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Budget Appliance Repair

Wierd voltage readings on a 240 volt plug

45 posts in this topic

I ran into something strange on a job this weekend and would like some feedback.

I was working on a built-in GE double oven that the control board was fried from a dropped neutral line.

The oven was at a hospital work shop in storage and that is were I was to work on it.

The pigtail whip is the hardwired conduit type and there was a 240volt plug near by so I grafted a pigtail on the the conduit pigtail whip to plug into the 240volt socket to test.

Before testing I needed to make sure the socket had power becuase the circuit breakers were off to the 240volt plugs and the guy that let me in didn't know what circuit breakers went to what plugs so we had to turn the breakers on and test the plug.

This being a hospital they have 3 phase circuits in the same box as the 2 phase. There are circuit breakers with with 3 breakers ganged together.

Anyway, when I tested the plug, this is what I got:

L1 to Neutral=119volts

L1 to L2 =238volts

L2 to Neutral=206volts

L2 to Neutral if I had hooked the clock power circuit to this power leg I would have fried the new clock.

I made sure the clock power circuit was on the L1-Neutral line to test the oven when I was done and all worked ok.

I would like to know what would cause the strange voltage reading on the L2 to Neutral side, but L1 to L2 still reads the correct voltage. Would this be something to do with the 3phase power circuit and maybe the breaker for this 240volt line is feeding off the wrong power line phase for the L2 side.

Also, should something be done about this? The oven won't be installed were I worked on it and I would have to assume this power setup has been this way all along and they haven't had any problems that anyone knew of.

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Need appliance parts? Call 877-803-7957 now!

when did the dropped neutral occur / and repaired ?

recently or the dryer has been sitting in storage for a while ?

what type of outlet was this ?  

3 or 4 prong dryer OR some type of 3phase outlet ?

is the power to this outlet shut off with one of the double breakers or one of the  triple breakers ?

check the voltages again to GROUND to see what they read, there may still be something wrong with an open neutral SOMEWHERE.

 

 

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when did the dropped neutral occur / and repaired ?

ANSWER: At the house where the oven was orginally hooked up, yes it was repaired and oven replaced with new one.

recently or the dryer has been sitting in storage for a while ?

ANSWER: It's not a dryer, it's a built-in GE double oven, only been at the hospital storage/shop for a couple weeks.

what type of outlet was this ?

3 or 4 prong dryer OR some type of 3phase outlet ?

ANSWER: A standard looking 240volt 3prong range outlet

is the power to this outlet shut off with one of the double breakers or one of the triple breakers ?

ANSWER: A standard double 240volt breaker, I didn't notice the amp rating of the breakers.

check the voltages again to GROUND to see what they read, there may still be something wrong with an open neutral SOMEWHERE.

ANSWER: Where I was working on the oven, (Hospital storage/shop), wasn't were the dropped neutral happened, and I carefully hooked up the oven making sure the L1-Neutral that had the 119volts went to the circuit that runs the ERC and cooling fan. The oven tested fine on this setup when I tested it. It will not be installed in this location, it will be going to another house to be installed, (not the original house where the dropped neutral happened).

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[user=36]Budget Appliance Repair[/user] wrote:

Also, should something be done about this? The oven won't be installed were I worked on it and I would have to assume this power setup has been this way all along and they haven't had any problems that anyone knew of.

Not necessarily. There could be a problem with that particular feeder line. A good test would be to see if you could do another voltage measurement at a different location, fed by a different feeder line.

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OK, sorry about the "dryer" reference...

The oven sounds OK, but I still think there may be someting wrong with that outlet wiring.

You're right, the L2 voltage sounds like it may be one leg of a 3 phase circuit.

For their safety and to put our minds at ease, I'd still check all (3) voltages (L1, L2, & neutral) to ground to try to determine if there's something wrong, and mention the results to an electrician or someone else at the hospital.

 

 

 

 

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Thanks, I will call the guy I was contracted to fix the range for, he is the contractor, (I think), for the hospital and explain to him what I found and leave it up to him to take it from there.

