UL508A Resistors in a Control Cabinet

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JWarner

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We currently have a control system that we are trying to get UL Approval on. The problem is, we have a couple of stepper drives that require 2.2K Ohm, 1/4 Watt, 250V, 5%, Carbon Film Resistors from our Digital Output to the Drive Enable as well as Step and Direction for the Drive. Currently we have a PCB with terminal blocks on it and thru hole resistors soldered in place where needed. None of which are UL Listed. I am trying to locate UL Listed 2.2K Ohm, 1/4 Watt, 250V, 5%, and am unable to locate them. Even if I move up to 1/2 Watt Discharge resistors. The only UL listed 1/2 Watt are for 1MW and up. This has had me stumped and doing a ton of searching to find replacement for the resistors when there may be a simpler way around it so we can get this control panel UL Approval.
 
JWarner said:
We currently have a control system that we are trying to get UL Approval on. The problem is, we have a couple of stepper drives that require 2.2K Ohm, 1/4 Watt, 250V, 5%, Carbon Film Resistors from our Digital Output to the Drive Enable as well as Step and Direction for the Drive. Currently we have a PCB with terminal blocks on it and thru hole resistors soldered in place where needed. None of which are UL Listed. I am trying to locate UL Listed 2.2K Ohm, 1/4 Watt, 250V, 5%, and am unable to locate them. Even if I move up to 1/2 Watt Discharge resistors. The only UL listed 1/2 Watt are for 1MW and up. This has had me stumped and doing a ton of searching to find replacement for the resistors when there may be a simpler way around it so we can get this control panel UL Approval.
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Take a look at 44.2.1.
 
A 2.2k with 250 volts across it will dissipate about 30 watts ... something doesn't fit. I cannot imagine UL listing such a thing.
 
GeorgeB said:
A 2.2k with 250 volts across it will dissipate about 30 watts ... something doesn't fit. I cannot imagine UL listing such a thing.
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Possibly the OP needs 250V insulation in the body of the resistor?
Or a resistor switched into a circuit to discharge a capacitor could have high instantaneous power but little heating.
 
GoldDigger said:
Possibly the OP needs 250V insulation in the body of the resistor?
Or a resistor switched into a circuit to discharge a capacitor could have high instantaneous power but little heating.
Click to expand...

I think the voltage rating is more about the construction of the series of resistors then it means that you can safely apply that much voltage to a specific resistor.
 
petersonra said:
I think the voltage rating is more about the construction of the series of resistors then it means that you can safely apply that much voltage to a specific resistor.
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I was thinking of being able to apply 250 DC to one end of the resistor and 249V DC to the other end of the resistor while mounting the resistor body over a grounded trace on the PC board.
 
GoldDigger said:
I was thinking of being able to apply 250 DC to one end of the resistor and 249V DC to the other end of the resistor while mounting the resistor body over a grounded trace on the PC board.
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The non-destructive voltage rating of the insulating material surrounding resistor body?
 
Smart $ said:
The non-destructive voltage rating of the insulating material surrounding resistor body?
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Yup. In most cases the published voltage rating of a resistor is not directly related to what voltage it can withstand steady state from terminal to terminal.
The latter part is taken care of by the ohm and watt ratings together.
Note that for a very high resistance the body insulation rating, as well as perhaps the breakdown of the resistive material itself, may lead to a far lower voltage rating than what could theoretically be applied terminal to terminal without exceeding the power rating.

P.S. The term resistor body, in electronics, generally refers to the lump between the two leads, not the resistive element itself inside that body.
 
JWarner said:
We currently have a control system that we are trying to get UL Approval on. The problem is, we have a couple of stepper drives that require 2.2K Ohm, 1/4 Watt, 250V, 5%, Carbon Film Resistors from our Digital Output to the Drive Enable as well as Step and Direction for the Drive. Currently we have a PCB with terminal blocks on it and thru hole resistors soldered in place where needed. None of which are UL Listed. I am trying to locate UL Listed 2.2K Ohm, 1/4 Watt, 250V, 5%, and am unable to locate them. Even if I move up to 1/2 Watt Discharge resistors. The only UL listed 1/2 Watt are for 1MW and up. This has had me stumped and doing a ton of searching to find replacement for the resistors when there may be a simpler way around it so we can get this control panel UL Approval.
Click to expand...

You won't find listed resistors. No one makes them. So how to get around the listing issue? Place this part of the circuit behind a LISTED Class 2 power supply, and all is well. I know it's dumb that you may have to install a separate Class 2 supply in the cabinet, instead of a 1/4A fuse, but the UL overlords say so.

The other way you can fix this is to insulate the resistor with heatshrink, and put across two adjacent terminal blocks. Done and done.

Talk to your field rep. The better ones can offer useful guidance, though some I've dealt with are little better than potted plants. Ain't 508A fun?!



SceneryDriver
 
petersonra said:
How does insulating a resistor get you past the component requirements?
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Resistors and like components are to be treated as live components, and must be insulated for the voltages involved.

