Tasco's Inspector III

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Leigh Simpson

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There is alot of interesting data when the Inspector III is plugged into a receptacle. On the second screen, one can find the amount of fault current in amps that will be created when there the hot and neutral are shorted. Also, displayed is the amount of fault current in amps when the hot and ground are shorted. Does anyone know the significance when the fault current is higher in either case?
 
I think this is a case of an instrument providing needless information just because it can. I've never had the need to know much of what that instrument provides.
 
mdshunk said:
I think this is a case of an instrument providing needless information just because it can. I've never had the need to know much of what that instrument provides.
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That should make a new member feel welcome.
 
080504-1254 EST

Leigh:

I would assume the instrument some how tests the loop resistance. Probably by applying a known load and from this estimating the source impedance.

Suppose everything is wired properly and the equipment grounding conductor is the same diameter and material as the nuetral, then the two readings should be close.

Suppose that in addition to the physical ground conductor there is in parallel with it a well bonded together conduit and it is thick wall, then I would expect a higher estimated fault current to the ground conductor.

On the other hand suppose there are some bad connections in series, then you would get lower readings and this would be a useful troubleshooting tool. Maybe they also display source resistance and that would be a more useful mode.

If you have a 150 ft run of #12, that is a loop length of 300 ft, then at 120 V source the maximum possible current at the 150 ft end is I = 120/(0.3*1.59) = 252 A. It would actually be less because there is some source impedance preceeding the input end of the 150 ft run.

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qcroanoke said:
That should make a new member feel welcome.
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I'm not on the welcoming committee. I know my limitations. :wink:
I was just offerring my unsolicited opinion on the product, which has nothing to do with the person who posted the question.
 
LarryFine said:
But... but... you're Megger Man! :cool:
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Compared to what their is to know about them, I know very little. I'm just a guy that recognizes how helpful they can be as a troubleshooting aid and I think everyone should have one. Brian John or Zog surely know the whole scoop on them. I sure don't. This Inspector III is certainly a neat gadget, but I'd be hard pressed to tell a fella that he needs to own one.
 
080504-1646 EST

Leigh:

I just ran some crude source impedance measurements. Test instruments -- Fluke 27, and a 10.6 ohm resistor (electric heater hot. its room temperature resistance is 8.6 ohms). All my wire is copper. Even to the pole.

Very close to main panel. At most 2.5*2' of #12. Includes main fuses and 1 QO breaker.
No load 123.5 V, loaded 122.5 V. I = 122.5/10.6 = 11.56 A. Voltage change 1 V. Source impedance = 1/11.56 = 0.087 ohms.

Kitchen counter. 30'*2 of #8 and about 25'*2 of #12. Same as above but one additional QO breaker.
No load 122.8 V, loaded 119.7 V. I = 119.7/10.6 = 11.3 A. Voltage change 3.1 V. Source impedance = 3.1/11.3 = 0.27 ohms.

At one of my shop benches. 50'*2 of #8 and about 25'*2 or more of #12 and several plugs. Main fuses, a breaker at main, breaker at first bench, followed by another breaker on second bench.
No load 123.3 V, loaded 116.7 V. I = 116.7/10.6 = 11.0 A. Voltage change 6.6 V. Source impedance = 6.6/11.0 = 0.6 ohms. I expect substantial drop in the several plugs.

My wire lengths and sizes are guesstimates. My service is 200 A.

Can your instrument give you this information directly, if not it can be approximately calculated from your fault current measurements? And do you have any comparable measurements to compare with mine? Almost any locations that would show close to the main panel value and a far away value.

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In Europe measuring the Prospective Short Circuit Current of a receptacle is required during installation.

The regulations specify the time that a breaker must clear a fault in, and if you have a long run of cable the short circuit current can be insufficient to clear the fault in the allowed time, because of the time curves of the breaker. You need to clear the fault in the prescribed time because of the overheating of conductors due to excessive current flowing.
 
080506-1012 EST

Some additinal measurements at my kitchen location. Today the tests are run with nothing else on the neutral except the test load.

