Originally Posted by cheezypsnookle
can u explain backfeed to me?
"Backfeed" is when the electricity from your generator or battery bank/inverter goes OUT the external power line to the outside world, instead of coming in. If your bus is unplugged, backfeed is a danger to you, as you will have live, lethal voltage on the exposed bare male pins at the end of the cord. If you touch them, or touch the bus when you have the cord coiled in a cabinet with metal walls, watch out!
A greater danger is if the shoreline is plugged into a live outlet. If the on-board source is not synchronized with the shoreline, i.e. the pluses and minuses of the alternating current not occurring at the exact same milliseconds, the shoreline will grab the windings of your generator and could do great harm. (The inverter generators that are capable of being paired may be able to survive this with electronic rather than rotational synchronization.) I have seen video of a utility generator bigger than a semi-tractor self destruct when it was intentionally connected out of phase as part of a test. The phrase "having the generator walk across the room" is often used to impress the seriousness of this.
The greatest danger is to electricians and utility workers. Imagine an electrician got a call to the campground, and started by disconnecting the mains. When working on the "cold" branch circuits, he/she could be electrocuted by the electricity "backfeeding" from the generator on your bus to all the other wiring on your circuit. You could also cook your generator (or hopefully trip the breakers) trying to power 50 or so other campers' rigs via your backfeed.
If you had a whole-house generator, and did not have a proper transfer switch, and lines were down in a storm, the backfeed from your house could kill a linesman picking up a "dead" downed line.
Grid-tied home or office solar systems must have "anti-islanding" circuitry to similarly prevent photovoltaic panels from energizing dead wires the same way, and creating an "island" of live wires back to the nearest break or disconnect.
This is why you have one of several methods to choose between shoreline or on-board power. The simplest is to have a generator socket, so that you either plug the bus wiring into the generator or to "shore" power, never both. Plugging the shoreline into a portable generator on the ground instead of a power outlet on a pole is a no-brainer. [EDIT: Sockets like Smitty posted and The Experience described can be used for an internal "switch" with permanent wiring]
A multi-pole manual switch that connects the bus wiring to either the shoreline or generator is the next level. You must be sure to break and transfer the neutral and ground, not tie them together and just break the hot leg. (And do not tie shoreline neutral to ground in the bus like wiring a house, the campground provides this link.) As a technical note, the switch must be "break before make" so the switch does not make the backfeed connection while it is being operated. Most common switches are of this type, but special "make before break" switches are out there.
The most advanced is a relay or series of relays working as an automatic transfer switch. A simple one uses one of two options. A transfer relay is pulled in all the time that shore power is present, and drops out when it fails. This kind could also initiate an automated "run request" to the genset. The other is to have the relay pull in whenever it sees the generator running. This option puts less hours on the relay coil, and allows the generator to crank at no load instead of full load. Most ATS have several stages and incorporate both, though the "run request" is now usually electronic and not necessarily a relay.
Originally Posted by New Earth
Ive been trying to figure out how to run electricity in the cabin of my skoolie,i thought maybe i could just run a high impendance circuit breaker to a heavy duty extension cord which i'd then plug into RV lot power sources along the way on my trip. Do you guys think this would work? Thanks ahead of time.
RV wiring and a "shoreline" (taken from yachting terminology, no doubt) is common. There are two standard campground outlets types. The first are "30 amp" outlets at 120 volts. These provide 30 x 120 = 3600 watts. The other are "50 amp" outlets at 240 volts. The 240 volt wiring has two 120-volt hot legs, and can actually provide up to 100 amps at 120 volts, with the load divided 50 amps maximum per hot leg. These provide 50 x 240 = 100 x 120 = 12,000 watts. There are also some parking areas with "20 amp" service, which would be like a common household outlet and heavy cord, providing up to 2400 watts maximum.
If you are describing circuit breakers in terms of "impendance," you should probably do a bit of reading to learn more, or get a friend/relative to look over your shoulder or review your ideas about wiring. For instance, you have to have the proper sized wire to safely handle the amount of current you are building for. Most home center extension cords won't handle 15 amps continuous draw very well. If you put a 30-amp plug and 30-amp breaker on one of these, you could have a fire.