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Ran D. St. Clair · 02-03-2010, 06:45 PM

Lets talk about protection…

Son, there’s a lot of fine ladies out there, oh wait, not that kind of protection. Circuit protection…

Now that I have my electrical system pretty much completed, I can walk through all the various protection features. It’s not at all like your house wiring. Your house probably brings the power in from the street, which then goes into your breaker box. You have a couple of big breakers or fuses on the mains, then a bunch of smaller breakers or fuses on each of the branch circuits. In the old days, that was pretty much it. More modern houses probably also have one or more GFI (Ground Fault Interrupter) in the bathroom and or kitchen. Your individual appliances will often have their own individual fuses or circuit breakers built in, but not always.

My truck is both more complex, and smaller. Let’s walk through the system from beginning to end and check out all the safety features. This is just my truck, which may be similar to your truck, or not. I am no certified electrician, so jump in if I am violating some code or standard practice. I designed everything with safety in mind, but I might have overlooked something.

If I am running off of shore power then the AC comes in through a watertight covered plug on the outside of the truck. It then goes to a socket inside the generator bay through a 15A fuse. The plug that feeds the battery charger, and only the battery charger, either plugs into this socket or into the generator.

The generator, a Honda EU2000i has a 20A button breaker on the front panel. It also has built in electronic protection circuitry that is reset by turning the motor off and then starting it up again. In practice, I have never popped the breaker, but I have tripped the electronics when the motor surges to meet a rapid change in demand. It is immediately obvious because the motor starts running happily as if under no load, which is exactly what it is doing.

The battery charger has 2 each 40A automotive type fuses which are intended to protect it from a reverse polarity connection to the battery., The DC output of the battery charger is protected electronically. Most modern power supplies have what is called “fold back” protection. If it detects a load that it can’t drive, the output voltage will “fold back” to zero or near zero, or in other words, it will give up. Depending on the supply, it will try again briefly every few seconds and if the load is back to normal it will begin working normally again. Some supplies have to be power cycled to reset the fold back circuitry.

A battery charger is more complex than a simple power supply because the voltage has to give to match the battery voltage. Put another way, it regulates current not voltage, depending on the battery state of charge. Regardless, it can detect if the voltage and current are outside of acceptable ranges and shut itself down if it needs to. Likewise it can shut itself down if it gets too hot. None of this should damage anything, but abusing anything will kill it eventually.

The batteries have 3 each 100A fuses in series with each of battery pairs. (6 each 6V batteries configured as 2S3P (2 in series and 3 in parallel). I went over those earlier.

The batteries feed the inverter which has a lot of intelligent protection circuitry built in. It will shut itself down if the battery voltage is too low, or the battery input current is too high, or the AC load is too heavy, or it gets too hot, etc. All of this electronic protection is great, but designers always worry that it will fail, so they tend to use something like a fuse or a breaker as a back up. The inverter has a 15A breaker on each of its two banks of two output sockets. You can also hardwire into a single output for the full 30A. It’s only rated for 2500W continuous, so that’s a little less than 21A, but the short term surge current could be much higher, hence the need for the circuit breakers.

In my case the inverter feeds what amounts to two each 15A branch circuits. Since the inverter is already protected, there is no need for additional circuit breakers. I do run the branch circuits through a GFI though. A GFI serves a different purpose from a fuse or breaker. Fuses or breakers protect the wiring from overload, which could cause them to get hot and possibly start a fire. A GFI aims to protect the human using the power. It only takes a tiny amount of current to kill someone if that current should pass through their heart. A circuit breaker or fuse will not limit the current to anything close to a safe level.

The GFI measures the current in the hot (black) and neutral (white) wires and makes sure that they are equal and opposite. As long as they are, there is no stray current passing through some hapless human, so all is well. If you should put yourself between the black and white wires, you become the load, and the GFI won’t help you, but if you should manage to get between the black wire and ground, then the current going through you won’t be going back through the white wire like it should, and the GFI will shut the circuit down.

