Evaluate your Electrics Knowledge
- Thread starterruinexplorer
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David Ashton
Well-Known Member
JD
Well-Known Member
Ah yes! It was simpler in the vacuum tube days where you could picture the electrons leaving the negative cathode and heading on their journey towards the positive anode (called "Plate")! Then those newfangled transistors came along and we learned about the "flow of holes!" Now everything is drawn Bass Ackward!
epimetheus
Well-Known Member
89.74%
The quiz is a bit wonky in that there are more questions than the results provided at the end take into account. I also got a popup midway through that said thank you for completing the quiz, even though there were many more questions to go. I also thought the question regarding meter category rating was misleading, though that could be because my day job involved "real" transmission circuits (high voltage), which a CAT IV meter is not rated for.
The quiz is a bit wonky in that there are more questions than the results provided at the end take into account. I also got a popup midway through that said thank you for completing the quiz, even though there were many more questions to go. I also thought the question regarding meter category rating was misleading, though that could be because my day job involved "real" transmission circuits (high voltage), which a CAT IV meter is not rated for.
David Ashton
Well-Known Member
there is a question about a circuit with zero or near zero resistance causing what current?, well unless you have super conductors the zero resistance is not possible, so the correct practical answer is that the current is not assessable, and not "infinite" which is the "correct" answer on the test.It's a bit like doing physics when the teacher says what we taught last year is not quite accurate because ......... and this is what really happens, then you move on to quantum physics and you have to forget all the logical stuff you learnt for years and light does bend etc.One just has to drop into the test at a particular level and assume you mean conventional current flow, basically the simple answers, while wrong, are of use for practical purposes with the possible exception of bringing infinity into it, infinity is a very problematic concept in mathematics, let alone theatre tech.Hence the reference to Mr Menkens quote.
STEVETERRY
Well-Known Member
I agree that the meter category question was misleading, because it lacked reference to use on "service entrance equipment" or "outdoor conductors", or "point of low voltage utility connection", which all describe the typical low voltage (1000V or less) application of a Cat IV meter, not "transmission equipment" (which is inherently high voltage) as the quiz states.
I noted a problem with the quiz section on tap rules which applied NEC section 240.21(B) tap rules to portable feeder cables (with single-pole connectors as shown in the photograph) used at their free-air ampacities. This is incorrect--section 240.21(B) tap rules apply only to permanently installed conductors contained in raceways, switchboards, or other containment. The only tap rules allowed on portable single conductor feeders are those of theatre sections 520.53 (H) (3) and (4), and for motion picture studios section 530.18(B).
I thought some other questions were confusing and possibly wrong in their answers. Notably, the list of harmonic-producing equipment includes items that are not inherently producers of high harmonic content such as transformers, LED's (not LED fixtures with switch mode power supplies), and motors. While the quiz asks which devices have "high harmonic content", it rejects answers that do not also include those non-harmonic-producing devices, stating "Almost everything we use in live event production produces some harmonics." Perhaps, but that's not what the question asked.
So, while a lot of this quiz might be informative and interesting, I urge caution about using it as an authoritative source of fact until such time as it undergoes appropriate peer review.
ST
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I disagree with the question relating to the effect of halving the throw distance. The illumination will increase as the distance decreases. It is a function of 1/r**2. From a mathematical standpoint, as r gets smaller, illumination increases. From a real world view, it is obvious that shorter throw is higher output.
This is why I created this quiz. A distorted current waveform produces harmonics that cause current to flow in the neutral conductor of a balanced 3-phase system. The current is not caused by an imbalance, it's caused by harmonics. Here's a picture from my Fluke 43B power quality meter of the harmonics generated by fluorescent fixtures, the type commonly used to shoot television. It was a balanced 3-phase system and we measured 170% of the phase current flowing in the neutral, meaning that if the phase current was 100 A, the neutral had 170 A.
John, you're right and so it the answer in the quiz. The correct answer is that the illuminance increases by a factor of four, because, as you correctly point out, the illuminance is related to the square of the radius. When you have the throw distance, you half the radius, which decreases the area by a factor of four, therefore the illuminance increases by a factor of four. That's what the correct answer says. If you chose (A) the beam size is halved and the illuminance is doubled, that's not correct.
Sorry about the wonkiness - I don't know why it would thank you for completing the quiz halfway through. This is new software and I'm still trying to figure it out. As far as the categories of meters, I don't know what you mean by "real" transmission circuits, but there is no category higher than IV.
Steve,
You're confusing the list of equipment with the correct answers. The correct answers do not include transformers or motors. Yes, I did include LEDs because, in my experience, about half of the LED fixtures I've tested have high harmonic content. You work for a manufacturer that happens to make LEDs that don't produce harmonics. That's not been my experience, and I test them everywhere I go.
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I believe we are now debating cause and effect.
Your empirical observation of a neutral current is produced using a meter that provides an average reading.
The observed neutral current is due to imperfect phase cancellation.
The harmonics do not cancel so they must be out of phase.
To this point we are in agreement. Where we diverge is on the conclusion. It is based on an association fallacy. It is incorrect to conclude that because some harmonics result in a current on the neutral, all harmonics will. For all practical purposes your conclusion is a reasonable answer but it is not a correct one.
Just as it is possible to combine complex sound waves from 3 different sources and produce silence, so is it possible to combine 3 complex current waveforms and produce no net current.
Your empirical observation of a neutral current is produced using a meter that provides an average reading.
The observed neutral current is due to imperfect phase cancellation.
The harmonics do not cancel so they must be out of phase.
To this point we are in agreement. Where we diverge is on the conclusion. It is based on an association fallacy. It is incorrect to conclude that because some harmonics result in a current on the neutral, all harmonics will. For all practical purposes your conclusion is a reasonable answer but it is not a correct one.
Just as it is possible to combine complex sound waves from 3 different sources and produce silence, so is it possible to combine 3 complex current waveforms and produce no net current.
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I will just make a few comments about the functionality of the quiz, and leave the content to the other, smarter, people on here (as some already are). I too got a "Thank you for completing the quiz" screen half way through the quiz. There's a typo on the sACN question, it says "Streaming CAN." In the evaluation section you cannot click any of the radio buttons plus, there is no explanation as to what the numbers indicate (is 1 good and 5 bad or vice-a-versa). Also, you cannot type anything in the comments box. None of the blue button links to outside websites (e.g. Ohm's law rescue kit or to subscribe to your newsletter) work. It would also be nice if the correct answers were explained in the end instead of just stated. Then I could actually learn from the quiz and not just know how dumb I am.
Thanks!
-Tim
Thanks!
-Tim
STEVETERRY
Well-Known Member
You are correct.
Only selected harmonics, those that are triplens (odd multiple of the third harmonic), also known as "zero sequence" harmonics, create neutral over current. Even numbered harmonics create no waveform distortion and thus no neutral over current. Zero sequence harmonics are in-phase and therefore add together and create no cancellation of current on the neutral.
ST
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