Voltage drops by 10V under load?

[jacob]

Afternoon all,

 

I'm trying to fix a recurring problem we're having with the electrics. Every now and then, a string of lights (all on the same 12V spur) will stop working. This happens pretty randomly - today I dropped something heavy on the floor and they switched off, but other times it's happened with no apparent cause.

 

I've done some digging in the cables, this is what I've found. The problem is with a spur of 3 lights - a 12V strip light and two 12V touch led lights. When this spur is disconnected, the voltage across the terminal block is 13.5V. Once I connect the spur, the voltage drops to just under 3V. At this low voltage, the lights just don't turn on. When the lights are working and on, it measures about 12-13V across the terminal block.

 

I'm struggling to work out what the problem is, and why it's so intermittent. I'm fairly sure that it's only those 3 lights connected, but I can't easily access the wires as they're buried into the ceiling.

 

Any ideas what the issue could be, and any possible fixes?

 

Cheers,

Jacob

[pearley]

Try connecting each lamp one at a time to see if it's one of them causing the problem.

 

[bizzard]

4 minutes ago, jacob said:

Afternoon all,

 

I'm trying to fix a recurring problem we're having with the electrics. Every now and then, a string of lights (all on the same 12V spur) will stop working. This happens pretty randomly - today I dropped something heavy on the floor and they switched off, but other times it's happened with no apparent cause.

 

I've done some digging in the cables, this is what I've found. The problem is with a spur of 3 lights - a 12V strip light and two 12V touch led lights. When this spur is disconnected, the voltage across the terminal block is 13.5V. Once I connect the spur, the voltage drops to just under 3V. At this low voltage, the lights just don't turn on. When the lights are working and on, it measures about 12-13V across the terminal block.

 

I'm struggling to work out what the problem is, and why it's so intermittent. I'm fairly sure that it's only those 3 lights connected, but I can't easily access themm wires as they're buried into the ceiling.

 

Any ideas what the issue could be, and any possible fixes?

 

Cheers,

Jacobmo

If any of them are let into the ceiling panels quite often roof condensation can short them out, more so during the winter.

[jacob]

2 minutes ago, pearley said:

Try connecting each lamp one at a time to see if it's one of them causing the problem.

 

 

Hmm, I've given that a go but the problem is still there no matter which of the three are connected (and even when they are all disconnected)

 

The only other thing I could think of is something else being connected, but the only other things that are remotely near are the horn and the water pump, both of which are on seperate circuits.

[nicknorman]

19 minutes ago, jacob said:

Afternoon all,

 

I'm trying to fix a recurring problem we're having with the electrics. Every now and then, a string of lights (all on the same 12V spur) will stop working. This happens pretty randomly - today I dropped something heavy on the floor and they switched off, but other times it's happened with no apparent cause.

 

I've done some digging in the cables, this is what I've found. The problem is with a spur of 3 lights - a 12V strip light and two 12V touch led lights. When this spur is disconnected, the voltage across the terminal block is 13.5V. Once I connect the spur, the voltage drops to just under 3V. At this low voltage, the lights just don't turn on. When the lights are working and on, it measures about 12-13V across the terminal block.

 

I'm struggling to work out what the problem is, and why it's so intermittent. I'm fairly sure that it's only those 3 lights connected, but I can't easily access the wires as they're buried into the ceiling.

 

Any ideas what the issue could be, and any possible fixes?

 

Cheers,

Jacob

There is an intermittent high resistance connection upstream (ie battery side) of the terminal block, probably caused by corrosion. Slight vibrations, changes in humidity etc can make the difference between a good connection and a bad high resistance connection. When the issue is manifest (voltage at the terminal block 3v), trace the wiring back towards the battery and find out where 3v suddenly becomes 12v - that is where the problem is. Most likely corrosion on a fuse holder or any other place where there are connections.

[jacob]

26 minutes ago, bizzard said:

If any of them are let into the ceiling panels quite often roof condensation can short them out, more so during the winter.

 

13 minutes ago, nicknorman said:

There is an intermittent high resistance connection upstream (ie battery side) of the terminal block, probably caused by corrosion. Slight vibrations, changes in humidity etc can make the difference between a good connection and a bad high resistance connection. When the issue is manifest (voltage at the terminal block 3v), trace the wiring back towards the battery and find out where 3v suddenly becomes 12v - that is where the problem is. Most likely corrosion on a fuse holder or any other place where there are connections.

 

Cheers both, I'll try tracing it back and see where that gets me

[Tony Brooks]

I am with Nick on this. What type of fuses/circuit breakers do you have? If they are torpedo fuses (pointed each end) they are very well known for causing such high resistance connections.

 

Depending upon how you connected the voltmeter it could just as easily be in the negative side. Check that all the connections on the negative bus bar are tight and clean.

Edited July 3, 2022 by Tony Brooks

[ditchcrawler]

On 03/07/2022 at 13:46, jacob said:

 

 

Cheers both, I'll try tracing it back and see where that gets me

What Nick said