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Trouble diagnosing shop power outage

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Forum topic by MNRelic posted 763 days ago 890 views 0 times favorited 11 replies Add to Favorites Watch
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MNRelic

5 posts in 1658 days


763 days ago

My workshop is in an outbuilding. Power is supplied from 100 AMP 240 VOLT breaker in the house breaker box to a 100 AMP servicer box in the shop via underground cable installed by an electrician 5 years ago.. We had been away and when we returned the power was out in the shop. The 100 AMP breaker in the house had popped. I reset the breaker but there was still no power to the shop. I reset the breakers in the house and in the shop Stiill no power to the shop. Used a volt-ohm meter. Found that ther was 120V on each side of the breaker in the house, but ony 10V coming into each side of the breaker in the shop. i suspected a break in the underground wire, so I dug a hole to begin trying to ‘guess’ where the break might be and found the underground cable. The non-contact voltage tester didn’t indicate any voltage in the underground line.

The breaker in the house stays set, 120V going out from the house breaker to the shop, 10V at the shop, no voltage indicated in the underground line. My best guess is a break in the underground line, but why is ther 10V showing incoming into the breaker box in the shop, but no voltage showin up in the underground line.

Could the 100A breaker in the house be defective and putting out 120V at a low amperage?

Any suggestions to explain or fix the situation would be appreciated.


11 replies so far

View waho6o9's profile

waho6o9

4824 posts in 1202 days


#1 posted 763 days ago

I would call an experienced electrician, that ’s a lot of amps to be messing with.

View brtech's profile

brtech

664 posts in 1548 days


#2 posted 763 days ago

The non contact meter can’t see 10 V. You probably have a break in the line.

I would:
a) turn off the breaker in the house. Check to see that the 10V goes away.
b) run an extension cord from the house near the panel to the shop near the panel. Don’t plug it in, it’s just a piece of wire. Just so I can explain it, put the plug in the shop and the socket in the house
c) run a little test – set your meter to volts, verify you have zero volts on at the socket. Switch your meter to ohms. You should read infinite (199 digital). No connection. Then connect a small piece of wire between the prongs of the plug in the shop. Go back and measure ohms at the socket. You should see a few ohms. You are verifying you can read resistance through the extension cord.
d) just to make sure we know the ground is good, put the meter between the center (ground) socket and one of the two power sockets, leaving the wire connected between the plug pins. You should see a few ohms. You are verifying the ground wire in the extension cord works.
e) Now, making sure the breaker is off in the shop and in the house, measure the voltage between the two “hots” at the house panel. They should read zero – you disconnected both ends by turning the breakers off. Then measure ohms. You should read infinite, no connection. There shouldn’t be a connection if it worked, but we’re just making sure
f) Now, put one side of the meter, set to ohms, on the middle (ground) side of the extension cord, and the other side of the meter on one hot at the main panel. You should read infinite, no connection.
g) Connect a small piece of wire between the middle (ground) pin of the plug to one of the hots. Read ohms. This is test measurement 1.
h) move the wire to the other hot in the shop, read ohms. This is test measurement 2.
i) move the meter to the other hot in the house, read ohms. This is test measurement 3.
j) move the wire back to the “first” hot in the shop, read ohms. This is test measurement 4.

Now for the conclusions. I think I have done this right, another jock can check me.

If tests 1-4 read infinite, the underground cable is completely severed somewhere.
If tests 1 and 3, but not 2 or 4 read a few ohms, the cable is good, the problem is somewhere else
If tests 2 and 4, but not 1 or 3 read a few ohms, the cable is good, the problem is somewhere else
If test 1 works, but not 2-4, the wire connected to the first hot works, the other one is broken somewhere
If test 4 works, but not 1,2 or 3, the wire connected to the first hot works, the other one is broken somewhere
If test 3 works, but not 1, 2 or 4, the wire connected to the second hot works, the other one is broken
If test 2 works, but not 1, 3 or 4, the wire connected to the second hot works, the other one is broken

If you have some other combination, you did something wrong.

We don’t know which hot on the house side matches which hot on the shop side, which is why we need 4 tests instead of two. We’re measuring the resistance of the underground cable, one leg at a time. A resistance check, like a voltage check needs two connections to the meter. The ground wire in the extension cable is one, and it’s known good. So we first try one hot in the house and one in the shop. Might work, but they could be reversed, so we try the other hot in the shop with the same on in the house. That’s tests 1 and 2. Then we try the other side the same way (tests 3 and 4).

View cutworm's profile

cutworm

1064 posts in 1419 days


#3 posted 763 days ago

Was the line to the shop spliced at any time. If so that is your problem. Look for discolor on the wiring at the breakers as well. This indicates a bad connection. Wiring once buried should be ok unless it’s cut by something.
Measure voltage on top and bottom of the breakers to make sure they are ok.

-- Steve - "Never Give Up"

View MNRelic's profile

MNRelic

5 posts in 1658 days


#4 posted 763 days ago

Thanks brtech for the detailed test plan. I’ll give it a try and let you know the results.

View TopamaxSurvivor's profile

TopamaxSurvivor

14721 posts in 2301 days


#5 posted 762 days ago

10 volts is probably a ghost voltage which is common issue (problem) in low impedance digital meters.

-- "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View MNRelic's profile

MNRelic

5 posts in 1658 days


#6 posted 762 days ago

UPDATE – Thanks to all who have responded. All became clear this morning when the Digger Hotline folks came to mark the existing utility lines in case we had to run a new line. When the lines were marked we noted that there was a line that we hadn’t seen before along the gravel road between our house and the shop. We had been away for a month so we checked with the neighbors and found that they had a new phone line installed that ran across our buried line. Well, we knew where to dig and sure enough we found the severed cables.
The electrician just left and the shop is back in business!

View Knothead62's profile

Knothead62

2364 posts in 1586 days


#7 posted 762 days ago

Glad the problem was solved. Personally, I would put any underground cable in conduit, no matter what the UL listing of the wire. Would the phone company be liable for your expenses? “Call before you dig.” or “Look before or while digging.”

View HerbC's profile

HerbC

1161 posts in 1485 days


#8 posted 762 days ago

I hope the neighbors (and the phone company) will pay the cost of the repairs.

-- Herb, Florida - Here's why I close most messages with "Be Careful!" http://lumberjocks.com/HerbC/blog/17090

View canadianchips's profile

canadianchips

1831 posts in 1622 days


#9 posted 762 days ago

Your telephone company and your neighbors were digging in YOUR yard without marking it first ? That is scary..what would have happened if you had GAS line underground ?

-- "My mission in life - make everyone smile !"

View dhazelton's profile

dhazelton

1173 posts in 922 days


#10 posted 762 days ago

Keep record of where that underground splice is. I had an underground line NOT in conduit broken by an excavator putting a french drain behind my house. They used an underground splice which lasted all of three years. The splice WILL fail – it’s just a matter of when.

View Gatorjim's profile

Gatorjim

203 posts in 830 days


#11 posted 761 days ago

OMG brtech do you work in managment or the goverment? LOL While your adviced was well detailed it was a little over the top. Power at one end and not at the other means some thing in between is broken. I don’t mean to be mean just reminds me of were i work while managment is figureing out a plan we do the work.

-- My theroy in wood working will be. If I'm not enjoying doing it i won't do it.

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