Many metal appliances have a ground wire (green) that connects to the outer chassis, which is the part you could possibly touch. The ground wire should never have any voltage on it, and should never have any current through it. Any current through the ground wire implies that something is wrong and will trip the GFI if you have one. Even if you don’t have a GFI, a dead short between the hot (black) and chassis (ground - green) will trip the breaker and hopefully save you that way.

Many plastic appliances have no ground wire, just hot (black) and neutral (white). They are “safe” because there is no way for you to touch the wires inside. Take them apart at your own risk.

My truck also has a 12V DC power distribution system. You might assume that 12V DC is inherently safe, but that’s not true. 12V will easily push enough current to kill you if it can get through your skin resistance. It’s true, you can grab the terminals of a 12V battery with both hands, and nothing will happen. Do not try it with wet hands though, and stay clear of any natural wet spots. The 12V batteries in my truck, be they the starter battery, or the house batteries, have tons of energy in them, more than enough to start a fire. Fuses or breakers are still necessary to protect the wires.

Beyond the link from the batteries to the inverter, which is protected by the 3 each 100A fuses, there is a 2nd link from the inverter to the DC distribution plate which is protected by a 60A circuit breaker. It is not a standard hardware store AC breaker though. It is made for DC use as well.

From there the current flows through 3 each 15A thermal breakers that feed the 3 DC branch circuits. These are mechanical devices that will break the circuit if the current gets too high, and will automatically reset themselves several seconds later when they cool down. If the short is still present they will open up again, and then close again, and then open again, you get the idea… They are convenient because you don’t have to run to the fuse box to manually reset them. They are not a total solution though, because they keep closing again. I also put 20A fuses inside the wall outlet boxes where the cigarette lighter sockets are mounted. If I blow one of those, then I have to remove 2 screws to remove the cover plate. The fuses themselves are the automotive blade type. They are convenient to use with matching crimp on lugs which are readily available at your local home improvement store.

To be continued….

