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Email This Page to a FriendPreview: Boat Wiring, Part 5 – Overcurrent Protection
April 29, 2014
Fire in a wire. This graphic presentation shows what can happen when too much juice flows through too small a wire and how the situation can be prevented with overcurrent protection. Even if your boat has little more than running lights or even just a bilge pump, you gotta see this one.
This video series concludes in Part 6 - The Two Battery System - Be sure to check it out.
– [Narrator] Havin’ a smokin’ hot boat is great. A smokin’ hot babe on your boat? Priceless. But smokin’ hot wires in your boat? That’s not so cool. So Don Eley joins us again to show you the basics of what you need to know to not have smokin’ hot wires like this.
– Today we’re gonna talk about over-current protection. Think most of you are familiar with fuses and circuit-breakers, and so we’re gonna look at how we get the power from the battery through the wiring to the load safely. And one of our biggest concerns is power coming from the battery to the load heating up the wire. We’ve looked at various sizes of wire: 18 I believe, 16, 12; and as the wire gauge gets larger, we get a greater cross-sectional area of the wire, and that allows us to put more current through the wire safely. So current is a ray that the electricity is flowing through the wire, and that’s measured in amps. If we try to put 10 amps through a small wire, we not only see a voltage drop as that runs along the wire, but that wire also can get hot. So we need to ensure that we pick the correct wire gauge for the load, and then what we need to do is protect that wire from getting hot during its operation. And we do that a couple of different ways. One, I think most of you are familiar with a standard fuse, and this is where we put a fuse in line, in series with the circuit. If that small piece of wire exceeds the amperage that it’s rated at, it will get hot, blow, and open the circuit, and that’s how we protect the wire is with a fuse like that. This is kind of a handy little arrangement here where it looks like it has two fuses in it. Actually only one fuse is in line, and then we’ve got a spare that’s always there, so that’s a pretty handy way to have a little fuse holder with a spare always with it. Another option for over-current protection is a circuit breaker. It is a switching device, we can turn it on and off. When it’s in the on position, it has a particular load rating and it operates magnetically, so the greater the amount of current that’s flowing through the circuit breaker, the greater the magnetic field. And when we exceed that current, the magnetic field then increases and we actually trip the circuit breaker into the off position. Now, typically, these are also designed so that even if we hold them in the on position, if the current exceeds the rated current, it will trip and turn on. That’s called a trip free circuit breaker, and it’s a safety device that even if we try to keep the circuit breaker in the on position, it will open the circuit. What we’re doing now is we’re looking at the back of the breaker panel, and it looks fairly complicated but it’s actually a simple arrangement. This is the newer convention where we use the red for the positive and the yellow for the negative. And so our power’s coming in from the battery, and then as it comes out of this terminal block, the negative comes around here to what we call a negative bus, and this is a common bus, all of our negative terminals are gonna come to a common place there. And on the positive side, if you can follow this, the positive comes in here and you’ll see it wind its way around to this circuit breaker. So the power’s gonna come into the circuit breaker and that’s our power in, and when I turn that circuit breaker on, that powers this entire bar here. So the power comes in here, through the circuit breaker, and powers this entire bar. If, this one happens to be rated at 15 amps, if the total amperage exceeds 15 amps, that magnetic field is gonna build up and then we’re gonna see the circuit breaker open and that’ll shut the power off for the entire breaker panel. Each individual load then is fed off of this bar and each one of those loads comes down to an individual terminal block. I’ve wired it this way so that we keep from having to always go back to the circuit breaker if we wanna change a load. We could come off each one of these individual circuit breakers to your individual loads, but it’s typically tough to get into that circuit breaker panel. What I like to do is bring it out to the terminal block and I have each individual circuit breaker here numbered, and then each load will come off of that terminal block, so if you do have to change anything out, you’re not going back to the circuit breaker and trying to get right in tight there. All of our switching and all of our over-current protection are always done on the positive side, so you notice that all of our circuit breakers are on the positive side, so whether we’re using a fuse or a circuit breaker we always do that on the positive side. We’ve got a whole set of circuit breakers here that all happen to be 15 amp, but what’s important there is that each one of these circuit breakers is protecting the wire in that circuit. So we wanna ensure that, whatever wire gauge we’re using for that particular circuit, that we have a circuit breaker or a fuse that’s protecting that wire from the maximum amount of current or amperage that that wire can handle. Here’s our typical breaker panel. Remember our top one was the main breaker that turned everything on, so we’re gonna turn it on, we see our battery voltage there. Now all of these circuits are available to us to turn on. And we’re gonna look specifically at the bilge pump circuit. That’s gonna turn the power on to our bilge pump. Now this one’s rated at 15 amps, so it’ll protect the wire from over-current, but we’re also concerned with a locked rotor condition with the bilge pump. This is where we get something like a piece of wood or something stuck in the bilge pump, and the bilge pump is on, and while the wire might be protected, the pump motor itself is not. So what we’ve done here is we’ve added a fuse that’s rated specifically for the bilge pump. And this fuse is in series with the