"36.2.2 When evaluating spacings in Section 10, Spacings, the body of a resistor is an uninsulated live part."

UL listed heatshrink rated to 600V works, according to my UL field rep. It gets a little weirder once the wattages involved preclude covering the resistor body with heatshrink, but for little 1/4W units, they just can't generate enough heat to matter.

Components like diodes, resistors, and the like simply are not UL listed, as they are components presumed to be built into larger assemblies or parts. So far, doing exactly what the OP describes hasn't caused a problem for us in our UL-listed panels.


SceneryDriver
 
SceneryDriver said:
Resistors and like components are to be treated as live components, and must be insulated for the voltages involved.

"36.2.2 When evaluating spacings in Section 10, Spacings, the body of a resistor is an uninsulated live part."

UL listed heatshrink rated to 600V works, according to my UL field rep. It gets a little weirder once the wattages involved preclude covering the resistor body with heatshrink, but for little 1/4W units, they just can't generate enough heat to matter.

Components like diodes, resistors, and the like simply are not UL listed, as they are components presumed to be built into larger assemblies or parts. So far, doing exactly what the OP describes hasn't caused a problem for us in our UL-listed panels.


SceneryDriver
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You can get away with a lot that the inspector won't notice or will just accept but that doesn't make it compliant with UL 508a. Lots of people use the heat shrink trick to insulate all kinds of things even things like ethernet cables.
 
petersonra said:
You can get away with a lot that the inspector won't notice or will just accept but that doesn't make it compliant with UL 508a. Lots of people use the heat shrink trick to insulate all kinds of things even things like ethernet cables.
Click to expand...

Unfortunately, I can only pass on what I was told by UL. After dealing with this exact resistor question via a week's worth of emails between me, my client, and UL, I'm convinced that UL doesn't always understand its own codes. I have to go by what I was told by my field rep, and he said the UL listed heatshrink over the body of a 1/4W resistor is acceptable.

In the case of larger resistors, he said that if you want to support them on a PCB, that PCB material must be "listed for the support of live components." Most all PCB material you buy (except for the EXTREMELY cheap imported stuff) is (or can be) listed as such.

You can use these as a UL-listed solution:

http://new.abb.com/low-voltage/prod...minal-blocks/component-holder-terminal-blocks

Just solder your resistor (or other component of choice) into the terminal block, and away you go. The only reason I don't like these TB's is that they're likely to confuse the crap out of a service tech that hasn't seen them before. At least with a "normal" pair of terminal blocks, and a heatshrink -covered resistor, you can see all the components. The ABB blocks get points in my book for elegance though, for sure.

I've used these for 24V Class 2 circuits, and fab-ed a Lexan shield for them. My rep had no issue with them.

http://www.bipom.com/products/us/2899938.html


SceneryDriver
 
he shouldn't have a problem with anything found in a class 2 circuit. it just does not require UL evaluation.

however, I doubt that it is kosher to just heat shrink something and claim it is OK to use it that way outside of a class 2 circuit.

my understanding is that a lot of drive and plc i/o are already listed as class 2 circuits so as long as you do not hook them up to something that is not a class 2 circuit you would be Ok anyhow. but I have never looked real closely at that possibility.
 
petersonra said:
he shouldn't have a problem with anything found in a class 2 circuit. it just does not require UL evaluation.

however, I doubt that it is kosher to just heat shrink something and claim it is OK to use it that way outside of a class 2 circuit.

my understanding is that a lot of drive and plc i/o are already listed as class 2 circuits so as long as you do not hook them up to something that is not a class 2 circuit you would be Ok anyhow. but I have never looked real closely at that possibility.
Click to expand...

You are correct, and I probably should have been more clear. Heatshrink over a resistor only applies to Class 2 circuits. It was my field rep's way of being OK with having a resistor hanging out between two adjacent terminal blocks. If the resistor is in a power circuit, the requirements are much more stringent.

That said, I've found that VFD braking resistors aren't UL listed either. You have to obey the regs regarding mounting and considering a power resistor as a live component (unless it's potted in a metal enclosure).

"Exception: A resistor embedded in a metal sheath and mounted to grounded metal that complies with the Dielectric Voltage Withstand Test in the Standard for Industrial Control Equipment, UL 508, is not required to be evaluated to this requirement."

It's one of the strange idiosyncrasies that resistors, caps, etc... don't have to carry a UL sticker, so far as I know. I think it's weird too. It's a lot like wire ferrules; UL wants everything in the panel listed/recognized, but ferrules aren't, and everyone is somehow OK with it as long as they're used appropriately (quantity and size of conductors).


SceneryDriver
 
UL has since rescinded the letter that they wrote that allowed the use of unlisted ferrulles.

I seem to recall that braking resistors are available in either listed or unlisted varieties. I've talked to post Glover about this a couple of times and they tell me the only difference between resistors they make that are listed and those that are unlisted is the sticker
 
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