Originally Posted by backinblue
And I think many are over-thinking this one!
Just to expand on this point a bit and to intentionally over think it regarding voltage drop vs checking resistance....
As a 36 year tech in aviation that includes basic electrical I was approaching this from my experience and perspective. I can say that in 36 years of troubelshooting aircraft systems that I could use my fingers and toes to count all the times we have used a voltage drop test. It has probably been 1000 to 1 doing resistance test VS voltage drop. And of course I cant recall any issues we eventually did not fix or uncover. So of course my "go to" procedure is to reach for the meter and check resistance. As a matter of fact in all the manuals and hundreds of references I have used I really cant recall of a prescribed test using the voltage drop. I even asked some of the senior techs in the hanger this past week if they have used the voltage drop and most have not. Only the dedicated electricians have used the voltage drop and not often.
So in thinking about this further I had to figure out why this was so and reconcile the reasons for this when the voltage drop clearly can be a better tool in some cases. The answer that seemed to come up was differences in the types of systems and the nature of when and how a problem is presented. Also how a maintenance schedule is used and applied.
Types of systems
- On aircraft open lugs and terminal strips are not the norm. Most systems are enclosed and use large sealed cannon plugs etc.
On boats many terminations are easily accessible. Especially terminal strips, switches, and relay lugs. Newer boats with sealed harnesses may present the same access issues. It may be required to pierce a wire to make reading. A practice I really dont like to do unless I have to. As such most of the time on aircraft we are taking apart a connon plg connector that may have 5 to 40 pins that render systems incapable of being powered up. In addition the current carrying is generally low. So resistance checks are the norm in these curcumstances.
In addition, how the system is operated and what operational condition it is in may also dictate if you you can do a voltage drop or if a resistance check would be better. Can you power up the system and load the system? Or is there a danger in loading the system in its dysfunctional state? In that case try the resistance checks.
*Edited to add* one of the drawbacks to resistance checks is that you typically want to isolate the component you are checking resistance on. Which of course means removing the wires. But this also means you can bench check components already removed or new ones prior to install. Voltage drop allows for quick checking in place if you have access.
How a problem is presented
- On aircraft it arrives at the facility usually with a particular system inoperative. When a system is inoperative the issue can usually be discovered using either a voltage drop or resistance checks just as easily depending on access.
On an old boat exposed to salt and other harsh conditions if a high current carrying system using large conductors is underperforming like battery charging and/or starting, Windlass components etc , then as stated by some here the voltage drop really does shine. If a system on a boat is completely inop or intermittently inop a resistance check is generally just as useful. Also to check minute performance degradation of larger load carrying components like terminals, crimps, switches, contactor/relays, wires, splices etc..again..this seems to be where the voltage drop would really shine. And in thinking about this now I want to do some basic checks on a few of the systems in the boat using this. A large contactor that is impeding a ton of current/voltage may well ohm out just fine. So it seems as Ronobrien pointed out ..as current requirements go up the larger the departure there is to the resistance check telling you what you want to know.
How a maintenance schedule is used
- One of the other factors regarding why we rarely use the voltage drop test in aviation is an interesting point that took me a bit to realize has to do with maintenance schedules. One of the main things I could see us using the voltage drop test in place of the usual resistance checks is of course large current carrying contactors/ relays. Well in quite a few aircraft these large current carrying contactors are actually life limited components. For instance on one particular type of aircraft the start contactors are changed every 7 years regardless of condition. The removed item is sent in as a core and will be overhauled and then made available for purchase again in overhauled condition. This practice alone eliminates many of the circumstances it would be likely we would want to do a voltage drop test since this nearly eliminates issues with these components. A good case could be made to apply something like this to a boat for mission critical components that do fail like contacts and relays in charging and starting systems.
I absolutely love thinking about these types of things and enjoy this aspect of having a boat or some of the other machines I own. So If I get a chance to expand or reorganize my own information of this nature it is only helpful in the long run. As a wise man once said " I dont want to be wrong about something a minute longer than I have to be"
So thanks to those who chimed in to add this useful info. Much appreciated.