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Old 12-12-2008, 02:12 PM   #1
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Blown Circuit and More


I recently started a circular saw from an outlet that also had a small "Makita" type battery charger plugged into it. Almost immediately the saw stopped and a flame shot through what appeared to be the location of the Makita charger. The saw was tried elsewhere and it works fine, the Makita charger is gone, but the battery in the charger is still good???? This 100amp service was in an outbuilding with a main breaker panel with the neutral and grounds wired together. This service has been set up and in use for almost 3 years without any problems up to this point. Circular saws and battery chargers and other combinations have been used frequently throughout the past 3 years without any such consequences. This service comes off of another 100 amp breaker box in another building which comes off of a 200 amp service breaker at the power pole. I since have been told that the neutral and grounds should only be connected at the pole (where the first service disconnect is), I have gone back into the service panel in question (where the circuit blew) and seperated the neutrals and grounds and connected the grounds to a ground rod. I reconnected a test outlet and it appeared to have power when I plugged in a circuit tester (2 amber lights), but I tried a small electric drill to test and I got nothing?? A voltage tester shows 240 when both hots are touched, 120 when 1 hot and neutral touched (same on both hots). Also get 120 when black side of outlet tested with white side of outlet. I'm just bewildered at this point why I'm not getting any power to the drill??? Do I need to go back further in the system and ground the second box (the one that this service comes off of and which the service from the pole goes into).

Another important point that I forgot to make is that NO circuit breakers blew in the box when this happened.

If anyone has any ideas, I'm at a loss at this point. I can supply further description if I've left out any information you might deem pertinent.

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Old 12-12-2008, 03:11 PM   #2
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Unless you have a 4 wire feed the neutrals and grounds should be tied togther in the panel.
The problem could have been a loose neutral connection. This will cause a high voltage situation when high current devices are used. That could be what blew the charger. The loose neutral could be anywhere between the POCO connection and the panel feeding this circuit.

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Old 12-12-2008, 04:24 PM   #3
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Originally Posted by joed View Post
The problem could have been a loose neutral connection.
With 1kW into your house and a reasonably unbalanced load (8A on one 120v circuit and 0.3A on the other 120v circuit), you'd have 135v on the side of your house that damaged the charger and the voltage across your neutral connection would be ≈ 15v instead of less than ≈3 mV, so you're not looking for something subtle, here.

Amber lights = neons? I wouldn't use neons for a load on the 120v.

Last edited by Yoyizit; 12-12-2008 at 04:55 PM.
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Old 12-12-2008, 04:33 PM   #4
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Originally Posted by Yoyizit View Post
With 1kW into your house and a reasonably unbalanced load (8A on one 120v circuit and 0.3A on the other 120v circuit), you'd have 135v on the side of your house that damaged the charger and the voltage across your neutral connection would be ≈ 15v instead of less than ≈3 mV, so you're not looking for something subtle, here.
Assuming the problem is an open neutral. The charger might also have just taken a crap, coincidentally.
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Old 12-12-2008, 05:05 PM   #5
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Assuming the problem is an open neutral. The charger might also have just taken a crap, coincidentally.
Yes.

the battery in the charger is still good????
Probably. You can check it for capacity and open circuit voltage.

it appeared to have power when I plugged in a circuit tester (2 amber lights), but I tried a small electric drill to test and I got nothing??
Circuit testers with neons fire on phantom voltages.

A voltage tester shows 240 when both hots are touched, 120 when 1 hot and neutral touched (same on both hots). Also get 120 when black side of outlet tested with white side of outlet.
Phantom voltages will also show 120v or 60v on a high impedance DVM.

I'm just bewildered at this point why I'm not getting any power to the drill???
Phantom voltages cannot support a current draw over 40 ľA.

Another important point that I forgot to make is that NO circuit breakers blew in the box when this happened.
So less than 15A was drawn.

You need to unbalance the loads on each side of the pole transformer's center tap to check for a high resistance neutral.

Last edited by Yoyizit; 12-12-2008 at 05:08 PM.
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Old 12-12-2008, 05:40 PM   #6
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What about testing from the neutral to a ground rod? If the neutral is bonded anywhere to ground, and is OK, you should (in theory) get a very low resistance. That said, and I think it has been said in other threads, the Earth is not a conductor.

Edit: Are you sure your drill is OK? Have you tried it in another receptacle?
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Old 12-12-2008, 05:58 PM   #7
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Turn off all 120v breakers that serve one side of the 240v, so zero amps flows.
Put a 10A load on the 120v line on the other side.

If the voltage drops <~3v across the load when the load is switched on, the neutral and wiring are OK.
If the voltage drops >6v across the load when the load is switched on, there is very likely a problem in the wiring or in the neutral connection.
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Old 12-12-2008, 08:24 PM   #8
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Quote:
Originally Posted by nprranch View Post
...I have gone back into the service panel in question (where the circuit blew) and seperated the neutrals and grounds and connected the grounds to a ground rod....
You have to undo that immediately! On a 3-wire fed building, those neutrals and grounds must be connected together, or else you can experience many more similar problems.

You can keep the ground rod connected, however.
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Old 12-12-2008, 08:32 PM   #9
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Originally Posted by Yoyizit View Post
Turn off all 120v breakers that serve one side of the 240v, so zero amps flows.
Put a 10A load on the 120v line on the other side.

If the voltage drops <~3v across the load when the load is switched on, the neutral and wiring are OK.
If the voltage drops >6v across the load when the load is switched on, there is very likely a problem in the wiring or in the neutral connection.
Wouldn't those be double pole breakers? Not too easy to shut off one leg.
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Old 12-12-2008, 08:37 PM   #10
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Wouldn't those be double pole breakers? Not too easy to shut off one leg.
He means turn off all the 120 V loads that are fed from one leg of the 240. In most panels that would mean turning off every other breaker.

Personally, I think he should just tighten everything in the panel up, and see if it happens again. If it were the service neutral, he would notice it by the lights dimming and brightening all over the house when a heavy load comes on. It may just have been the battery charger going south during the VD event caused by the saw.
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Old 12-13-2008, 08:45 AM   #11
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Quote:
Originally Posted by InPhase277 View Post
He means turn off all the 120 V loads that are fed from one leg of the 240. In most panels that would mean turning off every other breaker.

Personally, I think he should just tighten everything in the panel up, and see if it happens again. If it were the service neutral, he would notice it by the lights dimming and brightening all over the house when a heavy load comes on. It may just have been the battery charger going south during the VD event caused by the saw.
OK. Now I see that's what he was talking about. I guess I was too sleepy when I posted my reply last night<g>.
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Old 12-13-2008, 10:34 AM   #12
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Here's a better way. I ran it through a homebrew spreadsheet for a resi. power circuit and it seems to work.

If you switch on a ≈10A, 120v load on one side of the neutral, and the voltage noticeably increases (≈2v or so) on the other side of the neutral when the load is switched on, then the neutral connection is bad.

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Last edited by Yoyizit; 12-13-2008 at 10:39 AM.
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