bilge pump, if we exceed the current that’s rated on the bilge pump then the fuse will blow. So let’s look at the bilge pump. The bilge pump actually says right on it that it’s a 1.5 amp bilge pump that’s under its normal load, and it tells us right on the surface there that we wanna put a three-amp fuse in series with this circuit. So this 15 amp breaker may be protecting the wire, but the fuse itself is only gonna be three amps. So if we do get into a locked rotor condition with a pump, that fuse will melt, open the circuit, and protect the pump from overheating. What we have here is a three-position bilge pump switch. It’s off in the center, automatic on one side, and manual on the other. So if we put it in the automatic position, what that allows us to do is, the bilge pump has its own float switch; when the water rises,it’s gonna turn on, when the water drops down, the pump is gonna shut off. So that’s our automatic position. In the manual position, I can operate the pump manually from the panel itself without getting down into the bilge. And then the off position, the pump’s off. Normally I’d leave it in the automatic position. Let’s look at how that’s wired on the back of the panel now. We’ve come from our breaker to our positive side of our bilge pump. That wire comes around, and what we’re looking at here is the back side of that fuse panel, so here’s our wire coming into our fuse panel, that’s in series with the pump switch. And we have two wires going to the bilge pump: one is in the automatic position, so this one will supply power to the bilge pump through the float switch in the automatic position; and this one that’s solid brown here is the one that will supply power to the bilge pump in the manual position, so it can operate in either situation. Here’s our negative coming back from the bilge pump to the negative bus, back to this yellow wire, back to the battery, completing that circuit. So there’s a lot of wires here, so let’s take a look again and review what we’ve looked at. Here’s our positive coming in from the battery, it’s gonna come around and up to our main circuit breaker, which is gonna feed this bar, so this circuit breaker’s on, this bar’s gonna be supplied with electricity; across each circuit breaker to each load, so each one of these terminal blocks go to a load. This would go out to our bilge pump. From the bilge pump it would return, this actually happens to be the return from the bilge pump on a negative side, which is connected to this negative bus bar. It then returns through this yellow wire here and back to the battery, and that’s our complete circuit. This is all about protecting the wire from excessive amounts of current, which would overheat and then of course get us into a situation where we might have fire onboard. So let’s do a little experiment here to show you what happens if we exceed the maximum capacity of the wire. We’re gonna start from a battery, actually gonna come through a circuit breaker, to what we call a variable resistor, so we’re gonna be able to adjust the resistance in the circuit. And we’re gonna bring that current through a small, 18-gauge wire that’s rated at about 20 amps, its maximum safe load. This is our ammeter, and it actually is measuring the magnetic field through this wire and giving us a reading in amperage. This is measuring the temperature of the wire itself, so we have it attached to the wire here, and as we increase the amperage, we’re gonna see that the temperature goes up. We’ll turn on the breaker, and this variable resistor here is a carbon pile, and as I tighten that carbon pile the amperage goes up. Right now we’re only at three quarters of an amp, and as I tighten that up we should see the amperage increase. Here I am, my amperage is starting to increase now. Remember, this wire that we’re experimenting on is rated to operate safely at about 20 amps. So I’m gonna bring this up to about five amps. Well within its safe range. So here we are approaching five amps. Our temperature is still room temperature here, we have not exceeded the wire’s capacity, so its temperature is fine. We’ll bring this up to 10 amps now. Still within the range of the capacity of the wire. Temperature may be rising just a little here. We’re gonna bring it up to 15 now. We’re starting to see the temperature rise. Though technically temperature rose from the beginning. I’m bringing it up to 20, which is its maximum safe load. And the wire definitely feels warm to touch, and of course we’re seeing the temperature rise now. 90 degrees. It’s certainly safe, but if we leave it here for a while, we’d see the temperature continue to rise. But that’s its maximum ampacity to operate that wire safely. And at this point we should certainly have a circuit breaker or a fuse that opened the circuit so we didn’t exceed that amount of current through that particular gauged wire. Here we’ll see, in just in that little bit, we’re up over 100 degrees now, and I guess I’m at 22 amps, let me bring it to 25 amps, which definitely exceeds the capacity of that wire. And we’re well over 100 degrees now. Now we’re starting to smell that, as we would as we increase that temperature beyond its maximum ampacity. I’m gonna bring it up to 30 here and maybe even 35. And you can imagine if this wire’s buried in a bunch of other wires onboard, it’s getting hot, so it’s impacting not only this wire but others, and the insulation is actually expanding. You can see it, it’s getting greater and it’s too hot to touch now. We’re at 140 degrees, 32 amps, well beyond what the wire is rated at. And now you can see a fair amount of smoke coming off. Under 40 amps now. So we’re at 40 amps now, we’re seeing blistering of the wire insulation, we’re seeing quite a bit of smoke, the bubbling over here, we’re definitely in a situation where we can start a fire. And we’re approaching 200 degrees. You can see the insulation has completely melted. That was 42 amps, well beyond the wire’s rated capacity, but it gives you a good idea of what happens if we don’t have over-current protection appropriate for a wire gauge.
– [Narrator] Now that we know more about how to save our bacon rather than fry it, join us in the next video for the final installment in this series, where Don pulls it all together. He’ll outline a simple, two-battery system. See you then.