EC&M - Latest Comments

Re: Fundamentals of Harmonics — Part 3

ram kumar — Fri, 20 Nov 2009 15:49:18 -0000

we are having 30-32% of harmonics in our power system. we need to calculate the loss in terms of power cost per unit. can anybody help us to do the calculation

Re: Whats Wrong Here?

sparkmaster — Fri, 20 Nov 2009 14:56:42 -0000

Lets see, romex unbushed, routing in front of stud,[left] looks like a mix of 12-2 bx and 14-2 romex, ground missing, no way to bond to bx. no pig tail on nuetral, wire fill, box not approved for installation.

Re: Simplifying the Search for Unwanted Ground Current

meby — Fri, 20 Nov 2009 10:54:14 -0000

Here's an interesting exchange between a reader and the author of this article.

READER'S QUESTION

Tom,
Just read your article on simplifying the search for unwanted ground current and I have a question that I am having trouble finding an answer to.

When is it “too much ground current”? Having conducted a few ground audits in my career, I have measured ground currents from 0.1 amps to 20 amps. I typically flag on total current on grounds above 1 amp and recommend the problem be followed up on and cleaned up (typically an electrician has accidently put a ground on neutral bus or a neutral wire on the ground bus, etc.).
But I would like to tie my recommendation to an actual IEEE or NEC specification. For instance, something like “exceeds 500mA, and according to XXXX specification it should be corrected.”

Appreciate any help you can offer and again I loved your article for the information and its clear application.
--Ron

AUTHOR'S REPLY

Hi,
Boy, wouldn't we all like to have something in the Code - other than objectionable ground currents are not permitted?

We follow roughly your process. Here is the fun wrinkle.

UL limits leakage current to ground to 0.5mA to 3.5mA for plug connected equipment. There is no similar requirement for hard-wired devices. So, leakage current can get pretty large at times. Ironically, if all three phases have the same amount of leakage (at roughly the same phase angle), then some cancellation will occur and single phase leakage could dominate.

Europe is moving toward the required use of earth leakage breakers (GFI in our parlance). They seem to be the ones wagging the dog these days so that requirement might spill over to us.

Best regards,
Tom

READER'S FOLLOW UP RESPONSE

Thanks Tom,
Your response affirmed what I am seeing in the field. I just wanted to make sure I hadn’t missed something in NFPA 70 or even IEEE.

My current statement to the customers is:
“While in a perfect world we strive for zero current on the ground bus, we can see up to 500mA on many panels. Above this level, it not only interferes with many ground tests, it can become a serious safety issue to both equipment and personnel. Any grounds found with over 750mA also becomes a dangerous condition to the facility electrical infrastructure. Facility owners should immediately develop a Ground Cleanup Plan to reduce ground, leakage and net current levels from the facility infrastructure.”

I appreciate your candid responses and the time you took for your article.
Best regards,
Ron

Re: Whats Wrong Here?

meby — Fri, 20 Nov 2009 10:22:50 -0000

Here's a comment I received from a reader on this particular installation.

Two items jumped out at me while reading the code violation description for this installation.

The first is the implied assumption that the electrical installation was first in the space. After looking at the concrete pad the water meter is sitting on, it looks like the electrical installation was placed over an existing water meter.
This may very well be the electrician’s violation. You should have said you may not install electrical equipment in a space occupied by pipes, etc.

The second is that there is no meter bonding wire because one is not needed. Let me explain. It looks like the pipe with the white valve is a meter bypass valve. (Open the white, close the reds, remove and replace the meter.) This allows continuous water supply when the meter is removed.
Without seeing around the corner you jumped to the conclusion that no bond wire is automatically wrong.

Please think about the possible reasons for what you see before passing judgment.

Re: Whats Wrong Here?

meby — Fri, 20 Nov 2009 10:05:33 -0000

Here's a comment I received from a reader.

