elevated neutral voltages

[scroot]

Here is a problem that is coming up more and more.
I thought I would see what all you guru's can come up with. I have my own thoughts but collaboration always seems to help.

I have a middle school, 208V, 3 phase , 4 wire service. All grounding and bonding done per NEC. N-G voltage at mains = 0V. A panel fed from main gear has 3V N-G voltage. Some UPS's fed from this panel are going haywire switching back and forth frantically between line power and battery.

What to do?

 

[don_resqcapt19]

Re: elevated neutral voltages

That voltage is the voltage drop on the grounded conductor as a reasult of the load. A school is a occupancy with a lot of the load nonlinear, and there may be excessive grounded conductor current that results in the voltage drop. I would first check all the connections in the grounded conductor as a poor connection will also result in excessive voltage drop. I'm not sure why the UPS should be having problems because of the voltage between the grounding and grounded conductors. If the voltage drop is caused by high grounded conductor loads, you could replace the neutral with a larger one or pull in a parallel grounded conductor if the original is #2 or larger using Exception #4 to 310.4. A second solution would be to install a transformer at the panel to serve this load. You would be bonding XO to ground at this point and reduce the grounded to grounding conductor voltage to very near 0 at the panel.
Don

 

[scroot]

Re: elevated neutral voltages

Thanks Don.
Those are good solutions.
I have also had this problem with some battery exit signs. Some of the new electronic devices dont seem to sense the incoming power correctly and elevated voltages on the grounded conductor cause switching between battery and normal power. These UPSs seem to be doing this also.
I guess this is another reason for double sizing the grounded conductor or using more transformers to make more Xo's in the building.

 

[dereckbc]

Re: elevated neutral voltages

Sensitive electronic equipment is sensitve to the N-G voltage, especially if there are switch mode rectifiers invloved like a UPS.

I agree with Don, there are two ways to solve the problem. Least effective is to oversize the neutral. It should be designed to keep the N-G voltage below 2 volts at full load.

The most efeective way is to install a K rated isolation transformer with an oversized neutral close to where the power will be utilized. This will establish a new N-G bond.

Do not make the mistake a lot of quacks do by bonding the neutral again downstream to minimize N-G voltage.

[ March 18, 2003, 01:12 PM: Message edited by: dereckbc ]

 

[don_resqcapt19]

Re: elevated neutral voltages

Dereck,
Why is the UPS and battery back up lights even looking at the grounded to grounding voltage? I understand that this voltage is often a problem for interconnected electronic equipment, but this is the first time that I've heard of it causing a problem with UPS systems.
Don

 

[dereckbc]

Re: elevated neutral voltages

Scroot, what kind of UPS is being employed?

Is it a standby type where the load is normally connected to the utility, then switched if power fails? They are used for lighting where a mometary interuption can be tolerated.

Or is it a staic UPS where utility power is converted to dc through a rectifier, and converted back to ac through a inverter? These are used for critical electronics that cannot tolerate a power interuption.

 

[scroot]

Re: elevated neutral voltages

The UPS's are APC smart UPS's and I haven't actually seen them yet.
This is what has been reported to me from the school's electrician.
They are static UPS's serving some of their network servers at the school.
And yes I'm afraid to say that the short term fix that was used was the very dangerous N-G bond at the panel. I have instructed the electrician to remove this bond he created.
Also, one more point I have learned for what it is worth: The UPS's have 3 setting switch for the incoming power; HI, MED, and LOW sensitivity. When the UPS's are on high, they go into their psycho switching mode. When they are on MED sensitivity, they only intermittently go crazy. They are trying the LOW sensitivity mode today. I will be going on an invesigative mission this week.

Your comments have been and are greatly appreciated.

Scott (Alias scroot)

 

[dereckbc]

Re: elevated neutral voltages

Scott, I am familar with APC UPS, mostly the large ones used for data centers. If you can get the model number I might be able to help you.

Right now all I can do is take shots in the dark. Could be the fault wiring detector built into the "Smart Series" or could just be extremely noisy ground from being in a shared branch circuit. Beats me what is going on without some details.

If you get stuck, and need some help, give me a shout, or ask on the forum, I am sure someone will be able to help.

First thing to look at is go to the recept where the UPS is connected and see what is going on. L-N voltage, L-G voltage, N-G voltage, dedicated, shared, IG, harmonics, etc. If you have a Sure-Test meter use it and cycle through all the modes.

 

[dereckbc]

Re: elevated neutral voltages

Scott. was sitting around scratching my head (seem to be doing that a lot since I was laid-off) with your problem. You mention a large N-G voltage of 3 volts, which can be interpeted as a 6 volt line loss.

Now since the UPS has a sensitivity switch, it is possible the 6 volt line loss is being seen as a brown-out at the UPS causing the indicating circuitry to chatter.

Just a thought, if you can give the model number, and voltages I can look in my books and get a clearer picture.