I'm not an electrician, and wasn't in anyway responsible for the hospital wiring, but will let him know in case he wants to follow-up on it.

Thanks again,

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Was the glow stick the same color on the L1 as the L2 or did it seem a little brighter on one leg??:cool:

 Samurai has a valid point of checking it to a different opposite potential.  I would bet if you checked the neutral to a good ground that you would of found stray voltage sitting on on the neutral from a feed somewhere. 

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Didn't and don't use a glow stick, used my digital meter and I haven't had any problems with said meter.

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Glow stick reference was humor Willie :(.  I have seen stray voltage on neutrals as high as 65-70 volts.  Usually something else in the home, shop, etc. is the cause.  Somewhere, something was bleeding back through from the L1 side.  You did not read the increase on the neutral to L1 because the L1 you were testing and the bleed through were the same potential.  That is whey it only showed up on the L2.  I've always confiscated glow sticks from my techs unless they use them to look for breaks in L1 wires, or quick verifications of L1 potentials such as outlet polarity.  Usually I take it out of the pocket and ask what they use if for.  If they tell me voltage tests, I snap it or confiscate it.  The kids like rubbing them on their shirts and making them blink!! 

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Is the hospital using automatic inductance compensation? If that malfunctions (overcompensation) it can give you excess voltage.

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I've seen two-phase configurations (for sewer pumps) that provide a voltage of 208.  I think this is what you're experiencing.  In residential it's normally not a problem as most homes don't have more than one phase connected.  Having a standard 240V plug set up like that sounds a bit dicey.

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[user=8638]Chesterfoxes[/user] wrote:

I've seen two-phase configurations (for sewer pumps) that provide a voltage of 208.  I think this is what you're experiencing. 

That would be two phases out of three, 208V between, 120V from each to Neutral...   25% less power to a device designed for 240V, but should be in the acceptable range for most devices.

Or are you talking about a 3x208V sewer pump?

If I read it right, B.A.R. had normal looking readings from L1 to N and L1 to L2, but an abnormal reading of 208V between N and L2... if it was a 208V two phase feed, it would have read 208V between live legs.

We never came to know if the voltages normalized when under load. There apparently was a good Neutral hookup.

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[user=10234]Keinokuorma[/user] wrote:

If I read it right, B.A.R. had normal looking readings from L1 to N and L1 to L2, but an abnormal reading of 208V between N and L2... if it was a 208V two phase feed, it would have read 208V between live legs.

We never came to know if the voltages normalized when under load. There apparently was a good Neutral hookup.

You read correct Keinokuorma, And I don't know if for sure there was a good NEUTRAL hookup. I never went back out on it.

The hospitals wiring wasn't my job, I was there to fix the built-in oven that was just in storage at the maintenance shop and had to power it up to check and make sure all repairs were successful.

At the time I never thought to check each leg to ground, (the plug box or conduit). Just did my normal checks at the plug to see if it was a 240volt plug before tryng to hook the oven up to it and make sure there was power to the plug since the guy that let me in didn't know what breakers went to the plug and it had no power when I started until we found the breaker for that plug and turned it on.

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Midwest,

If (and apparently when) you are an appliance tech, contact the Samurai about upgrading your status from Grasshopper to Master Appliantologist. That way you can start your own topics, you'll later on upgrade to Sublime Master after posting enough, etc. You probably even get a free upgrade, while a "layman" upgrade to an Apprentice costs $5. Grasshopper and Apprentice statuses will not upgrade.

Also I'd like to note to you that by the Netiquette it isn't exactly advisable to post your company advertisements in an unrelated thread. You may post your ad here: http://applianceguru.com/forum18/ where people are pointed to look for local servicers.

EDIT: That forum appears to require the Sublime status, but I think it only takes 50 to 100 posts as a Master to obtain.

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Not sure how a grasshopper is posting in this thread but I am closing it down...