The Code reference is 110.26 (F), but the photo shows disconnect switches. This section of the Code appears to be applicable to "…All switchboards, panelboards, distribution boards and motor control centers…". Can you let me know how disconnect switches are also included under this requirement?

Re: Whats Wrong Here?

jbanana — Thu, 19 Nov 2009 20:55:16 -0000

Exposed Romex, no connectors, not secured.

Re: Code rules for low-voltage lighting.

meby — Thu, 19 Nov 2009 16:43:49 -0000

Here's a reply I received from the author of this article.

My comment on the comment is that it was indeed valid for prior editions of the NEC, but not now. The 2008 NEC amended 411.3 to create an entirely new category of acceptance that, with some restrictions, allows for mixing and matching listed components. The reader will likely to be able to accomplish what he is trying to do under 411.3(B), particularly if he is not relying on exposed bare conductors.

Re: Maximizing VFD and UPS Performance

meby — Thu, 19 Nov 2009 16:13:05 -0000

Here's a copy of a comment we received after publishing this article in the pages of EC&M magazine.

I’ve read your article and it was very informative. I had a question on something that has recently happened and was wondering if you have seen similar occurrences. I have two 130kVA Powerware 9315 UPSs and one 300kVA Liebert 610 UPS that are backed up by a 450kW Cat Generator (natural gas). During testing the generator had trouble holding frequency. I assumed harmonics were the culprit so I had some metering performed to measure. While we were measuring, one of the UPS technicians was on site and he turned off the filter in his UPS. This seemed to help the fluctuations in the generator. He also recommended to change the “charge rate” in all the UPSs to reduce the harmonics further.

My question to you is, have you seen something like this before and does it seem right that the filters in the UPS should be turned off?
— Douglas Obrist, P.E., Project Manager, RobsonWoese, Inc. Consulting Engineers

AUTHOR'S REPLY
Douglas:
Well, the world of harmonics and power quality continues to fascinate all of us.

Filters are designed (i.e. component values chosen) based on a specific system load on the UPS. In other words, a filter that blocks certain harmonics at one UPS load will not be as effective at other levels of UPS output. My guess is that by taking the one filter offline, based on system conditions and UPS loading at the time, harmonic levels were changed such that generator controls were not as severely effected.

I would assume the "charge rate" referred to is the charging rate of the batteries. If the batteries were recently heavily discharged the UPS would have to draw increased AC current to charge the batteries. By decreasing battery charging voltage (if possible), UPS load would be decreased, and thus, harmonic output decreased. However, it is important to charge batteries at manufacturer's recommended voltage and current.

Modern generator controls, such as the CAT EMCPs seem to handle nonlinear loads quite well. At least that is what I am being told in the field. To compensate for situations such as yours where harmonic loads are the primary load on the generator, generators are typically oversized. At 450kW, your unit has a kVA rating of probably 563kVA. Your nonlinear loads are 560kVA. So, I would consult with your generator supplier as well and get their thoughts on this problem.

However, problems that you are encountering I find are hard to diagnose without putting a power quality analyzer on the equipment and watching to see what happens. In this case it would be helpful if you could put an analyzer at the output of the generator and watch to see what is being reflected back into the equipment in terms of harmonic frequencies and levels. If possible, take note of what happens when: UPS load is changed, filters are switched off and on, and batteries are drawing large amounts of charging current. In addition to monitoring harmonic levels, also watch THD values. See if you can determine at what levels generator frequency begins to de-stabilize. With all of this information you should be able to start to developing some hypothesis and then determine what is the best course of action to stabilize the system.

Work safely,
Randy

Re: Code Q&A

don1 — Thu, 19 Nov 2009 14:38:54 -0000

Where are the places a regular receptacle can be used that is not tamperproof?

Re: Whats Wrong Here?

Bill Gallus — Thu, 19 Nov 2009 13:29:54 -0000

Receptacles mounted to the cover must have more than one(1) screw supporting them per 406.4 (C).

Re: Code Q&A

James F. — Thu, 19 Nov 2009 12:07:28 -0000

does exposed wires on outside of a bulding need to be in some kind of conduit?

