Confusing ohm readings

[George Stolz]

I had an emergency panel change over the weekend. FPE panel, FPE service (to be changed shortly.)

The old panel developed some sort of issue (that I never fully understood), basically it developed an internal short, blew a hole in the bussing and knocked out the utility transformer fuse.

As I was hooking up the old circuits in the new panel, I tested for continuity and found a home run that had 4 ohms. I did a little math and concluded that the circuit would be pulling 27A when I energized it. When I fired it up, it was only pulling 4A.

What gives?

 

[mivey]

I'll assume you did not measure a parallel resistance by putting your leads in the wrong place, or something else similar, so how about: DC ohms measured were not the same as the AC impedance.

add: 108 volts (27*4)?

 

[mivey]

How about resistance increasing as the circuit heated up. This probably makes the most sense.

add: example would be a 100 watt bulb with cold resistance of 9.5 ohms and hot resistance of 144 ohms

 

[iaov]

If you were looking at a small transformer out there some place you would see a short circuit with your meter. A transformer is a reactive device and you can't read what its real impedence is with an ohm meter.

 

[mikeames]

I had an emergency panel change over the weekend. FPE panel, FPE service (to be changed shortly.)

The old panel developed some sort of issue (that I never fully understood), basically it developed an internal short, blew a hole in the bussing and knocked out the utility transformer fuse.

As I was hooking up the old circuits in the new panel, I tested for continuity and found a home run that had 4 ohms. I did a little math and concluded that the circuit would be pulling 27A when I energized it. When I fired it up, it was only pulling 4A.

What gives?

Are you just teasing us George?

 

[mivey]

Are you just teasing us George?

I would not think so. Either my post #2 or #3 or a combination of both would explain these readings.

 

[mivey]

If you were looking at a small transformer out there some place you would see a short circuit with your meter. A transformer is a reactive device and you can't read what its real impedence is with an ohm meter.

Nope. Even a small transformer is going to show some resistance.

 

[gar]

090427-1020 EST

A Signal Transformer Co. 175 VA unit connected for 115 V input has a DC resistance of 1.7 ohms.
Same company but 43 VA and the primary for 115 V is 12 ohms.
A Stancor P8668 117 V input and 56 VA output is 6.2 ohms for the primary.

1000 ft of #12 is 1.588 ohms. 1000 ft of #14 is 2.53 ohms.

.

 

[gar]

090427-1024 EST

The 27 to 4 ratio 6.75 looks more like a transformer or motor than an incandescent. The ratio should be over 10 to 1 for an incandescent. A 100 W bulb at room temperature has resistance of about 11.2 ohms. At 120 V it will be about 144 ohms. Or a ratio of 12.9 to 1.

.

 

[Fulthrotl]

I had an emergency panel change over the weekend. FPE panel, FPE service (to be changed shortly.)

The old panel developed some sort of issue (that I never fully understood), basically it developed an internal short, blew a hole in the bussing and knocked out the utility transformer fuse.

As I was hooking up the old circuits in the new panel, I tested for continuity and found a home run that had 4 ohms. I did a little math and concluded that the circuit would be pulling 27A when I energized it. When I fired it up, it was only pulling 4A.

What gives?

1/2 hp motor load? the ratio between locked rotor resistance and running
resistance looks to be about 7 to 1, and that's about the ratio between
inrush current, and running current, at least in the small motors i've measured.

 

[mivey]

090427-1020 EST

A Signal Transformer Co. 175 VA unit connected for 115 V input has a DC resistance of 1.7 ohms.
Same company but 43 VA and the primary for 115 V is 12 ohms.
A Stancor P8668 117 V input and 56 VA output is 6.2 ohms for the primary.

1000 ft of #12 is 1.588 ohms. 1000 ft of #14 is 2.53 ohms.

.

Here's the primary ohms for the ones I could readily get my hands on:
117/24V, 120VA = 5.6 ohms
120/12.6V, 15VA + 3ft cord = 51.0 ohms
120/18V, 26VA + 3ft cord = 16.0 ohms
(2) 120/12.6V, 38VA each + 3ft cord = 21.0 ohms
120/16V 10VA = 186.3 ohms

 

[mivey]

motor ohms data:

115volt, 3/4 HP motor:
4.3 ohms DC, but ran 7.6 amps at 120.0 volts (lightly loaded)

 

[dereckbc]

George you are just seeing a transformer or motor coil out there. If you are really curious turn the breaker off and se what it turned off. My bet it is something like a computer power supply, stereo, or maybe even a cieling fan or something like that on the circuit.

 

[mikeames]

I would not think so. Either my post #2 or #3 or a combination of both would explain these readings.

I agree. I thought this was too simple. I thought he was just trying to start some debate.

 

[brian john]

AS for the panel blow up, something resulted in a breakdown of the insulating material between the bus, A over heated bus stab that results in arcing and the insulator chars. Taking the insulator from an insulator to a conductor.

Water is an other issue (actually the minerals in the water or dirt and dust brought in by the water.

 

[mivey]

I agree. I thought this was too simple. I thought he was just trying to start some debate.

George? A pot stirrer? No way. Moderators are above that sort of thing.

 

[George Stolz]

No, I just didn't think the difference between resistance and impedance was that vast, with the power off. Some things that some take for granted, some of us haven't really thought about before.

Thanks for the replies.

 

[iaov]

Nope. Even a small transformer is going to show some resistance.

An ohm or two. In my world, thats a short.

 

[mikeames]

Some things that some take for granted, some of us haven't really thought about before.

I can totally relate to that.

 

[mull982]

How about resistance increasing as the circuit heated up. This probably makes the most sense.

add: example would be a 100 watt bulb with cold resistance of 9.5 ohms and hot resistance of 144 ohms

I have seen this before as well.

If the light was on for a period of time and the element heated up to temperature then you could shut it off shortly and should be able to measure the resistance that would work for your theoretical calculations.

If the element was already up to temperature and you switched the bulb back on would there be any current transient since there would be no change in temperature?