Failed Capacitor/resistors

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raiderUM

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I've attached a picture showing on good resistor and one bad one.

My question is what causes these resistors to go bad? and why do they take out internal fuse protection and the fuse protection for the unit itself? Internal fuses are 100 amps and 600 amps for the overload protection on the unit itself.

The resistor is all burnt up. I checked all the Caps within the bank and they all checked out to be okay. So these resistors are taking out both fuses? The main 600 amp and the internal 100 amp? I would think the 100 amp would blow and that would be it.

IMAG0533.jpg
 
120710-1142 EDT

Excessive power dissipation. Looks to be a carbon comp resistor. The intact one is 15 k-ohms. A 2 W is 0.75 x 5/16 OD, 1 W is 0.625 x 1/4, and 1/2 W is 0.375 x 1/8. A wire wound 2 W is about same size as the 1 W carbon comp.

Assume 1 W and 15 k, then allowed continuous voltage is about 120 V.

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gar said:
120710-1142 EDT

Excessive power dissipation. Looks to be a carbon comp resistor. The intact one is 15 k-ohms. A 2 W is 0.75 x 5/16 OD, 1 W is 0.625 x 1/4, and 1/2 W is 0.375 x 1/8. A wire wound 2 W is about same size as the 1 W carbon comp.

Assume 1 W and 15 k, then allowed continuous voltage is about 120 V.

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Click to expand...

Ok???? So is this something that goes bad over time? And causes both overcurrent protection to fail?
 
120710-1255 EDT

I don't know where the failed resistor is in your circuit, but it is unlikely that its failure caused whatever problem occurred to occur. Rather it is likely that something else was the failure that caused the fuses to blow and that failure also cause the resistor to fail. If the resistor's resistance remained constant at its rated value and you applied 1000 V to it, then the current would be about 0.07 A, and the power dissipated about 67 W.

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raiderUM said:
I've attached a picture showing on good resistor and one bad one.

My question is what causes these resistors to go bad? and why do they take out internal fuse protection and the fuse protection for the unit itself? Internal fuses are 100 amps and 600 amps for the overload protection on the unit itself.

The resistor is all burnt up. I checked all the Caps within the bank and they all checked out to be okay. So these resistors are taking out both fuses? The main 600 amp and the internal 100 amp? I would think the 100 amp would blow and that would be it.

View attachment 7154
Click to expand...

I'm taking a wild guess here. Since you haven't told enough about what role the resister plays in the circuit. It is common for resisters to be required across the terminals of a capacitor in order to dissipate the voltage that is stored in capacitor after it has been de-energized.
This prevents an unwelcome hazard to those who come into contact with the terminals one a supposedly innocent looking capacitor.
As such Discharge Resistors: Each capacitor cell incorporates internal discharge resistors designed to reduce the capacitor voltage to 50 volts or less within one minute after removal from the circuit.
 
Besoeker said:
Looks like brown green yellow on my screen which would make it 150k.
Click to expand...

You must be of the old school and remember that saying for the color code values. It's burned in my mind as is the trig functions.
Then, is that a gold or silver band for the tolerance? 5 or 10%?
 
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templdl said:
You must be of the old school and remember that saying for the color code values. It's burned in my mind as is the trig functions.
Click to expand...
Yes, I am very aged....:D. And blessed and cursed with a fairly good memory for particularly for numbers. And a lot of it of no use to me whatsoever. I know an acre is 4840 square yards and that there are eight furlongs in a mile and stuff such as that I'll never use. Is it occupying brain capacity that could be put to better use I sometimes wonder.....

Back on topic - I remember two sayings for the colour code. One is far too coarse to be permitted here.
But I don't actually use them. I just know or somehow remember what the colours are. Then the resistors are specific values in what we usually encounter as either the E12 or E24 series.....10, 12, 15, 18, 22, 27, 33 for example. You get to recognise patterns and can identify a value without going through the band, band multiplier thought process. At least that's how it seems to me.


templdl said:
Then, is that a gold or silver band for the tolerance? 5 or 10%?
Click to expand...
Looks like silver - 10%.
 
templdl said:
I'm taking a wild guess here. Since you haven't told enough about what role the resister plays in the circuit. It is common for resisters to be required across the terminals of a capacitor in order to dissipate the voltage that is stored in capacitor after it has been de-energized.
This prevents an unwelcome hazard to those who come into contact with the terminals one a supposedly innocent looking capacitor.
As such Discharge Resistors: Each capacitor cell incorporates internal discharge resistors designed to reduce the capacitor voltage to 50 volts or less within one minute after removal from the circuit.
Click to expand...

Sorry for the lack of information. Here is a picture of how they are wired in the circuit with the Capacitors. As you can see in the Picture the bottom fuse is blown, but all the Capacitors within this bank and the entire unit test out as being perfectly good. I dont know what else would have taken the fuses out? Lighting? Power surges?



