250Volts and NO SPARK WHEN SHORTED

[bullheimer]

okay i quit. i couldn't figure this out so i walked away from a house (@1940s) because i could not stop the following sitsheasion: 3 lights no workie:

three switches in a 3 gang box. nutes tied together. hot in hot out to somewhere.

none of the lights work. Fluke meter says 220V to an UNGROUNDED metal box. Ideal meter says 245V to ungrounded box. whether from the hot, OR FROM ANY OF THE THREE SWITCH LEGS you get 240V to the metal box. i loosened all the cable clamps and pushed the cables down in case there was some kind of short and no loss of voltage. neutral and all phase voltages normal in panel. no breakers tripping or tripped.

now dig man, i touch each and every wire to said metal box and only getta teeny tiny spark of a almost NOTHING, instead of the huge blast i ASSumed i was gonna get, sending shrapnel across the room inna fireball. Nada!


but i gots ta know. I gots ta know!!!! WWWWWWWWWWWWWHHHHHHHHHYYYYYYYYYY IT DODAT?????

 

[charlie b]

Do the same test with a grounded metal box and you will get arcs and sparks, and I will get sued for making the suggestion, so don't actually do the test. But if the box is not grounded, then there is no complete path from that box back to the source. Thus, there is no reason to expect a surge of current, as the current has no place to go. Voltage is a measure of the difference in potential between two points. If the box is not grounded, then its potential will not be that of planet Earth.

It is like asking the difference between the height (above the street level) of the floor upon which you are standing and the height (above street level) of the floor inside the elevator. If you don't know where the elevator is at the moment, then how can you answer the question? Connecting a bonding wire or EGC to the metal box in which your switches are installed, and running it back to the panel, and from there back to the main panel, and from there to the ground rod, will essentially force the elevator to remain on the bottom floor. Then you will be able to measure the difference in height from any higher floor to the floor of the elevator.

 

[G._S._Ohm]

Phantom voltages?

 

[hillbilly1]

You are in series with another load, the box is not grounded, but is providing a return path (open neutral the likely cause) to the opposite leg.

 

[bullheimer]

diggama: i know what the definition of voltage is: difference in potential is 245Volts. No sparks. Got it? lets say i jump out of your second floor window but i forget to fall down.

anytime i see 250V in a 120V box there is usually an open neutral in the woodpile. thats why i had to double check the voltages at the breakers.

still, why am i floating up instead of falling down? stupid meters anyway!. i am basically wondering out loud how inthe He!! my meter will tell me there is 220 and NOT getta spark.

if i'm in series or something and there is no load i will get no spark? but if the lights dont work it's alittle hard to get current to flow. maybe if i was an EE i wouldn't have to ask this Q. maybe when i touch a wire to this box there is a light bulb somewhere that is going brighter. if i add a load that is virtually non existant (short) it would be hard to tell. the little spark should mean there is just a tiny little load somewhere else right? smaller than any light bulb. maybe a battery charger charging a fully charged battery? this bytes.

 

[Little Bill]

diggama: i know what the definition of voltage is: difference in potential is 245Volts. No sparks. Got it? lets say i jump out of your second floor window but i forget to fall down.

anytime i see 250V in a 120V box there is an open neutral in the woodpile.

still, why am i floating up instead of falling down. stupid meters anyway. if i'm in series or something and there is no load i will get no spark. but if the lights dont work it's alittle hard to get current to flow. maybe if i was an EE i wouldn't have to ask this Q. maybe when i touch a wire to this box there is a light bulb somewhere that is going brighter or dimmer. i should know which too but i don't. this bytes.

As has been said, (including you) check for loose, unconnected, or otherwise broken neutral.

 

[gar]

110316-2244 EDT

The strange result is 250 V. Go to the main panel and using a high impedance meter (10 megohms input) measure:

Phase A to B voltage. I expect about 250.
Phase A to neutral.
Phase B to neutral.
Neutral to earth. Could be a metallic water pipe on the street side of the water meter if the metallic pipe actually goes into the ground. Otherwise a 12" screwdriver in the earth outside.

Nest load one phase with a 1500 W heater, and redo the measurements. What happens to the neutral to earth voltage, and other voltages? And does the heater get warm?

This information should tell you whether there is a neutral and/or other problem.

The next step to answering your question will be based on those measurements.

.

 

[LarryFine]

Did you read line to neutral (excuse me, grounded conductor)?

You might also run an extension cord from a known-properly-wired receptacle, such as the laundry circuit, but only after verifying that its L-N, L-G, and N-G voltages read correctly. If not run it from a neighbor's house, after verifying the same there.

