Voltage reading at this point

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leeradney

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This is from some training software my students use. The trainer shows values of 120V here, but I think it is wrong. There is no path to Neutral. Both the normally open M1 Holding contact and the Load Contact are open unless M1's coil is energized. But since neither are, there is no path to neutral to register a potential difference. Am I missing something here?


120V.jpg
 
gar said:
180526-1625 EDT

leeradney:

In the real world might be anywhere from 0 to
something greater than 120.

.
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The OP is saying that you have two open contacts and an open start switch so can you explain how you end up with any voltage across the on/off switch?
 
180526-1746 EDT

infinity:

With a 10 M input impedance DVM and in a control cabinet with 480 and many different wires in close proximity many possibilities are possible from capacitive coupling and leakage. Most likely from capacitance.

In my kitchen, wood floor, one lead of my Fluke 27 in the 120 hot slot and I touch the other lead the reading is 39 V.

But I do agree that the author of the question made a mistake. The use of the value 120 implied a low impedance path to common, and there is not a real low impedance path.

.
 
leeradney said:
This is from some training software my students use. The trainer shows values of 120V here, but I think it is wrong. There is no path to Neutral. Both the normally open M1 Holding contact and the Load Contact are open unless M1's coil is energized. But since neither are, there is no path to neutral to register a potential difference. Am I missing something here?


View attachment 20425
Click to expand...
Is the training software simply showing that the control voltage being used is 120V?
 
gar said:
180526-1746 EDT

infinity:

With a 10 M input impedance DVM and in a control cabinet with 480 and many different wires in close proximity many possibilities are possible from capacitive coupling and leakage. Most likely from capacitance.

In my kitchen, wood floor, one lead of my Fluke 27 in the 120 hot slot and I touch the other lead the reading is 39 V.

But I do agree that the author of the question made a mistake. The use of the value 120 implied a low impedance path to common, and there is not a real low impedance path.

.
Click to expand...

I assumed that you were referring to capacitance but is that really helpful to the person using this software to learn the basics of following the circuit to see if it's open or closed and figure out the voltage at the switch?
 
leeradney said:
This is from some training software my students use. The trainer shows values of 120V here, but I think it is wrong. There is no path to Neutral. Both the normally open M1 Holding contact and the Load Contact are open unless M1's coil is energized. But since neither are, there is no path to neutral to register a potential difference. Am I missing something here?


View attachment 20425
Click to expand...
It has several meters in many places on the drawing, only the one is giving an reading - 120 volts.

It also says question 1 of 5 - what is the question? If asking if the 120 volt reading is correct - answer is no, assuming we disregard possible capacitive coupling, most training at beginner levels ignores capacitance, assumes zero conductor resistance, assues zero contact resistance in switches, assumes low impedance supply, etc.
 
kwired said:
It has several meters in many places on the drawing, only the one is giving an reading - 120 volts.

It also says question 1 of 5 - what is the question? If asking if the 120 volt reading is correct - answer is no, assuming we disregard possible capacitive coupling, most training at beginner levels ignores capacitance, assumes zero conductor resistance, assues zero contact resistance in switches, assumes low impedance supply, etc.
Click to expand...

In this example, the stop switch is shorted closed, which should not affect the value at the power switch. You click each meter and read the values in various states of the circuit to draw a conclusion.
 
gar said:
180526-1746 EDT

infinity:

With a 10 M input impedance DVM and in a control cabinet with 480 and many different wires in close proximity many possibilities are possible from capacitive coupling and leakage. Most likely from capacitance.

In my kitchen, wood floor, one lead of my Fluke 27 in the 120 hot slot and I touch the other lead the reading is 39 V.

But I do agree that the author of the question made a mistake. The use of the value 120 implied a low impedance path to common, and there is not a real low impedance path.

.
Click to expand...


Excellent point!
 
I’ll assume you mean a short to ground, as opposed to a closed switch.

Ashort to ground anywhere prior to M1 contacts, or M1 contacts that are closed for any reason, would account for such a reading.
 
Russs57 said:
I’ll assume you mean a short to ground, as opposed to a closed switch.

Ashort to ground anywhere prior to M1 contacts, or M1 contacts that are closed for any reason, would account for such a reading.
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Assuming that the neutral is at earth potential. Usually is but it is not shown to be so would be an assumption. Read what you are given, not what you think you are given.
 
Valid point Sir. In my defense I can only say that here in the US a neutral is a grounded current carrying conductor.

But I agree that assumptions are the mother of all evils when it comes to troubleshooting. The most common may be the assumption that one’s meter works!
 
Russs57 said:
Valid point Sir. In my defense I can only say that here in the US a neutral is a grounded current carrying conductor.
Click to expand...
Usually at or about earth potential here (UK) too. For domestic and light commercial the usual LV distribution arrangement is 11kV/400V delta/star with the star point earthed or grounded. Phase to neutral is nominally 230V, 50Hz and that's what residences get.
 
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