I was asked by my supervisor what was going on here, and was looking for some additional help from anyone who may have thoughts.

I can't really draw a picture here, so I'll describe it as best as possible. There is 120 Volt power supply leading out to a machine/load. The load is I believe a saw trimmer line that needs this voltage. Basically, one power supply, one resistor. Theoretically, the voltage on the ground end of this load should be zero; however, it is measured as 6.7 volts. So this is causing problems. The current measured was to be 16 amps just after the load, so on the ground end of the load. The length of the copper cable going from the load to ground is 600 feet. The cable used has this information on it "T90/TWN75 NYLON FT1 CSA LL35335 (UL) THHN OR THWN OR GASOLINE AND OIL RESISTANT II OR MTW 600V VW-1 4/0 AWG E101805".

I am to look at, and comment on why there is still voltage on the line when it is directly connected to ground, or what can be done to get the 6.7 volts down to 1-1.5 volts which the instruments on the ground end normally function at without malfunctioning. Any thoughts would be appreciated.

Thanks.

 

I'd like to know how the measurement of voltage was made. What was the reference point. 6.7 volts of IR loss over 600 feet does not seem excessive. 6.7V/16A = 418 milliohms or 700 milliohms per 1000 feet. If you check the properties of the power run to the trimmer you should find that this is within expectations. If not then there is somethine else going on that requires further investigation.

According to my copper wire table 700 milliohms per 1000 feet is typical of wire with an 8 or 9 AWG. If it really is 0000 AWG then something else is going on.

You don't say what kind of problems this is creating. Your best bet would be to step the voltage up, send it down the line, then step it back down.

 

First, are we speaking of the earth ground line, or the AC common? You have two or three lines of 4/0 out to the load? If you mean AC common, then skip to the rest of my comment. If you actually mean a voltage on an earth ground, then there is a problem with the wiring or the ground which could rapidly become a lethal shock hazard. The power should be cut and the problem throughly investigated.

Do you have the 6.7 volts only at the load end, or is that voltage also present at the origin of the ground line? If it's present at both ends, I'd say you have an electrical problem and need to see where there is a poor quality connection.

On the other hand, with 600' of even 4/0, there's going to be some resistance. According to this site - http://www.powerstream.com/Wire_Size.htm - it's .049 ohms/1000'., Your 600' length should only be .0294 ohms, and the voltage you see suggests .419 ohms.

By the way, the best-case voltage is going to be .470 volts at the load end of the AC common line.

 

I am NOT an electrician - you need one.
With that kind of length, you certainly would be better off running a 240v motor on the saw. When you double the operating voltage, you cut the current in half so that total power in Watts stays the same.
Ohm's Law states:
P = EI, or Power(Watts) = Voltage x Current
Since you are running a 120v saw, you should have three conductors going to it:
1) L1 or L2 (hot, black insulated wire)
2) Neutral (white insulated wire)
3) Ground (green insulated or bare copper)
The hot wire and the neutral wire are what complete the electrical circuit from the panel to the motor.
The ground wire is for safety; the protection of the machine's operator. It should never be used to carry current, as then there could be a voltage drop across it, causing a safety hazard.

At the breaker/power distribution panel, neutral and ground are tied together in ONE place, and that is the ONLY place that neutral and ground should meet. There should be a large ground wire leading to at least one long copper-clad stake pounded into the ground.

Coming from the power company, you get (in a home power system) L1, L2, and Neutral. L1 and L2 are opposite ends of a 240v step-down transformer, and Neutral is the center tap; therefore between either L1 and Neutral or L2 and Neutral, you get 120VAC, and between L1 and L2 you get 240VAC, as L1 and L2 are 180° out of phase.

Since ground is tied to the Neutral line at the breaker/distribution panel, Neutral will measure 0 volts to ground at the panel. Without the tie to ground at the panel, all three lines (L1, L2, Neutral) could "float" to arbitrarily high levels, which would be extremely dangerous.

You might have multiple problems going on there, such as a corroded/burned buss, defective breakers, corroded connections, load imbalances between L1/L2, and other items.

You really need to have a qualified electrician come out and look at the situation.

 

An electrician was the one who gave the readings. I am a student going into Electrical Engineering so they gave me this to see if I could figure anything out with it. Everything is set up to code at the mill I am working at and there have been a few electricians who have looked at this that work at this mill but haven't had any thoughts on it, or realistically... they just don't care. But such thoughts have came to mind myself such as the one's you have mentioned SgtWookie. I appreciate the other comments as well, they are helping me along with alternate ideas and possibilities of what they problem(s) may be.

 

lukas86,

First of all, no disrespect to everyone else here, but home wiring colors will not be the same as industrial. There are a ridiculous number of "standard" schemes. The electrician should tell you what is what.

Now for your edification, for any circuit you have the "hot" which will source the current to the load, and you will have a "neutral" or return that will complete the circuit. In addition, for power systems, you will have "ground". The ground line should not have current flowing through it in the normal situation. Its sole purpose is for safety. If you detect current flowing through it (use a clamp-type current meter to measure it), it is a dangerous situation, and this should be fixed immediately (as beenthere stated).

Did it work before? That is important. That means that it probably was set up correctly, and something has broken.

Generally bad cables will show larger resistance than spec'd. This is usually due to a partial break somewhere along the line. Typically, you will also notice with high currents that the cable is getting warm at that point. Also, note that many times the high resistance point could just be a bad connection between the cable and source or load, so check those.