Test each wire of your circuit, hot, neutral (you know), and EGC, against each slot of the cord. The only thing you could read more than one way would be line to line, where you could get either 0v or 240v.

 

[sameguy]

Put a "wiggy" on it not a meter and you will not read voltage.

 

[gar]

110317-0957 EDT

sameguy:

Put a "wiggy" on it not a meter and you will not read voltage.

This might make the reading go away, but there is something strange about the reading. 250 V is not something you should see in a system supplied from a 240 V secondary with the center tap connected to earth (grounded). Probably not much more than 125 to to 130 V as an absolute maximum should be read, and more likely something around 30 to 90 V.

Using a Fluke 27 with one lead in the hot slot of an outlet, and the other lead squeezed in my finger I read about:
90 V when standing on the cement basement floor,
35 V when standing on the wood floor of the first floor.
This results from current thru the capacitive coupling of my shoes to the floor.

The floating box, not intentionally connected to an EGC, is capacitively and/or resistively connected to earth thru long or short paths. Plaster and/or wood walls, etc.

Thru capacitive or resistive coupling with no resonating inductor you should not read a voltage higher than the source voltage. With a properly connected 120-0-120 supply you should not see greater than 120 or whatever is the actual voltage at the transformer.

Thus, 250 V is a misreading, or the supply has a big problem. This is why the high impedance meter reading is important. Something appears to be wrong here.

You can learn far more from a high impedance voltmeter reading than from a wiggy. If you want to check for low level currents without the possibility of blowing the fuse in your multimeter, then put a 1000 or 10,000 ohm resistor in parallel with the voltmeter leads. A 50 W incandescent lamp is another useful tool in parallel with the voltmeter. Since its resistance changes with voltage it has some advantages and some disadvantages.

.

 

[charlie b]

diggama: i know what the definition of voltage is: difference in potential is 245Volts. No sparks. Got it?

I got it. Now get this. The potential level of the ungrounded metal box is "floating," not tied to anything. So if you measure voltage from a live wire to that box, you can get anything, and it could easily vary from minute to minute. You measured 245? OK, on that day, at that time, that was the potential difference.

Now you touch the live wire to the metal box. What happens is that you raise the potential level of the box to match that of the wire. That might draw a tiny spark. But from that moment forward, the potential level of the box and the wire are exactly the same. No more current, no more sparks. You just called the elevator to your floor.

This is different from a grounded box in that the potential level of the grounded box is firmly tied to zero, by way of the EGC. When you touch the live wire to the box, you have the wire being driven (by the source) to a level of 240 (or whatever) volts, and you have the box being held at zero volts. That is why you get major sparks in that condition.

 

[charlie b]

Here?s another way to think about the situation. Take a roll of #12 THHN, and cut yourself a section about two feet long. Strip one inch of insulation away from each end. Set it on your garage workbench, but on top of something, so that the two exposed metal ends are not touching the bench. Now measure the voltage from the hot side of the bench?s receptacle outlet to one end of the wire. What voltage do you expect to get? The answer is that (1) It could be anything, (2) It might even be 120 volts, and (3) The voltage has no meaning, since the wire is just sitting in open air.

Now hold the wire by its insulation (using gloves and other proper protective measures) and touch one exposed metal end of the wire to a live source. Do you expect to see lots of arcing and sparking? I should think not.

In this experiment, an ungrounded metal box permanently installed in a wall would act no differently than a wire just sitting on a bench.

 

[sameguy]

@ Gar, yes most of the guys have covered what he should do including yourself; my wiggy was to his why do I see voltage on my meter. He needs to take the readings at the panel, isolate the circuit, ie. turn all off but that one and find the missing neut. most likely.

 

[bullheimer]

thanks to all. i connected a known ground to the the box and then got 120 from the hot as well as all switch legs to it. i wasted enough time in this guys house and blew it off. i wanted to run a new circuit to it but decided if there was a n connected to a hot somewhere in the house i did not want my name on any paperwork associated with it so i just ate it.

i totally agree with the above posts about phantom voltage, even from the first post it obviously had to be, since the box was connected to nothing. very aggravating but a fact of life. as i understand it the next guy did as i said and just ran a new wire to the switch box and all was well.....at least for now. i still think something is screwed up somewhere and running a new ckt isnt going to fix it. well it could, but it might NOT fix it, i.e. open wire somewhere and a potential arc. it could be in a box in which case who cares, but it could be a flying splice somewhere. i dont know. not my problem now.

sorry to disappoint any of you who think i should pitch a tent there but after a couple three times of trying to figure it out i was at the point of lighting the house on fire myself. Peace