02-03-2010, 06:45 PM	#152
Ran D. St. Clair Senior Member Join Date: Apr 2009 Posts: 212	Lets talk about protection… Son, there’s a lot of fine ladies out there, oh wait, not that kind of protection. Circuit protection… Now that I have my electrical system pretty much completed, I can walk through all the various protection features. It’s not at all like your house wiring. Your house probably brings the power in from the street, which then goes into your breaker box. You have a couple of big breakers or fuses on the mains, then a bunch of smaller breakers or fuses on each of the branch circuits. In the old days, that was pretty much it. More modern houses probably also have one or more GFI (Ground Fault Interrupter) in the bathroom and or kitchen. Your individual appliances will often have their own individual fuses or circuit breakers built in, but not always. My truck is both more complex, and smaller. Let’s walk through the system from beginning to end and check out all the safety features. This is just my truck, which may be similar to your truck, or not. I am no certified electrician, so jump in if I am violating some code or standard practice. I designed everything with safety in mind, but I might have overlooked something. If I am running off of shore power then the AC comes in through a watertight covered plug on the outside of the truck. It then goes to a socket inside the generator bay through a 15A fuse. The plug that feeds the battery charger, and only the battery charger, either plugs into this socket or into the generator. The generator, a Honda EU2000i has a 20A button breaker on the front panel. It also has built in electronic protection circuitry that is reset by turning the motor off and then starting it up again. In practice, I have never popped the breaker, but I have tripped the electronics when the motor surges to meet a rapid change in demand. It is immediately obvious because the motor starts running happily as if under no load, which is exactly what it is doing. The battery charger has 2 each 40A automotive type fuses which are intended to protect it from a reverse polarity connection to the battery., The DC output of the battery charger is protected electronically. Most modern power supplies have what is called “fold back” protection. If it detects a load that it can’t drive, the output voltage will “fold back” to zero or near zero, or in other words, it will give up. Depending on the supply, it will try again briefly every few seconds and if the load is back to normal it will begin working normally again. Some supplies have to be power cycled to reset the fold back circuitry. A battery charger is more complex than a simple power supply because the voltage has to give to match the battery voltage. Put another way, it regulates current not voltage, depending on the battery state of charge. Regardless, it can detect if the voltage and current are outside of acceptable ranges and shut itself down if it needs to. Likewise it can shut itself down if it gets too hot. None of this should damage anything, but abusing anything will kill it eventually. The batteries have 3 each 100A fuses in series with each of battery pairs. (6 each 6V batteries configured as 2S3P (2 in series and 3 in parallel). I went over those earlier. The batteries feed the inverter which has a lot of intelligent protection circuitry built in. It will shut itself down if the battery voltage is too low, or the battery input current is too high, or the AC load is too heavy, or it gets too hot, etc. All of this electronic protection is great, but designers always worry that it will fail, so they tend to use something like a fuse or a breaker as a back up. The inverter has a 15A breaker on each of its two banks of two output sockets. You can also hardwire into a single output for the full 30A. It’s only rated for 2500W continuous, so that’s a little less than 21A, but the short term surge current could be much higher, hence the need for the circuit breakers. In my case the inverter feeds what amounts to two each 15A branch circuits. Since the inverter is already protected, there is no need for additional circuit breakers. I do run the branch circuits through a GFI though. A GFI serves a different purpose from a fuse or breaker. Fuses or breakers protect the wiring from overload, which could cause them to get hot and possibly start a fire. A GFI aims to protect the human using the power. It only takes a tiny amount of current to kill someone if that current should pass through their heart. A circuit breaker or fuse will not limit the current to anything close to a safe level. The GFI measures the current in the hot (black) and neutral (white) wires and makes sure that they are equal and opposite. As long as they are, there is no stray current passing through some hapless human, so all is well. If you should put yourself between the black and white wires, you become the load, and the GFI won’t help you, but if you should manage to get between the black wire and ground, then the current going through you won’t be going back through the white wire like it should, and the GFI will shut the circuit down. Many metal appliances have a ground wire (green) that connects to the outer chassis, which is the part you could possibly touch. The ground wire should never have any voltage on it, and should never have any current through it. Any current through the ground wire implies that something is wrong and will trip the GFI if you have one. Even if you don’t have a GFI, a dead short between the hot (black) and chassis (ground - green) will trip the breaker and hopefully save you that way. Many plastic appliances have no ground wire, just hot (black) and neutral (white). They are “safe” because there is no way for you to touch the wires inside. Take them apart at your own risk. My truck also has a 12V DC power distribution system. You might assume that 12V DC is inherently safe, but that’s not true. 12V will easily push enough current to kill you if it can get through your skin resistance. It’s true, you can grab the terminals of a 12V battery with both hands, and nothing will happen. Do not try it with wet hands though, and stay clear of any natural wet spots. The 12V batteries in my truck, be they the starter battery, or the house batteries, have tons of energy in them, more than enough to start a fire. Fuses or breakers are still necessary to protect the wires. Beyond the link from the batteries to the inverter, which is protected by the 3 each 100A fuses, there is a 2nd link from the inverter to the DC distribution plate which is protected by a 60A circuit breaker. It is not a standard hardware store AC breaker though. It is made for DC use as well. From there the current flows through 3 each 15A thermal breakers that feed the 3 DC branch circuits. These are mechanical devices that will break the circuit if the current gets too high, and will automatically reset themselves several seconds later when they cool down. If the short is still present they will open up again, and then close again, and then open again, you get the idea… They are convenient because you don’t have to run to the fuse box to manually reset them. They are not a total solution though, because they keep closing again. I also put 20A fuses inside the wall outlet boxes where the cigarette lighter sockets are mounted. If I blow one of those, then I have to remove 2 screws to remove the cover plate. The fuses themselves are the automotive blade type. They are convenient to use with matching crimp on lugs which are readily available at your local home improvement store. To be continued….