PM 3520B: replaced inverter, still not working

My Thought as Well

Bill Howatt

If the damaged area is indeed the source of the problem, I'd be looking for seriously damaged copper conductor and perhaps some serious heat/smoke damage around the bad spot.

Bill

Re: Low or uneven voltage was the problem

mikeee12345

Some breakers don't trip and many times there is not a direct "short" between ground or neutral and the insulation continues to heat up and turn to carbon until there is a direct "short". This can take quiet a while, the moisture and carbon provides a path for the electricity to slowly erode the insulation once it is exposed.

Re:Time to call a professional.

Stuart Johnson

You might fix the immediate problem but leave a more serious one that might cause a fire.

Re: Low or uneven voltage was the problem

Dennis in Southern Oregon

Let me illustrate with an example. Suppose the conductors are 12 gauge copper, and the distance from the "leak" to breaker box, where the conductors become much larger, is 30-feet (60-feet round trip). From standard tables, the round-trip resistance of the feed wire alone would be 0.095 ohms. Now suppose the "leak" dropped the supply voltage by 5% from 240V to 228V, a drop of 12V. The current through the leak would have to be 126A to make that happen. The power dissipated in the "leak" would be about 29 kW (very briefly).

Re: Low or uneven voltage was the problem

John K Jordan

The flaky power I've found has been due to a bad connection at a screw terminal or at a wire nut inside a breaker or switch. Sometimes the screw loosens or was not tight enough initially. Sometimes there is corrosion from moisture.

The load can indeed cause heating at the bad connection. I saw a breaker in an outdoor box in pieces from overheating. There's a good reason breaker boxes are steel.

One thing I like to do a few months after wiring a panel is recheck the tightness of all screws.

I have started using dielectric compound on connections when wiring. This keeps the air and moisture away and helps to prevent corrosion. Dielectric "grease" is also great for every connection in vehicles, from trailer plugs to taillight bulbs. I use it on cars, 4-wheelers, tractor, etc.

As for nicking the insulation of a wire: if the wire is aluminum I've heard it can corrode rapidly if moisture is present. I used big copper 250' underground to my shop for 100 amps even though it was far more expensive than aluminum. Some electrical people said I was crazy but too bad. I did use aluminum 1000' underground for 70 amps but I worry about it. (BTW, that was one BIG spool of wire)

JKJ

Re: Low or uneven voltage was the problem

Chuck Engstrom

I am wondering what you intend by "dielectric": insulating or conducting? I have encountered confusion on this word in the past. Any particular brand-name product you could point to that you are using?

Thanks --

Turned you were right, Dennis: eroded cable strand

Aage Rendalen

I found the nick in one of the cable strands, but it was insignificant and patched to my satisfaction. I dug up the entire cable (a bit over-dimensioned perhaps, with four strands of wire and 7 strands in each one) and only then did I see what you mentioned--a cancer on one of the strands, a bulge on the line and the copper completely pulverized.

I suppose I shall have to replace the entire cable. I brought it on myself by only shielding the cable in pvc piping part of the way to my workshop. I must have hit the cable with a shepard's hook or a spiked tiki torch.

Re: Turned you were right, Dennis: eroded cable st

Dennis in Southern Oregon

Aage, where I live, the minimum burial depth in unprotected soil for non-metallic electrical conduit is 18-inches for just that reason

Re: Turned you were right, Dennis: eroded cable st

Josh Bowman

I think your chasing the wrong issue. I agree with John "One thing I like to do a few months after wiring a panel is recheck the tightness of all screws." I wired my house and shop and in time after circuits get warm and cool, had screws loosen at the breakers. Either open the main and use an insulated screw driver to check all breaker connectors or if you're not comfortable with that call an electrician to do it for you, they'll most likely only charge a minimum to come out and tighten them and in addition find any other issues you have with your shop power supply. That nick in the wire is a problem......it's just not THE problem.

Simply checking voltage with no load on the line is usually not going to do more than tell you the breaker is closed or the line is cut. Volts and amps are like water in a pipe. Pressure=voltage and flow from the tap=amps. A squirt gun has lots of pressure, but little flow. A tub of water sitting on the ground has lots of flow but little pressure. I said that to say this, you check your power line with a volt meter and it says you have full voltage, but if you have a loose connection somewhere, when you try to start a motor the bad connection acts like an orifice and reduces flow (amps) and pressure (voltage). But when the motor is turned off the pipe (wire) repressurizes to full pressure (voltage). I wish you were closer to Tenn., I feel this can be knocked out fairly quickly.

One entire cable strand was pulverized

Aage Rendalen

I don't know enough about electrical work to contest what you say, but in so much as one of the cable twines with 7 wires inside (aluminum, not copper) was eaten up, I would be inclined to think that that would do it.

Re: One entire cable strand was pulverized

Josh Bowman

Assuming you mean you have a cable with with 3 bundles of wires, each made up of 7 strands, I'm still inclined to believe that's not the problem. I'm not certain what gauge wire you have, but assume it's pretty big (small number for gauge) and continuing with the analogies, that's like one side of a 7 lane freeway and one lane is closed. And guessing you only have running on that cable lights and trying to get the lathe to run, I'd make that freeway 6 lanes in a small city, so it's not loaded up. BUT I hope I'm wrong! Now I don't disagree that the cable needs repaired, either a splice or replacement, I'd just like to see all the connections tightened before you go to all that trouble and find you still have a problem and it's only a connection screw. It would be nice if it were possible to plug the lathe into an outlet not associated with the shop by either moving the lathe or by using an extension cord. Good luck and keep us posted.