IMAG0535.jpg
 
These may be used to discharge the capacitors upon denergization such as a bleeding resistor.

But, I am still strongly spsect that you are leaving a lot of about what the actual application, as it this were a puzzle.
But your picture show that it may be being using as a voltage balancing resistor on a reservoir for a high voltage DC bus where it is common to use a series combination capacitors?
 
gar said:
120710-1447 EDT

Very orange and gold on my CRT screen.
Click to expand...

Mine is a fairly new LED monitor and, as far as I can tell, the colour rendition is pretty good.
I do a lot of photography and things like this tiny flower just wouldn't work otherwise:

July12batch04blossom.jpg


A more technical point. I'm assuming that the lowest voltage the capacitors would be connected to is 120Vac.
At 15kΩ, the resistors would operate at close to their maximum rated dissipation continuously. They would get hot and age.
The intact resistor doesn't look particularly distressed. In any case, prudent design would be to provide a good operating margin especially on components that cost no more than a few cents.
Taking probable voltage and margins into account inclines me to think that 15kΩ is too low.
And Raider's latest picture makes it look more like brown grey yellow, so 180kΩ.

Maybe raider could kindly confirm the operating voltage.
 
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templdl said:
These may be used to discharge the capacitors upon denergization such as a bleeding resistor.

But, I am still strongly spsect that you are leaving a lot of about what the actual application, as it this were a puzzle.
But your picture show that it may be being using as a voltage balancing resistor on a reservoir for a high voltage DC bus where it is common to use a series combination capacitors?
Click to expand...


480V fixed capacitor bank. There are 150kvar in this bank split into three sections of 50 kvar each. These are connected by a 400 amp disconnect with 350 amp fuses, which is direct connected to the main buss. These are for secondary power factor correction. I have other locations where the same thing has happened but the kvar bank is 250 and 100. The 250 kvar bank main disconnect is fused with 600 amp fuses and the 100 kvar is fused with 250 amp fuses
 
Besoeker said:
And Raider's latest picture makes it look more like brown grey yellow, so 180kΩ.
Click to expand...
I have a high-end graphics workstation and all of the pictures indicate this color coding to be correct.

One of the things I have observed over the years is that when these resistors fail slowly due to being operated too close to their power rating, then the outer case tends to blister before the carbon fails. When the power overload is sudden, then the outer case tends to shatter or split in half like this due to the sudden thermal stress.

One possible cause for such a thing is a very short duration break in the circuit. The capicitor could still be fully charged (near peak voltage) with a given polarity, but power gets restored at the opposite polarity that is also very near the peak amplitude. This instantaneous reversal from one peak to the opposite peak represents a voltage difference that exceeds the normal rating of the resistor. It would also explain the possible reason for tripping an overload device.
 
Besoeker said:
I remember two sayings for the colour code. One is far too coarse to be permitted here.
Click to expand...
:thumbsup: I have very few memories that still stick with me from back during my high school electronics class. One is the 300VDC arm-to-arm shock that caused me to throw the metal plate I was holding over my head and striking a wall 20 feet behind me. The other is "Bad, Boys,........Violet, Gives, Willingly" He was a very gruff and politically incorrect teacher, but I could get away with murder with him. :p
 
Rick Christopherson said:
:thumbsup: I have very few memories that still stick with me from back during my high school electronics class. One is the 300VDC arm-to-arm shock that caused me to throw the metal plate I was holding over my head and striking a wall 20 feet behind me. The other is "Bad, Boys,........Violet, Gives, Willingly" He was a very gruff and politically incorrect teacher, but I could get away with murder with him. :p
Click to expand...

Violet and the bad boys is the one I remember also...:p
 
clearly 180k on my screen here too.

but perhaps the failed resistor is not related to the fuse blowing at all.... might be it overheated slowly over time, failed, and no one would have known it - until big fuses blew and caused you to look inside the case and see the unrelated 2 year old failed resistor?
 
It does appear to be a discharge resistor then. I probably would consider replacing it with a higher wattage rated resistor then if the existing one is degrading because of heat and age.
 
120710-2121 EDT

The first post was totally misleading. If it had been described as a bleeder resistor across a 480 V supply, then it would have been clear it was not 15 k. The second photograph is yellow on my screen.

We still don't know the resistor wattage rating. If there is really 480 V applied to a 180 k resistor, then power dissipation is 1.28 W and the resistor would have to be a 2 W. Is it a 2 W?

Resistors in this package size with 400 V applied were common in the tube days and were reliable.

Small package wire-wound resistors do not exist in a 180 k value.

Did the failed resistors really burn up, or just fracture in the middle?

A 3/4" long 2 W carbon comp resistor should be good for about 3 * 120 * 1.414 for over the surface leakage or 500 V.

The leads are rather short so was repetitive mechanical stress from large current loads on the capacitors a possible contributor to the failure. The fuse failures were not caused by the resistor failures unless an arc developed between the two resistor wires.

Did any capacitors fail?

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