Followup on the 200% neutral thread / harmonics and real world

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bbaumer

bbaumer

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Indiana
Have any of you actually measured neutral current that is higher than the phase current on an otherwise reasonably balanced 208/120Y system?

The reason I ask is I researched this quite a bit several years ago when K-rated transformers and 200% neutral busses were relatively new and being pushed heavily by the gear reps.

I used to work at a MAC school university with 18,000 students, 40 buildings and dozens (100's?) of computer labs. Most of the buildings were older and the labs were converted classrooms or other spaces. No new K-rated transformers were installed nor were any 200% neutrals. Power for the labs was just added to whatever was available.

I took numerous measurements with a Fluke 41 or 41B (can't remember now) harmonic analyzer. Not once did I ever find true RMS neutral current near any one of the phases or close to exceeding the conductor ampacity (pretty much all "full size" neutrals). I would routinely see high current distortion but voltage distortion was almost always under 3%. If it was higher than that it was not by much.

I do agree that, on paper, triplen harmonics will add but the distortion from PC's and other power supplies occurs at random angles and ends up having a cancelling effect anyway. In order to reach the 173% theoretical maximum neutral current you'd have to have condition just perfect for that with your loads and that just will never happen.

Again, anyone actually seen a transformer failure that you were absolutely certain was due to harmonic current and/or higher neutral current than phase current?

I never have and I've measured a bunch of locations. I'm wondering how much of this "problem" is scare tactic and how much is real. I know the trade magazine sure make a big deal out of it and have for years.

thanks,
bbaumer
 
bbaumer said:
I'm wondering how much of this "problem" is scare tactic and how much is real. I know the trade magazine sure make a big deal out of it and have for years.
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Very interesting post. :)

I have no doubt in my mind that the harmonics issue was/is very hyped up by the manufacturers and Copper.org.
 
Another member brian john does a lot of electrical troubleshooting and testing.

He has said in the past when he has found overloaded neutrals where in facilities that had a poor design to start with. Buildings that had changed use without electrical up grades etc.

You may want to PM him. :smile:
 
In short, yes, but not commonly.

My experience is in 415/240 land. But I would see no reason why 208/120 is any different.

In data centres, its quite common to find neutral currents over the highest phase current. And now that much equipment has multiple power inputs and often master/standby power supplies, the situation is not static. Balancing the load between all power supplies is much more polite, and is now more common than the old normal / redundant arrangement.

In brokerage firms where there is lots of electronics on the trading floor, then yes, high neutral currents common. Have seen an incidence of smoke issuing from vertical busbars, requiring the addition of an on-floor K rated tranny.

In most buildings, even given the prevalence of high frequency fluorescent lights, PC, printers etc, 100% neutrals seem to survive happily.

The other places this happens is theatrical light dimming, and since currents can be adjusted with the push of a slider, you can play and measure neutral currents. Large power amplifier installations can also have screwed neutral currents; I had a good second hand report of this but cant find it now I need it!

To see a simple example of light dimmers with neutral current greater than phase see this slide. The first line of this forum post also refers, albeit from Australia.
 
dbuckley said:
In short, yes, but not commonly.
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How are circuits usually loaded in your area?

Here we are limited to 80% if the load is considered continuous and no engined prints I have worked with will even come close to that level of loading in data centers. Often one circuit per equipment and the feeders will be well over sized.

I have seen an concert audio system feeder really load the neutral badly well beyond the phase conductors. Unfortunately the entire feeder was inadequate to start with. At the same venue the lighting guy was dragging the 120 volt down below 90 volt and causing him issues with his control system.
 
In the good old days, a rack would be fed with a couple of 16A (240V, so good for 3KW) sockets, and the load would normally be well under 3KW for the rack. Many data centre were designed for 2KW per rack heat extraction, so 3KW per rack would not be acheivable over a significant space.

So for this kind of installation the overall rating will be well below 80% anywhere, and usually the lowest rated component (and thus the component operating nearest its limits) is the UPS.

These days its much leerier, with racks wanting 25KW...

I cant help but post a link to this thread on a non-electrical forum...
 
dbuckley said:
In the good old days, a rack would be fed with a couple of 16A (240V, so good for 3KW) sockets, and the load would normally be well under 3KW for the rack. Many data centre were designed for 2KW per rack heat extraction, so 3KW per rack would not be acheivable over a significant space.

So for this kind of installation the overall rating will be well below 80% anywhere, and usually the lowest rated component (and thus the component operating nearest its limits) is the UPS.

These days its much leerier, with racks wanting 25KW...
Click to expand...
When I was in IT we started out with six 6U servers per rack and one 4U 1500 UPS in the bottom.

When I left that field, we were putting 34 1U servers from the top down and 4 2U 1500 UPSes from the bottom up...

Multiply that increase by 30 racks and you can see how the power requirements substantially changed over the years.

The A/C could never keep up- it never got upgraded...
 
We have won lots of bets ($200 ++),from groups claiming the phases are balanced to within 5% and therefore the neutral current has to be almost 0,for sure not greater then the phase currents.

Typically installations are data centres and audio/video centres.We have measured many installations where the neutral current is approaching 200% compared to the phase conductors.:smile:
 
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