Fuses in Instrument Circuits in Class I Div. 2 Locations

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pburger

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From NEC 501.115(B)(3):

"For the protection of motors, appliances, and lamps...fuses shall be permitted if they are within general purpose enclosures, and if they are of a type in which the operating element is immersed in oil ..... or the fuse is a nonindicating, filled, current-limiting type."​

From NEC 501.105(B)(5):

"...fuses for overcurrent protection of instrument circuits not subject to overloading in normal use shall be permitted to be mounted in general purpose enclosures if each such fuse is preceded by a switch complying with 501.105(B)(1)."​


QUESTION:
I occassionally have people tell me that any fuse in a Class I, Div. 2 area must be per NEC 501.115(B)(3), or specifically, "nonindicating, filled, current-limiting" However, that section seems to apply to "motors, appliances, and lamps".

I'd appreciate any comments on using standard 5x20mm glass fuses in control panels based upon NEC 501.105(B)(5). I think that this section allows the use of fuses under certain conditions (instrument circuits not subject to overloading), with certain requirements (each such fuse is preceded by a switch...). I also think that this section does not restrict the type of fuse to use. One example is the Allen-Bradley SLC-500 PLC: The power supply for the SLC-500 has a glass fuse next to the power terminals (under an access door).

I see a lot of control panels with GMA-style fuses (5x20mm glass) used in Class I, Div. 2 areas, but I still hear people quote the NEC 501.115(B)(3) to say that this practice is not allowed.

What do others out there do?

If anyone has read this far, what about the use of indicating fuse terminals? I'm referring to the terminal blocks with fuse holders that also have an LED to indicate when the fuse is blown. I see these used all the time in control panels, too. The little LED circuit is not necessarily rated for a Class I, Div. 2 area, so is this okay or not? What do you think?
 
I would endorse your fuse analysis. I am a bit surprised you omitted another viable option since glass fuses have an “…operating element … enclosed within a chamber hermetically sealed against the entrance of gases and vapors,” Glass fuses, reed switches, "ice cube" relays and similar constructions are what we had in mind.

With regard to LED’s, it’s more complex. LEDs are diodes and diodes are essentially resistors with non-linear voltage-current characteristics. They are still resistors though and would be analyzed under Section 501.120(B)(3). Obviously, a diode's resistance is not “nonvariable,” virtually by definition.

However there is still some room for further analysis. The key is the word identified in Section 501.120(B)(3). It is a defined term in Article 100. Read it carefully and recognize "listing" and/or "labeling" are not the exclusive means for “identifying” something. Section 500.8(A)(1) gives two additional methods.

This was a big issue in the 1999 and 2002 cycles when one organization submitted a multitude of Proposals to make "listing" the exclusive means of identifying products for Classified Locations. Of course, that would have essentially prohibited "ordinary location" products in Division 2.
 
Bob,
"ice cube" relays
Click to expand...

While I have seen ice cube relays that were sealed and suitable for Class I, Division 2 locations, I don't think that what most of us call "ice cube" relays have their contacts "enclosed within a chamber hermetically sealed against the entrance of gases and vapors".
Don
 
You're right of course, not all "ice cubes" are suitable. The ones that are though generally provide a "hermetic" rather than "explosionproof" seal. That's why they are typically listed only for Division 2. Thsnks for keeping me honest.
 
Thanks for you comments, they are appreciated. I'll look into the LED issue further. The conservative position is to just not use indicating fuse terminals (with LEDs) in CLID2 applications.

Concerning "ice cube" relays:
I typically call ice cube relays the small, square, P&B (or whatever brand) relays that have clear plastic cases. I always assumed these were called ice cube relays because they resemble ice cubes due to the clear plastic. These are most definitely NOT hermetically sealed.

I think you are referring to the relays of the same size and shape that are in metal cans, which ARE hermetically sealed and listed for use in CL1D2. I suppose some people call these relays ice-cube relays, but I just say "metal can relays" because it eliminates confusion, and most people know I'm talking about the hermetically sealed relays.

Glass Fuses:
If you can find a fuse manufacturer that states that their glass fuses are hermetically sealed, please let me know. Although they look like they are hermetically sealed, I can't say for certain. I dropped several in a cup of water and left them for a few days. I couldn't detect any moisture inside the glass tubes, but that's hardly a conclusive test. I'm pretty sure most (if not all) reed switches are listed as hermetically sealed by the manufacturer, but I can't say that for glass fuses.
 
I probably should have just said “hermetically sealed relays.”

Incidentally, most of hermetically sealed relays are simply “recognized components.” Per the UL “White Book”: “The Recognized Component Mark does not provide evidence of listing or labeling, which may be required by installation codes or standards.” Since they are not listed or labeled, 110.3 (B) does not apply. But 110.3 (A) still applies.

Most glass fuse manufacturers could readily provide you with a statement that the fuses are hermetically sealed.

Depending on the jurisdiction, they could be “identified” per 500.8(A)(1)(3).
 
rbalex said:
I would endorse your fuse analysis. I am a bit surprised you omitted another viable option since glass fuses have an ??operating element ? enclosed within a chamber hermetically sealed against the entrance of gases and vapors,? Glass fuses, reed switches, "ice cube" relays and similar constructions are what we had in mind.

With regard to LED?s, it?s more complex. LEDs are diodes and diodes are essentially resistors with non-linear voltage-current characteristics. They are still resistors though and would be analyzed under Section 501.120(B)(3). Obviously, a diode's resistance is not ?nonvariable,? virtually by definition.

However there is still some room for further analysis. The key is the word identified in Section 501.120(B)(3). It is a defined term in Article 100. Read it carefully and recognize "listing" and/or "labeling" are not the exclusive means for ?identifying? something. Section 500.8(A)(1) gives two additional methods.

This was a big issue in the 1999 and 2002 cycles when one organization submitted a multitude of Proposals to make "listing" the exclusive means of identifying products for Classified Locations. Of course, that would have essentially prohibited "ordinary location" products in Division 2.
Click to expand...

The glass fuses are NOT hermetically sealed. You can remove the cap with a little force and the caps are friction fitted without any potting or sealing material. Although there is no danger for the gasses to force themselves into the glass tube, the arc is not securely confined within the chamber in case of sort sircuit and the escaping hot gasses could be a source of ignition.

There are filled fuses that are listed for Class I, Division 2 use.

Relays would need to have a listing for Class I, Division 2 use. Most often reed contacts are employed and they have very small current carrying capability.
 
We appear to have been responding at the same time. I essentially addressed your comments here. I did however cite the appropriate text from which I derived my analysis.
 
hermetically sealed relays -- CID2 fuses

hermetically sealed relays -- CID2 fuses

I don't know about "most" hermetically sealed relays, but the gray, hermetically sealed metal can relays that I have used, both from P&B in the past and now Magnecraft, have all been listed by UL for use in CID2 locations. BTW: These are not reed relays, but regular mechanical relays that have been hermetically sealed in a metal can. Some of the larger ones that use an octal socket have contact ratings up to 12A.

Ice Cubes:
I will say that I pulled up a data sheet for one of the small square can relays, and it said "hermetic ice cube relays" at the top. And, I was discussing a project with someone today, and they said something like, "why don't we use ice cube relays..." and they meant the metal can relays. So, I guess the ice cube moniker applies to all the small square relays whether or not they have the clear plastic.

CID2 Fuses:
Are there any small (5x20mm) fuses that are listed for use in CID2 areas?

I still think that 501.105(B)(5) coupled with 500.8(A)(3) allows the use of glass fuses in instrument circuits not subject to overloading in normal use. I would really like to hear further comments on this.
 
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