Machine Design

Letters 5/24/2012

Readers debate GFCIs and call out manufacturers
One reader has a question about GFCIs and the increasingly common practice of using nonconductive pipes in bathrooms. A couple of other readers blame manufacturers for not taking the time to train employees. And another reader says the imminent death of the lead-acid battery is greatly exaggerated.

GFCI chatter
Lanny Berke’s column on ground-fault circuit interrupters ( “GFCIs — What They Are and Are Not,” March 8) brought up a question I’ve had that no one has been able to answer.

GFCIs work by measuring the amount of current going out one wire and checking that it all comes back through the other wire. So, if a person were to become part of that circuit and some of the current goes to ground, the GFCI would trip because all of the current did not return to the GFCI. That being said, GFCIs only work if there is a path to ground.

GFCIs are required in bathrooms and kitchens but there is no ground path in a bathtub or sink. The drain pipes today are PVC and with the increasing use of CPVC and pex tubing for water lines, there is no longer a path to ground. This being the case, would a GFCI trip?

People are under a false assumption that if they were to drop a radio or hair dryer in the tub or sink, they would be safe and in my opinion they are not.

Any thoughts?

Don Heim

GFCIs and policies concerning them can have some interesting idiosyncrasies. For example, GFCIs are not required in operating rooms in the U.’S. but are in some other countries. This led to an interesting situation.

For example, a few years ago, we got a complaint from a hospital in Hong Kong. It seems the GFCI would open every time they tried to turn on equipment used during eye surgery made by my employer. After a lot of skullscratching, we concluded the power-factor correction circuit on the equipment’s power supply was causing a burst of 30-kHz switching when the device was first turned on. This, in turn, was at a frequency so high that the EMI control circuitry shunted a few milliamps of current into the ground lead for the first hundred milliseconds or so.

One particular brand of GFCI used in that hospital apparently lacked the HF filtering on its trip circuit (which would ignore 30- kHz “noise” as it is not a fibrillation risk) and activated the start-up current.

The short-term fix was to add NTC inrush limiters to the power supply. The long-term fix was to find a vendor selling power supplies with better main input stages, which wouldn’t need to switch the inrush surge at 30 kHz.

Interestingly, we tested GFCIs similar to the one in the Hong Kong hospital, even from the same manufacturer. We could never get one to trip in our lab, nor would any commerc ial / residential-grade GFCIs that we bought locally ever trip. We never could get the GFCI manufacturer to admit that its circuit was overly sensitive to HF currents or that there was a difference between the ones sold in Hong Kong and those sold in Britain.

Alan Ritter

Manufacturing workers
I’ve read several articles, blogs and letters from readers regarding manufacturers that say they can’t find qualified help. Perhaps rather than wringing their hands looking forward, they should look backwards at the guilds of the 15th to 18th centuries.

When I couldn’t find the “ideal” candidate while I was manufacturing manager at several companies, I hired the best I could find, often out of school, and trained them. This had several advantages: I didn’t have to break bad habits they had learned at previous employers, and new hires came with an enthusiasm often lacking in those hired from other employers.

Their “apprenticeship” would last from three to six months, during which time they worked at a reduced salary. The salary was then gladly increased when they successfully completed the apprenticeship. Various states also have programs where they pay part or all of the trainee’s salary while they are learning.

Vernon Lowery

The kids are all right
You make some great points in your editorial (‘Old codgers always gripe about “the kids,”’ March 8).

Contrary to popular belief, universities have never produced large numbers of graduates who could be immediately productive in the workplace. The difference between then and now is that companies once hired recent grads, then made sure they were gradually exposed to increasing responsibilities based on a demonstrated ability to handle ever-more complex problems.

Companies don’t want to do that any more, and they shift the blame to the education system. What’s worse, in their efforts to appease industry, universities waste valuable time teaching students to use CAD tools at the expense of teaching them the basics, such as free-body diagraming real-world systems or applying thermodynamics to engineering problems.

Rajan Ramaswamy

Lead-acid batteries aren’t dead yet
Detractors of lead-acid batteries rarely know enough about them to be taken seriously (“Death knell for lead-acid batteries,” Feb.„9). Modern lead-acid batteries are designed with 20-year operational life, are 95% efficient, can be inexpensive, and the main component, lead, is completely recyclable.

Speculation about replacing lead-acid batteries most often comes from proponents of a technology that might replace them for some applications. They are hardly objective.

Sure, lead is heavy, and electric cars, currently a small section of the battery market, undoubtedly need more energydense batteries. But debates about which bat ter y i s bes t should deal with all the issues: efficiency, longevity, cost, recyclability, ease of manufacturing, availability of materials, and total energy budget.

Lead-acid batteries are miles ahead of the pack in many of those issues, and they’re also quite difficult to beat in efficiency (in terms of energy in versus energy out), a basic parameter with renewable energy.

Geoff Harris

I saw the ar ticle we wrote for MACHINE DESIGN, “How Servos and Steppers Stack Up” in the Feb. 9 issue. The text was fine. However, the stepper versus servo profiles graph, which was kind of the basis of the article, has the stepper and servo profiles swapped.

Tom Kutcher

© 2012 Penton Media, Inc.

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