January 2010 Quiz Results
The results are in! Thanks to everyone who took our first "What is Your Cable Testing IQ?" challenge. After collecting a month's worth of anonymous data you can now compare your results against a large cross-sampling of others who took the quiz. See which questions were most often answered correctly and incorrectly. Then check out the links provided for additional training on many of the subjects.
Look for "What is Your Cable Testing IQ - Challenge #2 in a future issue of this newsletter.
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How did you do?
Questions answered correctly:
- 1-3 Electronics can be a shocking experience for you!
- 4-5 You're in the "half that makes the top half possible!"
- 6-7 Not bad, this cable testing stuff is easy, isn't it?
- 8-9 Don't agree with one or two answers do you?
- 10 "...A flux capacitor is overdue Doc!"
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"Cable Testing IQ" - Challenge #1 Quiz Answers! Correct answers in red. |
Question #1:
If you put your ohmmeter leads at points a and b on the wire below, what kind of resistance reading would you expect to see?
- (16%) A. Absolute zero resistance
- (80%) B. Low resistance
- (02%) C. High resistance
- (02%) D. Off-scale
Most people got this question right, but 16% of you thought that "absolute zero" was the answer. This can't be right because you have to account for the resistance of the wire and contacts in the cable, however small. The resistance of your test fixtures comes into play as well. To see how the effects of length and gauge of wire and fixturing add up check out our "resistance of wire" calculator here.
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Question #2:
If you put your ohmmeter leads at points a and b on the two wire below, what kind of resistance reading would you expect to see?
- (19%) A. Absolute zero resistance
- (08%) B. Low resistance
- (10%) C. High resistance
- (63%) D. Off-scale (resistance to high for standard meter to read)
Ohmmeters read Offscale, which means "too high to measure" when the leads are not connected, or are connected to something with too high of a resistance to measure. Since wires that are supposed to be insulated from each other don't touch, you should get an Offscale reading for question # 2. See more about the results of such measurements on various assemblies at: http://www.cirris.com/testing/guidelines/testresults.html
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Question #3:
Assuming the two wires below are both 1 meter long, which one would produce a higher resistance measurement?
- (72%) A. The smaller diameter wire
- (16%) B. The larger diameter wire
- (06%) C. They would be the same
- (06%) D. Not enough information to answer
The smaller the wire, the more resistance it has to current flowing through it. The bigger the wire the lower the resistance. See the relationship between wire gauge and resistance at: http://www.cirris.com/testing/resistance/wire.html
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Question #4:
Which wire has the largest diameter?
- (96%) A. 10 AWG
- (01%) B. 14 AWG
- (00%) C. 20 AWG
- (03%) D. 26 AWG
Only 4% of the people missed this one. Pretty much everyone knew that the higher the gauge the smaller the wire diameter. See the resistance of different wire sizes at: http://www.cirris.com/testing/resistance/wire.html
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Question #5:
You're riding in the hotel shuttle bus and notice the interior lights flicker on/off everytime the driver hits a bump. What do you say?
- (29%) A. Looks like he's got a short
- (64%) B. Looks like he's got an open
- (03%) C. Looks like he's got a mis-wire
- (04%) D. Are we there yet?
This one was a bit tricky. 29% of our test takers said "short," but this is not likely. Shorts generally blow fuses and the lights stay out. A high-resistance short could dim the lights, but the best answer is "open." When a circuit is open the lights are definitely out and the fuse is not going to blow.
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Question #6:
What property of electricity kills?
- (09%) A. Volts
- (02%) B. Ohms
- (00%) C. Cost
- (89%) D. Current
This question was answered correctly most often, although 10% of you still thought Volts was the right answer. While volts are required to overcome the resistance of a person's body, it is current flow that kills. It is the "resistance" of our bodies that helps protect us, somewhat. Using ohms law you can calculate when the risk factor becomes dangerous. I (current) = V (voltage) / R (resistance). You can generally "feel" current in the range of 10mA. At 60mA you might freeze up and not be able to let go. Above 60mA and you are in serious danger. If you are standing in water your resistance is lower so the risk of injury or death increases, but you still need enough volts to create dangerous levels of current. (If you saw the CSI episode where someone was electrocuted with a car battery in a swimming pool you now know enough to cry "Shennanigans!") For more information on high voltage and safety issues go here: http://www.cirris.com/testing/guidelines/hipot_safety.html
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Question #7:
Three of seven strands are broken in a seven-stranded wire. What kind of electrical test will detect the broken strands?
- (15%) A. High voltage - Insulation resistance
- (45%) B. Very low resistance (4-wire Kelvin) continuity test
- (04%) C. High-speed flex testing
- (36%) D. None of the above
Toughest question on the quiz. Most of you thought that "very low resistance (4-wire Kelvin)" testing was the answer. If the question had been worded "is most likely to detect broken strands" we'd have to give you credit, but the phrasing was "will detect broken strands" so we're sticking with our answer! While very low ohm resistance testing is good for detecting many types of wiring defects, unfortunately it can't detect if only some of the strands are broken. The resistance of the remaining strands is sufficient to give a low enough resistance reading to still pass the test. The resistance of the wire itself, including temperature effects, is enough to swamp the effects of the broken strand resistance. See our detailed findings on this subject here: http://www.cirris.com/testing/resistance/broken_strands.html and a follow up report titled "Will high current catch broken wire strands? Here: http://www.cirris.com/testing/resistance/broken_strands-2.html
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Question #8:
The purpose of high voltage testing is to find:
- (59%) A. Weaknesses in insulation that separates conductors (wires and contacts)
- (05%) B. Weaknesses in the continuity of conductors (wires and contacts)
- (03%) C. Nicked or cut strands
- (34%) D. Both A and B
Another tricky question! The purpose of Hipot (high potential or high voltage) testing is to verify the integrity of the insulation or isolation between wires, or unterminated points, that should not be touching. It does nothing concerning the continuity or resistance of connected wires. Get a good, basic understanding of Hipot testing here: http://www.cirris.com/testing/guidelines/hipot_testing.html
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Question #9:
How can a Cirris tester be considered safe when testing at 1000 volts DC when a person can be electrocuted with 120 VAC from a wall plug?
- (12%) A. The tester is using (DC) rather than (AC) current
- (21%) B. The tester limits the current so as to be non-hazardous
- (08%) C. The tester quickly shuts down if the safe current is exceeded
- (59%) D. Both B and C
The voltages that Cirris testers use could be lethal if there were not safeguards in the design. Both current limiting and quick shutdown are used. Look for the details at: http://www.cirris.com/testing/guidelines/hipot_testing.html#h8
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Question #10:
My test spec. calls for a 1000VAC high voltage test, what DC voltage would be equivalent?
- (20%) A. 1000 VDC
- (10%) B. 2000 VDC
- (46%) C. 1414 VDC
- (23%) D. 707 VDC
Another difficult question. About half of you got this one right. When converting AC voltages to DC equivalents you need to make sure that the DC voltage is equivalent to the peak AC voltage. The formula for doing this is "AC peak voltage = AC RMS voltage x square root of 2 (1.414). Here are the simple conversion formulas:
To convert: AC to DC - Multiply the AC voltage by 1.414
DC to AC - Multiply the DC voltage by .707
For a detailed look at the math behind it all go here: http://www.cirris.com/testing/guidelines/ac_hipot_testing.html
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