Here is how to diagnose this situation. First check all of your connections as noted above. Then check the safety ground current as stated above. Assuming that is okay, check the voltage between the return and the ground at the source of power. If should be small if not zero, but a couple of volts is not atypical. And check the power to return voltage at the power source. If they are too low, then the problem is before this, not in the cabling. If that is okay, move to the load. If no significant current is flowing through the ground, then the difference between ground and return at the load vs. the source should only change due to resistance along the return cable. If the voltage drop between ground and return is the same (or really close) at both points, then that cable is good. Now measure the voltage from hot to return at the load side. If it has a different voltage than at the other end, then that is because there is a voltage drop on the hot cable.

 

Voltage drop across 600 feet of 4/0 at 16A should be less than half a volt.

As has been noted by others, check the connections. Don't just wiggle them and say "yeah, they seem okay." Have the electrician take them apart, clean them, and put them back together. Add a dab of antioxidant when putting them back together. Corrosion is often the culprit when symptoms like these are observed.

 

Several others along with myself have mentioned corrosion.

I've helped a few friends out with problems in their home wiring. A couple of the problems lead right back to the breaker/distribution panel. When the front panel was removed and the breaker of the problem circuit was popped out, it was discovered in two of the cases that the aluminum buss connectors had corroded, leading to a high resistance connection.

In one case, it was the L1 connection, resulting in reduced output voltage under load and high temperatures at the spring buss connector of the breaker, causing it to lose tension and burn even more.

In the other case, it was the neutral buss connection, resulting in a neutral that floated away from ground level under load, and similarly burned buss connector on the breaker.

I wouldn't be surprised if you found this latter case to be quite similar to your own.

Corrosion is a big problem with busses that are made from aluminium, particularly if they haven't been treated with antioxidants. Tinned copper busses used to be the norm, but copper prices have been skyrocketing for quite a while.

 

Apparently this problem has been there ever since the mill started up which was about a year and a half ago. The original ground was put in and this was where the 6.7 volts was measured. A separate ground was put in and the original ground was jumpered to the 'new' ground to lower that 6.7 volts.

I will tell the electricians to do a few measurements as you stated Caveman. I am not sure how long it will take them as they are busy with wiring problems in the building of the new planar mill on site as well.

 

If you are reading voltage between the neutral and ground wire at the motor then your problem is most likely to be a bad connection with the neutral wire possibly at the distribution panel or any connection between point of mesurement and the panel. A good way to prove it is to stop the motor then voltage should drop back to zero.

 

This is my attempt to draw the diagram, which is how my supervisor drew it for me. Vs is the source power, 120 volt supply. L stands for the load which requires this power. Now apparently between A and B there is 113.3V, leaving 6.7V on the ground side. So J stands for the jumper where another ground cable is temporarily in place to lower that 6.7V. So on the diagram he showed me, there is 6.7V across J, the jumper.

Apparently, this doesn't want to work, because the spaces that should be in there collapse and doesn't look right. So I'm going to throw in some hidden white letters so there's spacing.

|-----------------------------------------------------|
| aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaA |
| aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa|
Vs aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaL
| aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa|
| aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaB |---J---|
| aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa|aaaaa |
|-----------------------------------------------------| aaaaa|
| aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa|
|-------------------------------------------------------------|
|
GND

 

Please measure the current through J and post it.

 

I think this is what you meant to post.
Without the CODE brackets, the board really messes up ASCII schematics.

 |-----------------------------------------------------|
 |                                                   A |
 |                                                     |
Vs                                                     L
 |                                                     |
 |                                                   B |---J---|
 |                                                     |       |
 |-----------------------------------------------------|       |
 |                                                             | 
 |-------------------------------------------------------------|
 |
GND

 

Ya thanks, I will try to find an electrician to measure that. I've only been here for 2 weeks so not allowed to measure anything myself. I am not sure how long it will take for them to get back to me.

Supervisor is going to check, he said he'd let me know in 10 mins or so

 

Ok, lets see if I can draw this one. My supervisor came back and had a little picture with values on it as well.

               |----------------------|
               | Optimizer End Column |
|--------------|                      |
|              |                      |
|   A|--N      |                      |
|    |         |                      |
|----O---------|                      |
               |                      |
               |                      |
               |                      |
        |------O                      |
        |      |----------------------|
      B |
        |
        |             16.7A @ 60.0 Hz               To PDC
<-------O----------------------------------------------->

There we go, and N is the neutral, and A I believe is the jumper and has 5.7 amps across it. B has 4.2 A across it which is the machine ground. The "To PDC" is I believe a common ground for the mill. I am not sure.

 

Did the 113v come back to 120v when J was connected? If so I refer back to my earlier post, check the B wire coing back to the power source.

 

The ground conductor should NEVER carry current, in this case it seems that it's doing the job of the neutral.

 

The 113v did go back to 120v when J was connected. Apparently there are 2 other grounds connected to that large metal piece of machinery "Optimizer End Column" which I'm thinking has to do with the 1.5A missing from the 5.7A cable to the 4.2A cable. The electricians are out taking measurements or will be some time today though.

 

Have the electricians measure the current on L1 and L2 at the distribution panel that's the source for the optimizer. It could be that there is a fairly large imbalance.

 

Ya I told my supervisor that if the phases are a bit off, or there's some fluctuation between the different phase voltages that could mess something up, but the electricians are somewhat new so I'm just waiting to see what they bring back. Like I asked them to measure the resistance of a 4/0 wire, they're like "1 ohm" ok... well it should be way less, want to try again... thanks again for the help/advice on this though, I'd be fairly lost without it.