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FDR wrote:
"w_tom" wrote in message ... The investigator was a personal friend who we stumbled upon as he was returning from the exploded house. He immediately explained what had happened - the broken neutral wire inside transformer and no building earth ground. It was one of those, "Of course. I never realized that could happen." moments. As it would when one is familiar with 'how and why' buildings and utilities are earthed. I'm sorry, but if the neutral broke then there could be no return current for the single phase legs. Power would go out at the house. Only if all the devices are one a single phase. With an open neutral there is still 240 volts available at the structure. If the load on each phase is relatively balanced, the error may go unnoticed. If an alternate ground path exists for the neutral things may appear normal until it fails. The current in the neutral can be much lower than either of the phases. Craigm |
"FDR" wrote:
I'm sorry, but if the neutral broke then there could be no return current for the single phase legs. Power would go out at the house. ----------------------------- Sorry but that is not true. 15 years ago the neutral failed at the transformer and our aAC was cycling up dan down smoething wierd. My low voltage alarm tripped, and woke us up. I killed the breakers and called the utility. The other 3 homes had damge to their refridgerator and heat pump compressors. Motors don't like AC that runs from near 0 to well over normal. Terry |
"craigm" wrote in message ... FDR wrote: "w_tom" wrote in message ... The investigator was a personal friend who we stumbled upon as he was returning from the exploded house. He immediately explained what had happened - the broken neutral wire inside transformer and no building earth ground. It was one of those, "Of course. I never realized that could happen." moments. As it would when one is familiar with 'how and why' buildings and utilities are earthed. I'm sorry, but if the neutral broke then there could be no return current for the single phase legs. Power would go out at the house. Only if all the devices are one a single phase. With an open neutral there is still 240 volts available at the structure. Yep. If the load on each phase is relatively balanced, the error may go unnoticed. First, how likely is that? Secondly, I still don't get how the current on the one phase makes a return path if the neutral is disconnected. I know it's AC, but what's being described is like disconnecting one of the leads to a battery. Maybe somebody can better describe the term 'broken neutral wire'. If an alternate ground path exists for the neutral things may appear normal until it fails. What alternate path? The wire is broken supposedly so there can never be a return path. The current in the neutral can be much lower than either of the phases. How do you reconcile that with Kirchoff's laws? Craigm |
Secondly, I still don't get how the current on the one phase makes a return
path if the neutral is disconnected. I know it's AC, but what's being described is like disconnecting one of the leads to a battery. Maybe somebody can better describe the term 'broken neutral wire'. The 120V service in your house is between one of the hot legs of the line from the pole and ground. If you look inside your breaker panel, you'll see that the neutral from the pole, the white "neutrals" from all the branch circuits, and the bare copper or green-jacketed "ground" wires from the branches are all bonded together and to a grounding electrode driven in your yard somewhere. So, assuming there is still power at the hot legs of the service from the pole, there will still be 120V from either leg to ground. It'll just be the ground at the electrode in your yard rather than the ground at the transformer on the pole. |
"Brian Running" wrote in message m... Secondly, I still don't get how the current on the one phase makes a return path if the neutral is disconnected. I know it's AC, but what's being described is like disconnecting one of the leads to a battery. Maybe somebody can better describe the term 'broken neutral wire'. The 120V service in your house is between one of the hot legs of the line from the pole and ground. If you look inside your breaker panel, you'll see that the neutral from the pole, the white "neutrals" from all the branch circuits, and the bare copper or green-jacketed "ground" wires from the branches are all bonded together and to a grounding electrode driven in your yard somewhere. So, assuming there is still power at the hot legs of the service from the pole, there will still be 120V from either leg to ground. It'll just be the ground at the electrode in your yard rather than the ground at the transformer on the pole. Still, if the neutral is broken at the transformer, you still don't have a return path. |
Still, if the neutral is broken at the transformer, you still don't have a
return path. You have a lot of confidence in your position, so I think an experiment is in order, to verify the conclusion. First, go up the power pole and snip the neutral at the transformer. Then, go down in your basement, open up the main panel, put a finger on either one of the hot buses, and then grab a nearby copper or steel water pipe with your other hand. If we don't hear from you again, then we'll conclude that there can indeed be current without the neutral at the transformer. Otherwise, please report your findings. All seriousness aside, if there's power on either of the hot legs, then you don't need a "return path." Even when it's connected properly, the neutral is grounded, anyway, so the "return path" is from hot to ground -- any old ground will work, and that's why it's a potentially dangerous situation. |
"Brian Running" wrote in message m... Still, if the neutral is broken at the transformer, you still don't have a return path. You have a lot of confidence in your position, so I think an experiment is in order, to verify the conclusion. First, go up the power pole and snip the neutral at the transformer. Then, go down in your basement, open up the main panel, put a finger on either one of the hot buses, and then grab a nearby copper or steel water pipe with your other hand. If we don't hear from you again, then we'll conclude that there can indeed be current without the neutral at the transformer. Otherwise, please report your findings. Ha ha. All seriousness aside, if there's power on either of the hot legs, then you don't need a "return path." Kirchoff would be intrigued by that. Even when it's connected properly, the neutral is grounded, anyway, so the "return path" is from hot to ground -- any old ground will work, and that's why it's a potentially dangerous situation. If what you are trying to say is that the neutral wire was broken, but the neutral to ground connection was still good, then I can get your point. However, saying the neutral is broken is ambiguous. I just don't get how that happens on the line. If the break is internal, as it sounds, then there cannot be a properly grounded transformer. |
