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RussWhite · 10-21-2010, 12:49 AM

Richard and Brad,

Well I slept in my Newell last night and I am an electrical engineer :-)

I have to take exception with Richard's explanation of ohm's law. As an example think of a heater. Imagine the voltage slowly going all the way to zero, what is the current at that point - zero right? So no, current does not always increase when voltage decreases, in fact they normally are going the same direction. There are exceptions. A load that is trying to deliver a constant amount of work is one of those. Imagine a motor which is trying to maintain its rated rpm's because it is related to the number of motor poles and the frequency of the applied AC power. As the voltage goes down in that case the only way to balance the input with the output is for the amperage to go up. So in the case of a motor home, devices with AC motors ( air conditioners, household refrigerators, air compressors,... ) are adversely affected by low voltage due to heating from increased currents. Other devices however are not. Lamps, heaters, and all the other resistive loads will not be harmed.

That all said, the single biggest reasons for massive electrical problems in a coach is a bad neutral connection on a 240vac system. Without a good solid low resistive neutral, the 240 or whatever voltage you have, will not be evenly divided 120 to 120 as expected. Depending upon what appliances you have turned on at the time of the defective neutral the voltage could divide very unevenly, 200 on one leg, and 40 on the other. All the devices on the 200 volt leg that are turned on will suffer serious damage.
There are system that can be added to your coach that attempt to detect and disconnect before damage is done. They are probably not perfect in all cases that could develop, but at least they give you a fighting chance.

I chose http://www.progressiveindustries.net/ems_hw50c.htm Progressive Industries EMS-HW50C. It does provide a very nice hand if you are trying to convince a campground they have a problem. If the EMS will not energize, they need to get their power post repaired. I have used mine for just that and had good results with getting the post brought up to specs.

Hope some of this helps.
Russ - EE #530

10-21-2010, 12:49 AM	#3
RussWhite Senior Member Join Date: Jan 2010 Posts: 346	Richard and Brad, Well I slept in my Newell last night and I am an electrical engineer :-) I have to take exception with Richard's explanation of ohm's law. As an example think of a heater. Imagine the voltage slowly going all the way to zero, what is the current at that point - zero right? So no, current does not always increase when voltage decreases, in fact they normally are going the same direction. There are exceptions. A load that is trying to deliver a constant amount of work is one of those. Imagine a motor which is trying to maintain its rated rpm's because it is related to the number of motor poles and the frequency of the applied AC power. As the voltage goes down in that case the only way to balance the input with the output is for the amperage to go up. So in the case of a motor home, devices with AC motors ( air conditioners, household refrigerators, air compressors,... ) are adversely affected by low voltage due to heating from increased currents. Other devices however are not. Lamps, heaters, and all the other resistive loads will not be harmed. That all said, the single biggest reasons for massive electrical problems in a coach is a bad neutral connection on a 240vac system. Without a good solid low resistive neutral, the 240 or whatever voltage you have, will not be evenly divided 120 to 120 as expected. Depending upon what appliances you have turned on at the time of the defective neutral the voltage could divide very unevenly, 200 on one leg, and 40 on the other. All the devices on the 200 volt leg that are turned on will suffer serious damage. There are system that can be added to your coach that attempt to detect and disconnect before damage is done. They are probably not perfect in all cases that could develop, but at least they give you a fighting chance. I chose http://www.progressiveindustries.net/ems_hw50c.htm Progressive Industries EMS-HW50C. It does provide a very nice hand if you are trying to convince a campground they have a problem. If the EMS will not energize, they need to get their power post repaired. I have used mine for just that and had good results with getting the post brought up to specs. Hope some of this helps. Russ - EE #530