Gradual Amperage Increase

[A-1Sparky]

I put an ammeter on a 20 amp site lighting circuit that recently started tripping. I'm reading about 16 amps upon energization, and then the amperage gradually increases after that. It jumps up to about 19 amps within 10 minutes or so, then increases VERY slowly after that (about .01 amps every minute or so). It eventually trips the breaker and kills the circuit. None of the other site lighting circuits do this. They all stabilize within a minute or two. Any ideas as to what would cause this?

 

[boboelectric]

What type of lights?

 

[A-1Sparky]

These are all 100watt HPS lamps.

 

[Smart $]

These are all 100watt HPS lamps.

Odds are one ballast (or more) is "seeing" an dielectric breakdown (aka partial short) as it warms up.

 

[A-1Sparky]

That was my initial thought, but I wasn't quite sure. From my calculations, this circuit should only be drawing around 11 or 12 amps tops.

 

[LarryFine]

One time-consuming process would be to eliminate one light at a time, or if convenient, split the load in half, and monitor the readings. Keep going until you end up with one isolated fixture.

 

[Chamuit]

One time-consuming process would be to eliminate one light at a time, or if convenient, split the load in half, and monitor the readings. Keep going until you end up with one isolated fixture.

We had to do this on a parts house. Bought inline fuses and put them on all 10 fixtures (25-30 feet in the air) to determine which one was the cause of the circuit tripping. The extra cost of the fuses & holders and connecting them was less than the time we would have used.

 

[dbuckley]

If the fittings are electrically in a line, then go half way down and measure there. That'll tell you if the problem is in front or behind where you're measuring. Repeat the process.

Computer people call this approach the Binary Chop.

Works well with those "think of a number between one and a hundred" type of games too.

Of course, the assumption here here is that there is a fault: maybe this is the correct behaviour for these ballasts...?

 

[mivey]

Take temperature readings to see if it is isolated in the fixture or is in the supply line as well.

 

[A-1Sparky]

Unfortunately, whoever originally installed these lights ran direct burial wire and made numerous underground split-bolt splices just outside the pole bases instead of INSIDE the poles (they have hand holes). So, I would have to eliminate each ballast one at a time to find the culprit. Do you think I could use an IR gun to find it (based on Mivey's suggestion )?

 

[Cow]

I like Chamuit's suggestion.

The time you spend trying to find the problem now could be spent installing individual fuseholders. But on the plus side, if this ever happens again it'll simply blow the fuse and only take just a few min. to troubleshoot.

 

[A-1Sparky]

That's a good point. I'll see how the powers that be want to handle it. Thanks guys.

 

[mivey]

Unfortunately, whoever originally installed these lights ran direct burial wire and made numerous underground split-bolt splices just outside the pole bases instead of INSIDE the poles (they have hand holes). So, I would have to eliminate each ballast one at a time to find the culprit. Do you think I could use an IR gun to find it (based on Mivey's suggestion )?

I doubt you would find the underground stuff as the earth is a pretty big heat sink. But, an IR camera might be sensitive enough.

It might be able to identify the problem light after a trip as it should have an elevated temp as compared to the others. The wires might even show up. You would have to make sure the temperature range excluded the normal deltas as hot spots.

I'm not that familiar with the IR guns as I just bought a cheap one to play with. I am spoiled by the camera. The problem with it is the expense.

 

[LarryFine]

Unfortunately, whoever originally installed these lights ran direct burial wire and made numerous underground split-bolt splices just outside the pole bases instead of INSIDE the poles (they have hand holes).

Now that we know that, I have the funny feeling your problem is going to be underground current leakage.

I suggest checking all circuit lines for matching currents, and maybe see what the no-load current is, if you can disconnect all ballasts.

 

[broadgage]

Since it is stated that the extra current persists for many minutes, until the breaker trips, I rather doubt that a defective ballast is to blame.
The extra current, beyond the expected load is about 10 amps, that is well over a kilowatt, if that much extra power was being disipated in a ballast, it would have gone up in smoke long ago.

I would stronlgy suspect a defective underground joint allowing earth leakage, this may get hot locally, but would be concealed by normal temperature soil above, and thus not be readily located.

Such faults can be challenging to locate.
One possibilty is a voltmeter and a long lead.
Connect one side of the voltmeter to a good ground, such as the service entrance ground bar. Connect the other to a metal spike which is inserted into the earth at different points alonlg the cable run, the fault is probably close to the point that gives the highest reading.
This is by no means completly reliable as leakage/ground currents from elswhere can give misleading results, and of course the circuit must be energised for testing.
It would be advisable to wear linesmans gloves when using the tempory test connection, in case the faulty cable is very close, and a dangerous voltage is present.

 

[hardworkingstiff]

I tend to agree with suspecting the underground splices.

My 1st thought on troubling shooting based on that assumption would be to disconnect each light, then one at a time, connect a resistive load at the hand hole. Note the voltage drop. Do this for each light.

Calculate what the voltage drop should be based on the load, length and size of the conductors and compare it to the actual VD. Hopefully that will give you an idea of how many and where your problems are.

Thinking just a bit more, I wonder if you couldn't just but a load at the end of the circuit and check the voltages along the path, probably would tell you the same information without having to move the load to every pole.

Good luck, please let us know what you found and how you found it.

 

[A-1Sparky]

Thanks for all of the suggestions. I probably won't get back to this particular job for a couple of days, but I'll post what I find when I do.

 

[kwired]

Someone correct me if I am wrong but it is my understanding that a HPS lamp draws more current as it ages (from use). I always understood that this is the reason why the lamps cycle on and off as they approach end of life. The lamp draws more current and eventually the ballast can not provide enough current to maintain the arc and it goes out. After short cool down it restarts and the whole process starts over.

I have noticed after installing new HPS that they usually do not draw the amount of current that they have marked on them. I am guessing after a certain amount of use they will.

Based on this it is possible you may have too many luminares on the circuit.

The current draw when new was probably acceptable but now they are overloading the circuit.