Current limiters: We talk about them and we test them. But what do they do for us, and how can we protect them?
Let’s start with defining a current limiter. It’s a device that restricts or limits electrical flow to a predetermined maximum amount that prevents damage to components.
The most talked about current limiters in King Airs are the 325 ampere limiters found in the dual-fed electrical system design (see Figure 1) so that’s what we are going to focus on in this article.
What do the 325a current limiters protect? They control the amperage flowing between the isolation bus to the left main bus and the right main bus. The first time we check the current limiters is during our cockpit preflight checks. The battery is on and we press the button on our load meters to show the voltage to the left and right main buses. If the voltage reads battery volts, we know that our current limiters are intact. If one of the volt meters shows zero volts, the current limiter on that side has failed and will need to be replaced.
How can we protect 325a current limiters?
The time that the current limiters are most vulnerable to damage is during engine start. Let’s look at how that happens.
The generator-assisted start that I will describe offers the best protection for your current limiters and can be used in any King Air. Newer King Airs (BB-1444 and newer) and triple-fed bus electrical system King Airs can perform a true cross-generator start without damaging the current limiters. Check your POH to make sure your plane is capable of the cross-generator start. If you have doubts, using the generator-assisted start technique will help keep your current limiters safe.
After completing your before-start checklist, go ahead and start your first engine. It does not matter which side unless you are in a 90, A90 or B90 with a supercharger on the left side. In those models, the right engine first is the way to go.
Once the first engine is started, most pilots bring the generator online to recharge the battery after the start (prior to BB-1444). Here is where the problem arises. When you turn on the second starter, it will ask for approximately 800a to get the engine turning from a dead stop. The electrical demand will reduce dramatically after the engine begins to turn. The problem is that the operating generator is going to produce quite a bit of that power and send it through the current limiter on its way to the isolation bus (see Figure 2). Remember that the current limiter was rated for 325a. Forgetting to turn the first generator off prior to initiating the second engine start will set you up for a failed current limiter.
Passing 800a through a 325a current limiter is possible but not ideal. The current limiter might be able to withstand that a time or two but eventually passing that much current through it will cause it to fail.
Now that we have used some power out of our battery, we have a choice to make. If you are happy with the max motoring N1 speed you had on the first engine start (18% or so), then you can skip the battery recharge step and go straight into the second engine start. Hit the starter for the second engine and once N1 reaches 12% or greater, bring the first engine’s generator online. The generator will now assist in the starting of the second engine. I would expect to see 20% or greater N1 for max motoring with the generator assisting. The second engine start will be significantly cooler than the first engine. Remember the faster N1 is before you introduce fuel to the engine, the cooler the start will be. Alternating which engine starts first is a good way to even out wear and tear between the two engines.
With both engines running but only one generator online, we are going to do our second current limiter check. Press the button on your load meter again and check the volts on both. With one generator on and intact current limiters, you should see 28v on both volt meters. If you see 28v on one and 24v on the other, you have blown a current limiter during the start, most likely on the side that you started first. Time to shut it back down and get the failed current limiter replaced. Taking off in this condition is prohibited as you no longer have a dual-fed electrical system and the loss of one generator will cause other systems to fail with no means of restoring them in flight.
The third and last check of our current limiters is performed after shutdown but before we turn our battery off. With both generators offline, press the button on the load meters one last time. You should see 24v on both voltmeters. If one reads zero the current limiter on that side needs to be replaced before your next flight. If they both read 24v they are good and you can turn off the battery.
What about 60a current limiters?
The last set of current limiters we are going to talk about don’t get much attention from pilots but they are a good thing to check once in a while. The current limiters I’m referring to are the 60a limiters between the dual-fed buses and the left and right generator buses (see Figure 3). The dual-fed buses are protected by 50a circuit breakers as well as 60a current limiters. The circuit breakers are located on the right-side CB panel in King Air 200 and B200s and the fuel CB panel under the fuel gauges. Circuit breakers are easy enough to see when the have popped but the current limiters are a little harder to tell when they have had a problem.
So, how do we check them? On the ground with battery power or with a ground power cart hooked up to the plane, we are going to pull the 50a circuit breakers one by one and see if we have any dual-fed bus failures with only one CB pulled.
Let’s walk through the procedure using Dual Fed Bus #1 as an example. The CBs for Dual Fed Bus #1 are on the right-side CB panel, along with Dual Fed Bus #2 (Dual Fed Bus #3 & # 4 are on the fuel panel CB). First, turn on the battery; ground power helps but is not required. Pull one of the No. 1 CBs. Dual Fed Bus #1 items should still be powered; if not, the current limiter on the opposite side has failed. Assuming pulling the first CB has had no effect on Dual Fed Bus #1 items, pull the second No.1 CB on the right-side CB panel. All Dual Fed Bus #1 items should go dead, this verifies that the CBs are disconnecting power from the #1 bus. Third, reset the first Dual Fed Bus #1 CB and power should be restored to the #1 bus. If power is not restored the opposite side current limiter has failed and will need to be replaced. Repeat the process for the remaining three dual-fed buses to check all the 60a current limiters and CBs.
As you can see, protecting our current limiter during start is not a difficult thing to do. Using a generator-assist type start will protect the 325a current limiters in all dual-fed style King Airs. Occasionally checking the 60a current limiters protecting our dual-fed buses is a good way to check that our dual-fed buses are actually dual fed!
Zach Cleaver, a Gold Seal flight instructor since 2009, started teaching in King Airs in 2010. He has worked for King Air Academy in Phoenix, Arizona, since 2013 and flies all models of King Airs.