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Post by the light works on Jan 11, 2013 16:41:36 GMT
I ran into a rather odd trouble call this week. the plumber had called me because he was getting shocked off of the steel chassis of a mobile home.
my first line of endeavor was to check all the appropriate voltages - I was suspecting a broken neutral which was sending load current to ground through the trailer chassis, but ALL of the terminal to terminal readings were exactly perfect.
then I suspected a faulty appliance was sending stray voltage to the grounding system - but again, got perfect readings on all the terminals.
Finally, I started looking away from the home, and found the problem occurred even with all power to the home disconnected.
looking even further away, I found the neighboring houses were showing the same symptoms.
still, at this point we suspected the problem was with the service wires to the home. (underground leads) so we disconnected the home - and STILL had the problem.
eventually, we studied every home connected to the same transformer, and determined that in every case, shutting off the home reduced the problem, but no one home made it go away completely.
the symptom was that every time we tested from the dirt to a grounded conductor we would get AC voltage, ranging up to 40VAC. this was with standard voltmeter probes, not with ground testing equipment.
I'll leave it like that for a while to allow speculation - then come back with what we finally found the problem was.
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Post by rick4070 on Jan 11, 2013 16:46:45 GMT
Shorted telephone lines???
Just guessing...
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Post by watcher56 on Jan 11, 2013 18:15:53 GMT
Well, what ties the 'grounds' of all the trailers together...
Electrical grid, but you said you disconnected that (I assume you also disconnected the neutral) and the problem remained.
Telephone - telephone would need to be shorted (or have induced voltage) from the power lines, as the phone itself uses DC.
Plumbing - Depending on the age, plumbing may be all metalic therefore connecting the units, or it may have plastic that would isolate....
Internet/Cable TV cable? Not uncommon to find voltage on the shield of internet/cable tv cable - particularly if it is not properly grounded at the entrance. TVs, cable boxes, modems, all have ground leakage currents that need to be properly diverted.
I'd guess it had something to do with the last.
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Post by watcher56 on Jan 11, 2013 18:25:26 GMT
Would still need to have a loss of ground somewhere. Ground at the transformer itself compromised?
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Post by GTCGreg on Jan 12, 2013 0:55:33 GMT
Had a similar problem at my dad's house a number of years ago. In that case, is was the result of severe storm damage. What usually causes this type of problem is a fault that causes large ground currents. In the case of my dad's, one of the 120 volt leads from the transformer shorted to the neutral, as a result, the neutral burnt open at the transformer connection. This put 120 volts between the neutral and earth ground at each house connected to the transformer. There were 5 homes in this case and all had the same problem.
Since the neutral was bonded to the ground connection in each service entrance, you ended up with 120 volts between earth ground (such as a water pipe) and electrical ground, such as the case of anything with a 3 wire plug. In my dad's case, you could take a light bulb and connect it between his kitchen sink faucet and the refrigerator door handle, and it would light. Even with the main service breaker turned off.
The strange thing is the even though each house had a ground stake, the ground resistance was too high to effectively ground the system.
Because of the storm damage in the area, it took three days before the power company sent out someone to check. I enplaned to the lineman that showed up what was going on but he didn't believe me. He then proceeded to grab the meter box to remove the electrical meter and got knocked on his butt from the 120 volts on the box. My only response was a big grin and an "I told you so."
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Post by the light works on Jan 12, 2013 4:00:09 GMT
not bad. in this case, we suspected a transformer failure, but it turned out the return conductor on the primary had broken loose; so any current was flowing back in the ground; instead of on the conductor. I'd turned the problem over to the linemen, before they found that, so I'm not sure exactly where it was. the end result was the ground was energized, and looking for someplace to go.
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Post by silverdragon on Jan 12, 2013 8:27:25 GMT
After reading the original post...
I was going to suggest disconnecting any "Ground" that actually made it to Ground to see if that changed anything...
Electricians first rule "Is it plugged in", as in chase the power, if you disconnect something and you STILL have power, disconnect any possible source of power, INCLUDING the ground....
The phrase "Borrowed neutral" springs to mind there... I was working on a site one time where we had that problem, the site was separated into different phases whit different breaker boxes, but the problem we were fixing, the initial contractors had a lazy (deleted) who just picked ANY neutral when wiring in the lights.....
This caused a problem with "Live "wiring when one phase was turned "off".....
I was therefore suspecting SOMETHING MUST be plugged in?... as in who had borrowed the Ground wire as a possible neutral in some wiring disaster....
Now I have a question, Mobile home towed Caravan.. plumbing, why is the SINK connected to Ground?..
I accept that its normal household rules to ground out any metal parts on a sink or Pipework, "Just in case", but here we have a MOBILE mobile home, stood on Rubber Tyres, with no actual into the ground ground, so why ground out the plumbing?...
This is a 12v car battery system caravan, not plugged into mains at any point, no is there a mains jack available on the outside.
Is this just unnecessary wiring "Just in case" they decide to go for a mains jack?...
Surely if that ground is connected to the chassis, if you have any electrical fault with any appliance, it will make a circuit back t the power supply?... is this a case where Ground circuits could CREATE a problem?.... shouldn't all appliances used in that type of situation be "Double insulated" anyway, and have their own fault circuit to ground through the power supply?....