I think the cable is 8/4

Aage Rendalen

My 110 volt circuits work fine with one cable strand (out of 4) non-functional (and I have cut off the destroyed bulging wire section). After I clean-cut the cable strand, no electricity reaches the lathe, to judge from the RPM light.

Question 1: Would it be possible to rewire the 55 foot cable with the remaining 3 strands (each with 7 internal strands) and still have access to 220 vols for the lathe?

Question 2: I never draw more 30-50 amps at any one time in the workshop. What gauge wire would you recommend I buy for that 55 foot run from my garage panel to the woodshop sub-panel?

Re: I think the cable is 8/4

Barry Irby

It can be done, physically, but not to Code. Three wire systems used to be common, but not anymore. (Assuming all four conductors are the same size.)

Re: I think the cable is 8/4

Dennis in Southern Oregon

Aage, I think you may be treading on shaky ground there. I don't expect that there is any reasonable way to repair that cable safely or within code. You don't want to put yourself in the position of having an insurance claim denied in the the case of a fire or personal injury arising from a patch. The 8-gauge, 4-wire cable sounds about right, but I suggest that you visit a local electrical supply dealer and talk to a knowledgeable representative about your project.

I agree

Aage Rendalen

Dennis, a neighbor who is a contractor suggested 8/4 or 10/4 and I will replace the entire cable. Neither house nor workshop is fireproof. Thanks to you and everybody else who has offered helpful advice.

Re: I agree

mikeee12345

When you replace the cable, install in conduit or bury it deeper, or pour concrete over the top.

Those are your choices to meet code, pvc pipe is cheap, cuts easily, and glues together.

Just my 2 cents

John K Jordan

For direct burial, I like to bury wide plastic caution tape a few inches above the cable then another piece about 6" below the ground. That way when digging the caution tape is exposed before you hit the cable.

For conduit, I prefer to use 2", bigger than probably needed but easier to pull wire through. For heavy wire (I ran #1 copper 250 ft) rather then pulling I built the conduit around the wire - laid out the wire beside the trench and slid each section of conduit down the length and glued it into place.

When I ran wire underground to my shop I put in an extra 2" conduit with just a rope inside for future needs. (And another conduit with ethernet.) This is easier to do now than dig again to do it later.

JKJ

Re: I agree

Josh Bowman

If you do the conduit thing and make it about 1.5 to 2 inches you'll be able to change the cable or make it bigger in the future without digging.

Re: I think the cable is 8/4

Brad Vietje

Just to reply to the ampacity question: 50 amps can be carried by a #8 wire under certain (lower temp) conditions*, but really should be a #6 wire for safety. The distance of the run has to do with voltage drop, which can drive the need for a larger wire for long runs, but over just 55 feet it would be negligible.

This has to do with how it's contained, and where it runs. Free air allows higher ampacity, but conduit lowers that rating somewhat, and something like an exposed run on a roof could get really warm, so they want a bigger wire to handle that.

Just a few centavos....

Brad Vietje

Newbury, VT

Lathe spinning happily again: here's the inquest

Aage Rendalen

To all of you who offered me good advice last year on the problems I had with my PM3520B, here is a summary of the problem and how it was resolved:

1. Lathe stopped working, inverter code said low voltage

2. I called PM and they said I had an inverter problem, so I ordered a new one.

3. I replace the inverter but lathe still did not work. I put the old inverter back on.

4. PM service guy spent 3 hours on the lathe, declared the inverter seem okay but could not find out what was wrong. I said I had nicked the insulation of the 8-4 aluminum power cable to the workshop a month before. He declared that to be the cause, took my money and left.

5. I returned the inverter to PM and paid a restocking fee.

6. The nicked insulation (covered in electric tape) was not the problem. When I pulled up the entire power cable, I saw that one strand had been spiked (Tiki light?) and chemistry had pulverized it.

7. I replace the power cable. The lathe did not budge. The on-off switch was a problem and I replace it. Which did not resolve the problem.

8. A friend determined it was the inverter. I ordered one from the manufacturer at half the price charged by PM.

9. A couple of you guys supplied with me the parameters that PM charges $250 dollars for and in ten minutes it was spinning happily.

It took a long time, about 8 months, but I had cancer surgery last summer and haven't had much energy to push things along, but now I hope to spend a few final months at the mustard machine. Few things have given me more pleasure in life than turning.

Finally, when you need a PM service guy, make sure you get a certified electrician. Mine was not and it cost me $300 out the window.

Re: Lathe spinning happily again: here's the inque

John C Lucas

Glad you got it running and hope younget to spend many hours out there.

Re: Lathe spinning happily again: here's the inque

Don Orr

Thanks for the update and glad to hear of the successful revitalization of your mustard monster. Best wishes for many, many hours of turning fun!