FDR wrote:
Still, if the neutral is broken at the transformer, you still don't have a return path. ----------------------------------------- You really ned to talk to an alectrician. I just asked my electrician friend frind and he siad ti is all too common. OK lets look at it another way. We have a 240V CT transformer. Wiht the CT being neultral/ground. Here we have two wires going from our house to the pole, that join at the pole witht he center tap. We cut the CT open. We now have 240V hitting the breaker box. The old neutral will be somewhere between on side of the 240V and the other. If the loads are balanced, then it is possible, but not real likely that you could have 120V across each side, from either side of the 240V to the centern. I hope you will agree that 240 devices don't need a newtral return. My water heater has the element, a big honking resistor from one side of the 240V to the other. Let's put 1 100 watt light bulb on each side of the 240 transformer in series, with the junction being the house neutral. Now even without connection to the pole center tap/neutral/ ground we will ahve 120V across each bulb from common junction of the bulbs to either side of the transformer. My electrician friend says that around here, most transformer failures are caused by severe imbalance between one side of the 240V split and the other. Around here most transformers have 4 ot 8 homes. If all of those homes have the same imbalance, "BOOM". The transformer fails in a ball of fire. Properly designed installations go to great lengths to match the current per leg or phase. He hates the word phase because too many people confuse the "2 phases" of the common house transformer with the "3 phases" of commercial installs. In the typical house your have one transformer, energised by a single primary and the side of the 240CT are 180 degrees out. In real 3 phase, the phases are 120 degrees out. Draw a diagram of your pole to house circuit. Showing the ~12KV(varies by area) primary, a 240CT transformer, witht eh CT being ground or neutral or both, with the 3 (or 4) wires coming nto your breaker panel. The CT/Neutral/ground forces the sides to be 120 with refference to the CT. Without the CT, then the common, broken neutral, will follow ohms law and form a simple resitive divider. Motoros and non constant loads make it anything but simple, but just visualise each breaker feeding a differnt value resistor. Solve each side for parallel value, place the larger resistance over the lowest and multiply that percentage by the incoming 240V. It almost certainly won't be 120V. And when a motor turns on, it really screws things up. The motor can't draw enough current to get up speed, so it thermals out, and removes it's self from the circuit. Assume that after some arbitary period of time every thermal controlled device, fridges, heat pumps, furnace blowers, are all trying to start and the resulting voltage swings become very complex. This is a good way to ruin motors, blow bulbs and is a "real bad thing." And it happens. |
wrote in message ups.com... FDR wrote: Still, if the neutral is broken at the transformer, you still don't have a return path. ----------------------------------------- You really ned to talk to an alectrician. I just asked my electrician friend frind and he siad ti is all too common. OK lets look at it another way. We have a 240V CT transformer. Wiht the CT being neultral/ground. Here we have two wires going from our house to the pole, that join at the pole witht he center tap. Actually, three, including neutral. Maybe you are saying it a different way. We cut the CT open. We now have 240V hitting the breaker box. The old neutral will be somewhere between on side of the 240V and the other. If the loads are balanced, then it is possible, but not real likely that you could have 120V across each side, from either side of the 240V to the centern. I hope you will agree that 240 devices don't need a newtral return. I agree with that. My water heater has the element, a big honking resistor from one side of the 240V to the other. Let's put 1 100 watt light bulb on each side of the 240 transformer in series, with the junction being the house neutral. Now even without connection to the pole center tap/neutral/ ground we will ahve 120V across each bulb from common junction of the bulbs to either side of the transformer. I see where you are going with that. I did the math and you are correct. My electrician friend says that around here, most transformer failures are caused by severe imbalance between one side of the 240V split and the other. Around here most transformers have 4 ot 8 homes. If all of those homes have the same imbalance, "BOOM". The transformer fails in a ball of fire. Properly designed installations go to great lengths to match the current per leg or phase. He hates the word phase because too many people confuse the "2 phases" of the common house transformer with the "3 phases" of commercial installs. In the typical house your have one transformer, energised by a single primary and the side of the 240CT are 180 degrees out. In real 3 phase, the phases are 120 degrees out. Yep. Draw a diagram of your pole to house circuit. Showing the ~12KV(varies by area) primary, a 240CT transformer, witht eh CT being ground or neutral or both, with the 3 (or 4) wires coming nto your breaker panel. The CT/Neutral/ground forces the sides to be 120 with refference to the CT. Without the CT, then the common, broken neutral, will follow ohms law and form a simple resitive divider. Motoros and non constant loads make it anything but simple, but just visualise each breaker feeding a differnt value resistor. Solve each side for parallel value, place the larger resistance over the lowest and multiply that percentage by the incoming 240V. It almost certainly won't be 120V. And when a motor turns on, it really screws things up. The motor can't draw enough current to get up speed, so it thermals out, and removes it's self from the circuit. Assume that after some arbitary period of time every thermal controlled device, fridges, heat pumps, furnace blowers, are all trying to start and the resulting voltage swings become very complex. This is a good way to ruin motors, blow bulbs and is a "real bad thing." And it happens. I got it. |
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