With no load the ground wire to water pipe at sink voltage is 0.003 V.
With no load the ground wire to neutral voltage is 0.000 to 0.001 V. From the measurements below it is clear that the ground and neutral are separate back to the main panel.

I am using the same 10.6 ohm load as yesterday.

No load voltage hot to ground is 123.5 V.

With load between hot and neutral.
Hot to ground 121.6 V or 1.9 V drop on the hot line. In other words the measurement is relative to common (neutral and ground) at the main panel.
Hot side resistance = 1.9/11.47 = 0.17 ohms.
Neutral to ground 1.05 V.
Neutral resistance = 1.05/11.47 = 0.092 ohms.

With load between hot and ground
Neutral to ground 1.75 V.
GEC resistance = 1.75/~11.47 = 0.15 ohms.

Note: Voltage measurements exclude the voltage drop of the plug and socket by using a separate plug for the voltage measurements. The hot wire has two breakers in series and I did not check the possible voltage drop across the breakers. Since the same size wire is used for the hot and neutral the results are consistent. When the house was built the ground wire was smaller (#16) than the load wires.

Based on my assumed lengths of wire of yesterday I would calculate somewhat lower resistances than measured, but we are close.

Measuring a QO15 at room temperature and no current flow I get about 0.1 ohms resistance. Thus, at a 15 A load the drop is 1.5 V.

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mdshunk said:
I'm not on the welcoming committee. I know my limitations. :wink:
I was just offerring my unsolicited opinion on the product, which has nothing to do with the person who posted the question.
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That's OK Marc. I'll take care of it for you. :wink: :smile:

Leigh Simpson....Welcome to the forum.

Whew....I feel better.

fpn. Marc is an ok guy, don't let his table manners put you off.

:D
 
080506-20128 EST

Larry:

If your question was directed to my experiments, then the answers are:

The question about Inspector III stimulated my interest to see how they might be making their measurements. My measurements are approximately what there are doing, except they are not using a 1200 W heater.

My experimental results are in the ballpark of what I estimate from calculations. This technique can be useful in identifying wiring problems, but you really need to know approximate wire lengths to make an accurate judgement. Obviously if you have a 10% drop over a typical distance in an average house you have a problem. But for more subtle problems you need additional information.

I was a little surprised in the 3 MV difference between my ground wire and the cold water pipe since there was zero difference between ground and neutral. I may look for the reason in the future.

For the most part you can do what Inspector does with a good voltmeter, a useful load like a heater, paper, and a calculator.

I did not expect problems in my house because I know how it was built. But I have seen problems other places where a test such as this could be useful. The previous house my son bought originally had aluminum wiring and because of this he had to completely rewire it for safety. His new home has many very cheap duplex outlets and dubious quality of workman ship. So I will probably run some tests there. When you can buy a house that basically has what you want and at a good price, then you accept some of these problems and fix them.

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080506-2051 EST

Leigh:

Looking back at your original question you asked the significance of a different fault current to neutral vs ground.

My above experimental results show you why. The total measured loop resistance thru neutral is 0.262 ohms, and thru ground it is 0.32 ohms. Thus a lower fault current thru the EGC. At an assumed 120 V source the respective maximum fault currents are 458 A and 375 A.

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gar said:
080506-20128 EST



For the most part you can do what Inspector does with a good voltmeter, a useful load like a heater, paper, and a calculator.



.
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The difference between using a voltmeter and the Inspector III is the voltmeter does not have the green, yellow and red lights. A customer generally does not understand the values shown on a voltmeter, but they can certainly understand how they see the lights go from green to red. When the technician shows them that red is bad, they can clearly see that.
 
080507-0708 EST

Pierre:

I do not think Inspector III has LEDs. It has an LCD that displays numerical data, and flashes on out of tolerance.

I think Leigh's question was what does the displayed information mean.

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080508-2108 EST

Leigh:

Are you still around?

Has our discussion stimulated any thought on whether the Inspector III would be useful to you? In your normal work would you have any need for this and how would you use it? Have you experienced problems where the device would be useful? Would it pay for itself?

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