[ron]

Dereck,
Honestly, I don't understand the entire reason, but a project a while back, the UPS manufacturer required 3 phase 3 wire in the input to the modules, and 3 phase 4 wire on the output of the paralleled modules. They said it was required since they were paralleled. The neutral conductor only carried a very small current, so we we able to sneak a small conductor in each raceway after the fact.
So now, I just do it that way.