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Post by the light works on Jan 12, 2013 16:08:58 GMT
all potentially electrifiable parts are to be connected to the ground reference. that way all non current carrying components are at 0 potential difference. otherwise, say you have an appliance that accidentally grounds to the sink - now the sink it as whatever your line potential is, and when you have one hand opening the refrigerator and touch the sink with your other hand, you become the grounding conductor. (in US parlance, the term is "bonded" when you are connecting things that should never see electricity to your grounding network)
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Post by Lonewolf on Jan 13, 2013 4:43:38 GMT
Some years ago I did inspections of metal pole streetlights that weren't working and at one location found 220Vac on not only the pole but also on a nearby guardrail and chainlink fence. We realized that a car or truck had hit the pole and discovered a hot lead had broken inside and touched the inside of the pole. Luckily this was on a highway at an underpass by some RR tracks with the nearest pedestrian walkways well over 100 yards away so no one got hurt before it was found.
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Post by silverdragon on Jan 13, 2013 8:13:38 GMT
That makes sense... of course.... I didnt see it that way. (me=Duh!)
Sometimes the simple is escaping you?...
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Post by the light works on Jan 13, 2013 15:21:40 GMT
when you don't have your inspector/instructor hammering the importance of electrical bonding into your head, you have a good excuse for overlooking it.
I once crossed paths with an electric baseboard heater that had shorted to the housing in the middle of the element. it took us a while to figure out why it was not ever turning off (it had a single pole thermostat) when the thermostat was on, the entire heater was running at full voltage. when the thermostat was off, the second leg was still energized, and running the part of the element from the terminal to the short at half voltage.
(for european members, remember, our residential power is 120/240V single phase - a center tapped transformer so our 3 wire mains system has each line 120V to the center tap, or 240V line to line. - electric heaters are connected line to line, and the earth ground is bonded to the center tap)
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Post by the light works on Jan 13, 2013 15:23:57 GMT
to further clarify, the benefit of this is that rural power systems only need to have 2 wires on the poles, instead of 3 or 4 like rotary power requires. urban power systems use the 4 and supply rotary to large commercial and industrial customers and single phase to residential and small commercial.
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Post by c64 on Jan 14, 2013 2:01:14 GMT
to further clarify, the benefit of this is that rural power systems only need to have 2 wires on the poles, instead of 3 or 4 like rotary power requires. urban power systems use the 4 and supply rotary to large commercial and industrial customers and single phase to residential and small commercial. The benefit of the rotary current system is that twice the amount of wires can carry 3 times the power  |
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Post by the light works on Jan 14, 2013 5:40:17 GMT
to further clarify, the benefit of this is that rural power systems only need to have 2 wires on the poles, instead of 3 or 4 like rotary power requires. urban power systems use the 4 and supply rotary to large commercial and industrial customers and single phase to residential and small commercial. The benefit of the rotary current system is that twice the amount of wires can carry 3 times the power doubling the transmission line voltage can quadruple the power capacity with no extra wires. and I don't need 400 Amps in my house. |
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Post by c64 on Jan 14, 2013 13:19:29 GMT
The benefit of the rotary current system is that twice the amount of wires can carry 3 times the power doubling the transmission line voltage can quadruple the power capacity with no extra wires. and I don't need 400 Amps in my house. We have twice the voltage AND twice the wires. A common household is connected by 3x50A or 3x100A main fuses. |
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Post by the light works on Jan 14, 2013 15:29:16 GMT
doubling the transmission line voltage can quadruple the power capacity with no extra wires. and I don't need 400 Amps in my house. We have twice the voltage AND twice the wires. A common household is connected by 3x50A or 3x100A main fuses. here, a common house is connected by two 200A breaker poles, and an oversize house has 4 200A breaker poles. (our standard breaker panel is a 200A panel - oversize houses just use 2 panels) |
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Post by c64 on Jan 14, 2013 16:56:14 GMT
We have an additional power box in the cellar equipped with the main fuses. This box is locked and sealed for a very good reason. It's connected to a transformer which feeds several city blocks and messing with it is a real bad idea! Even at the power distribution box which houses the smaller circuit breakers, a 2-phase short circuit can be highly dangerous since the main fuses are not fast enough to prevent a major explosion. For industrial purposes, there is a system available which detects the arc using light sensors and causes a massive short circuit by driving copper rods into a power rail using an explosive charge - just like an airbag. The system even disables the part of the system with the arc and restores power within a split second. And just for fun, a "green" fireworks machine which causes no garbage at all: |
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Post by the light works on Jan 14, 2013 17:13:25 GMT
our systems are a bit less exciting. big commercial services are required to use Ground Fault Interrupters on the mains - any wattage that comes in on the line side and does not go out through the neutral triggers the service gear to open the circuit. I've seen an 800 amp 277/480 service open a bolted fault (too solid to burn clear) with no fireworks (except the aftermath of demanding to know why a pulled cable set was left with all the conductors bound together)
in residential service, I have seen a couple times the insulation burned off the service conductors and they were arcing inside the meter enclosure with 90% containment. (here the metering equipment opens to outdoors, and everything upstream of the main disconnects is rated for the maximum fault current the transformer can deliver, to ensure the breakers can open the system no matter how bad the fault. of course, at 120V our arc flashes are about a quarter the intensity of 240V arc flashes.
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Post by c64 on Jan 14, 2013 18:03:54 GMT
everything upstream of the main disconnects is rated for the maximum fault current the transformer can deliver, The problem is that we have much bigger transformers feeding 400V (Phase to phase) with up to 50kA into the lines to the blocks. |
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Post by the light works on Jan 14, 2013 18:10:40 GMT
those are the tradeoffs we deal with when we design an electrical system.
my breaker panels are rated for 100,000 AIC (Amps Interrupting Capacity) our average residential transformer is a 50KVA transformer. it will generally carry as many homes as you can reach within reasonable secondary runs.
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