Re: Harmonic Distortion: Definitions And Countermeasures

futureFun — Wed, 18 Nov 2009 22:22:10 -0000

Hi,
Thanks for the information. I hear about THD, but what causes it, can you give me an example of when it occurs in signal output, how to identify signal harmonics from the output, and how to correct THD, please? Thanks.

Sincerely,
Jean

Re: Top 2008 NEC Changes

roberteknox — Wed, 18 Nov 2009 10:24:28 -0000

How does Art 680 address Pool Remodeling Contractors adding 1 " remodeling brick pavers to an existing, 20 year old pool, with an existing poured concrete deck, already bonded to the pool structure. What is the requirement? How would a paver installer access 4 additional bonding points on the pool? How could it be buried 4" below grade?

Re: Whats Wrong Here?

doughemrich — Wed, 18 Nov 2009 03:06:08 -0000

LoL.. thats what I thought yesterday , when i was working on some privet owned street lights , LOL 6 light poles spread out over 100 yards and 10/2 romex pulled through pvc all the way...omg i thought who in the ___ did this....lol

Re: Whats Wrong Here?

biker — Tue, 17 Nov 2009 22:29:44 -0000

Are this people serious. I think that they don't know anything about the NEC.

Re: Whats Wrong Here?

biker — Tue, 17 Nov 2009 22:26:02 -0000

Everything is wrong,from the wire being like that, to the pull chain, to the bulb.
All against code. Home honers they don't know out they are doing.

Re: Low-Voltage Lighting Primer

Mark Endresen — Tue, 17 Nov 2009 12:52:02 -0000

can I use regular and burial 12/2 electrical wire for my low voltage light system?

Re: How Much Working Space Is Enough?

Gary H. Lucas — Tue, 17 Nov 2009 09:56:18 -0000

We build industrial waste water treatment equipment. Often the only space left for a control panel for the pump on a skid is the space above the motor. Can the panel be mounted there? Would the panel have to be located such that the pump and skid base don't extend more than 6" in front of the panel?

Re: Line Reactors and VFDs

Bikash — Tue, 17 Nov 2009 07:42:37 -0000

Dear Arun
Can u tell me the make and if possible the type no of drive , So that I can suggest U exact solution

Re: Line Reactors and VFDs

Bikash — Tue, 17 Nov 2009 07:41:04 -0000

How to calculate the Line reactor or output filter dimension from the KW and cable length of Motor, Please suggest theoritical as well as mathematical calculations for different length of cable where single motor and parallel motors rae present

Re: Natural Gray Isn't Gray

Louis — Mon, 16 Nov 2009 22:50:00 -0000

What makes you be the decion maker of what natural gray is. If natural gray is not allowed why hasn't it been removed from the code book in the last 75 tears?

Re: Code Q&A

Fritts — Mon, 16 Nov 2009 16:49:13 -0000

Good answers for new dwellings. The confusion comes with additions, rehabs, or adding one circuit or outlet for tamperproof, and arc fault. It would be nice to have guide lines for these.

Re: Understanding variable speed drives

Ron — Mon, 16 Nov 2009 14:32:46 -0000

Are the moter starts per hour and min time between starts different for VFDs as opposed to accross the line starting? re MG1-12.44

Re: Whats Wrong Here?

Matt — Mon, 16 Nov 2009 14:23:39 -0000

Electrical Panels Adjacent to Water Main/Meter Controls?

Re: The How's and Why's of Harmonic Distortion

dvshah — Mon, 16 Nov 2009 07:20:18 -0000

In the measured output from any harmonic analyser, the values of Harmonic currents in absolute Amps and harmonic voltages in absolute Volts is given. Is there any linear relationship between the respective harmonic voltage and current ? Can this give an idea about system impedance ? I have tried to divide individual harmonic voltages with respective harmonic currents, but the impedance values I get for each harmonic level are widely different. Please help.