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Our friend, Midwest, has joined the ranks of the Grasshoppers and so, sadly, we will no longer enjoy his spammy posts. However, it is my fervent hope and prayer that by re-opening this topic, we can all, somehow, get over the collective grief that I'm sure we all feel at the loss of our dearly departed friend, Midwest. :moon:

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Blessed A. :dude:

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Thanks, was not figuring out how the dude was posting in new threads...so better safe than sorry.....:poison:

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I ran into a similar situation in an HVAC scenario in Atlanta where I was expecting 208 VAC on all 3 lines of the 3 phase system.  I got 208 208 and 242.  Asked my tech school instructor and he advised that it was the type/configuration of the transformer serving the location.  Cant remember how exactly it went; however, a particular Wye or Delta configuration at the step down transformer to the building's panel will give you the 1 line that is over and above the other 2 legs and outside of the 10% tolerance for motors and 2% tolerance between legs. 

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Wait... this may be calling for another unintelligible hand sketch accompanied with easily misunderstood writing that contains overly technical jargon... be quick to say NO if you don't want it...

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Ptrue = 1.73 E LINE  I LINE  COS   (power factor)

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Hmm... on a 3 phase system,

E(Lx-Ly) = SQR(3)*E(Lx-N)

I was thinking of a special stepdown that has 3 split phase outputs... that layout could be wired to give 6x120V, 3x208V, or 3x240V feeds... and at least theoretically, if not all phases have a common center tap, it could be wired to produce abnormal voltages too... I'll try to figger a possible wiring...

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That formula is for a delta-connection....... and we all know, that delta connection provides an increase in current but no increase in voltage....A wye connection provides an increase in voltage but no increase in current.... We nad to learn all of this stuff in the Technical college, years ago........ Your brain is like a piece of cheese  in the rear of the drawer in the refer.   if you use the cheese regularly it stays good , but forgotten and when you want it, it is now moldy....    P.S  I had to take a peek in the book to get the statement correct.      USE IT OR LOSE IT   ( my headcheese)

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Uh,

Hmm.

It's primarily the voltage fed to the load that increases when you go from wye to delta. In Ameedica, typically 120V => 208V, in Yurup, typically 230V => 400V.

The higher voltage yield allows to transfer more power by the same wire gauge, or the same magnitude of power with smaller gauge.

Note that these are rough nominal RMS values for the voltage output. Expect the RMS 120V sinusoid waveform to have peak and bottom values of +/- 170V!!! The RMS (Root Mean Square) value correlates with the efficiency factor of said waveform: Steady 120V DC would deliver the same power to a pure resistive load, as RMS 120V AC sinusoid voltage. Or, for the matter, if there was triangle, sawtooth or square AC wave fed to a load, equal RMS waveforms would have the same efficiency, but different peak values.

Efficiency factor (RMS/PEAK) for sinusoid wave is 1/SQR(2) or roughly 0,71. The peak value would be SQR(2) or roughly 1,41 times the RMS value.

Here's a graph with phase voltage vectors, N for the neutral or "equilibrium" point, origo, whatever... L1 is marked for the upstraight vector, the other two are 120 degrees off of it and each other. full cycle = 360 degrees, 1/3 turn = 120 degrees. So the black vectors  originating from the origo (how come???) depict the main voltage vectors. The blue vectors drawn between the main vectors depict the live-to-live voltage. The distance each vector covers in the graph is proportional to the voltage output between the vector's endpoints.

You could measure straight off with a ruler, and calculate that the blue vectors are roughly 1,73 or SQR(3) times as long as the black vectors.

Further, if you were to draw this graph to a piece of cardboard, cut it off along the edges of the triangle, and be able to suspend it exactly by the Neutral point, it would (in an ideal case) hang entirely flat and balanced. That point would be the gravitational equilibrium point of the triangular cardboard piece.

And you know what, this crappy sketch is actually drawn with a little help from the computer. :dude:

wyedeltagraph.bmp

wyedeltagraph.bmp

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