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I just completed an FAA required recurrent course for the aircraft I fly. The Power Point program & instructor teach that a wire with 75 amp rated current limiters at each end will provide 150 amps of load before one or both current limiters melts and opens the circuit. The use of the current limiters is a modification to replace a single 450 amp rated reverse current circuit breaker. Instead of one line with a 450 amp RCCB between the main battery and the main bus, three parallel lines are used with current limiters at each end of each line. The advantage of the three parallel lines is to not lose all power to the bus if only one or two of the three parallel lines opens.

The explanation given is that total resistance in a series circuit is the sum of the individual resistances; therefore, six 75 amp current limiters (one at each end of the three parallel lines) will allow the same sustained current as the single 450 amp reverse current circuit breaker.

I disagree and I've read that each current limiter is rated at 150 amps and each of the parallel lines will allow a sustained current of 150 amps. By analogy, if a line has current limiters at each end, one rated at 75 amps and one rated at 50 amps, the circuit will open with a sustained current in excess of 50 amps, not a total of 125 amps.

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  • \$\begingroup\$ Different types of current limiter have different tolerances, a thermal one may indeed be rated to carry 75A continuous, but require 150A for x seconds to blow. But for sure, with a limiter on each end a line, whatever the specifications are, the currents will not add, but be the lowest of the two ends. Can you post a link to the specifications for the limiters? A schemtic of how they're connected would be good as well, I can think of how they might be driving parallel loads which means the currents would add with only a slight abuse of english, a picture would clarify. \$\endgroup\$ – Neil_UK Feb 7 '17 at 15:59
  • \$\begingroup\$ I don't have a link I can post. The DHC-6 Twin Otter Flight Safety Manual has schematics which simplistically show the two electrical systems: One with the 450A RCCB and the other with the current limiters. Either design is protecting the battery from a high current after the engines are started and the generators are brought on line. The flight crew can easily miss an opened RCCB until the generators are shut off prior to engine shutdown. The battery will be isolated due to the open RCCB and without either generator there will be no power to the main busses. It gets dark fast at night. \$\endgroup\$ – Ralph Feb 7 '17 at 21:44
  • \$\begingroup\$ Well in that case our advice will only be as good as your description of the circuit. If the breakers are truly in electrical series, which is what your description sounds like, then your analysis of how their limits combine, as min(limit1, limit2) is correct. \$\endgroup\$ – Neil_UK Feb 8 '17 at 6:13
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I am not familiar with aeronautical current limiters, but it sounds like they are similar to fuses or circuit breakers. Regardless of the details, any series connection through two current-limiting devices will limit at the lower of the two ratings. The current-carrying capabilities will not add.

However, I just looked at this document (FAA - Aircraft Electrical Systems), and on page 9-59 they show a "main battery bus" and an "isolation bus", connected in series, with a current limiter at each end. These two busses are connected together and fed at the junction. This may look like a single series connection, but in this case the current feeding the junction can equal the sum of both the current limiter ratings.

I would make sure that you and your instructor agree on what kind of connection you are looking at.

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  • \$\begingroup\$ The current limiters are basically a somewhat large metal link which acts like a slow blow fuse instead of using a typical circuit breaker such the reverse current circuit breaker (RCCB). Resetting an open RCCB is not recommended if it can be avoided. \$\endgroup\$ – Ralph Feb 7 '17 at 21:50
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I agree with you. I'm not sure what these 'current limiters' are (fuses, resistors, some sort of active solid state device, etc.), but if they are in series on the same physical wire, adding up their ratings will not increase the total current limit. The effective current limit would be the lowest of them. Unless they are resistive, in which case the current limit would actually drop. E.g. for a 24VDC bus, a 1 Ohm resistor would limit the current to 24A max. Putting two of them in series would limit the current to 12A.

I assume though these are more like fuses or breakers, in which case again, the effective limit would be the lower of them. Of course if placed in parallel, then the limit would increase, but it's hard to predict the limit because they may not share the load evenly. So in the case where three 150A circuits are expected to provide 450, well OK up until one of them trips (which might be at less than 450A if they do not share the current evenly), and as soon as that one trips the others will soon follow.

(What equipment is that? 450A is a lot more than what I would see in the small planes I fly!)

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  • \$\begingroup\$ de Havilland DHC-6 Twin Otter \$\endgroup\$ – Ralph Feb 7 '17 at 21:53
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Current limiters, fuses and PTC or resetable fuses usually have 2 current specs: rated current and tripping current. Rated current is at which the limiter will operated normally without tripping. The tripping current is the current at which the manufactures specs the limiter, fuse or PTC will trip in an specified amount of time. So in your case 75A is the rated current for the limiter and the 150A is the tripping current. So they are correct to say that the system will provide 450A before tripping and there's most likely 2 limiter on each end just as redundancy. Having redundancy in aeronautical system is common especially in their electrical system.

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