C1D2 Enclosure and fuses

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fredericktex

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This subject has been broached many times, but I think I may have a unique scenario that hasn't been addressed here before.

I work for a utility company in the natural gas side of the house. We have RTU (Remote Terminal Unit) cabinets that are classified as C1D2, Group D. The cabinet's innards consist of SCADA electronics, TBs, power supply, UPS, batteries, Rosemount transmitters and various hand switches and breakers. We currently use Phoenix contact disconnectable fuse blocks with glass fuses in the XMTR circuit. This is to primarily prevent the SCADA electronics from experiencing a current overload if the hot wire is accidentally shorted to ground during maintenance. Such a condition would ruin the SCADA pack and since it has no internal fuses and is single-ended, it would have to be replaced. No salvage is possible as I understand it. No moving to a spare input, nothing.

Now, we are not in a classified area. The inside of the RTU is the classified area because there are pilot regulator vents and instrument tubing inside the cabinet with ranges anywhere from 60 - 600 PSI. The RTU enclosures are vented to the atmospehe and installed on street corners all over CA.

Here's the question: Can we use glass fuses because this is a nonincendive circuit and we also have the main AC disconnect as well as a DC breaker to remove power to all the instruments at once, not one at a time? It seems like we should be fine by my interpretation of the code, but I would like a more experienced person's input.

I feel this is a unique question because all other threads seem to be concerned with an enclosure itself inside a classified area.
 
fredericktex said:
This subject has been broached many times, but I think I may have a unique scenario that hasn't been addressed here before.

I work for a utility company in the natural gas side of the house. We have RTU (Remote Terminal Unit) cabinets that are classified as C1D2, Group D. The cabinet's innards consist of SCADA electronics, TBs, power supply, UPS, batteries, Rosemount transmitters and various hand switches and breakers. We currently use Phoenix contact disconnectable fuse blocks with glass fuses in the XMTR circuit. This is to primarily prevent the SCADA electronics from experiencing a current overload if the hot wire is accidentally shorted to ground during maintenance. Such a condition would ruin the SCADA pack and since it has no internal fuses and is single-ended, it would have to be replaced. No salvage is possible as I understand it. No moving to a spare input, nothing.

Now, we are not in a classified area. The inside of the RTU is the classified area because there are pilot regulator vents and instrument tubing inside the cabinet with ranges anywhere from 60 - 600 PSI. The RTU enclosures are vented to the atmospehe and installed on street corners all over CA.

Here's the question: Can we use glass fuses because this is a nonincendive circuit and we also have the main AC disconnect as well as a DC breaker to remove power to all the instruments at once, not one at a time? It seems like we should be fine by my interpretation of the code, but I would like a more experienced person's input.

I feel this is a unique question because all other threads seem to be concerned with an enclosure itself inside a classified area.
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This is unusual for me. I often find myself telling users they have over-classified a location when they discover the full implications of doing so.

A few comments:

As a utility, you may not be directly subject to the NFPA/NEC world; however, if you were:

  • Depending on the location of the vent exhaust, you probably ARE in a classified location. There is always a classified envelope around an exhaust port from a flammable or classified source.
  • Any gas pressures inside the enclosures operating above 100 PSI should be of great concern for valves, flanges and fittings.
  • A hot to ground is an overcurrent, specifically a ground-fault or short-circuit depending on the system, NOT an overload (even for nonincendive systems). This is one reason classified location grounding/bonding requirements are still the same for nonincendive systems. A nonincendive system is "safe" only under "normal" conditions.
All that said, if the enclosure interior is properly classified, then glass fuses should be acceptable. [Section 501.115(B)(1)(1)]
 
rbalex said:
This is unusual for me. I often find myself telling users they have over-classified a location when they discover the full implications of doing so.

A few comments:

As a utility, you may not be directly subject to the NFPA/NEC world; however, if you were:

  • Depending on the location of the vent exhaust, you probably ARE in a classified location. There is always a classified envelope around an exhaust port from a flammable or classified source.
  • Any gas pressures inside the enclosures operating above 100 PSI should be of great concern for valves, flanges and fittings.
  • A hot to ground is an overcurrent, specifically a ground-fault or short-circuit depending on the system, NOT an overload (even for nonincendive systems). This is one reason classified location grounding/bonding requirements are still the same for nonincendive systems. A nonincendive system is "safe" only under "normal" conditions.
All that said, if the enclosure interior is properly classified, then glass fuses should be acceptable. [Section 501.115(B)(1)(1)]
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I do not believe we are subject to NFPA/NEC as you mentioned and we are definitely in a classified area; we were never mistaken about the C1D2 status. My PE agrees. That being said, we don't want there to exist a condition where we do something that would get us in trouble or worse yet, someone hurt. Just because we aren't necessarily subject to the NEC, doesn't mean we should just throw it out. Agreed?

Anyway, we use 3/8" SS tubing, and our valves are rated to 5000 PSI. This is of very little concern for failure at the pressures we operate. I think we can use the glass fuses with a phenolic tag on the door stating that the breaker and AC switch should both be turned off (well, maybe just the breaker to the XMTRs, wouldn't want the radio to go dead and have an alarm in Gas Control) and then you can open the fuse block and replace the fuse as needed.

Also, thanks for pointing out my error regarding overcurrent vs. overload as well as your input in general. Cheers!
 
The 5000 PSI rating relieves a lot of concerns; in fact, it may be justified to call the interior unclassified. (I don't know the application that well though).

You still may want to review the envelopes around the vent ports. I'd review NFPA 497.
 
If the pilot regulator diaphragm fails, and it's happened before, we think it's an abnormal condition that warrants the classification. The RTU itself isn't bigger than 48" x 48" x 36" so the entire inside of the cabinet is classified C1D2. I'm more confident than ever that the glass fuses will be okay with the breaker and switch in the circuit as well.
 
The glass fuses should definitely be OK, even if it weren't nonincendive. The rest of switching devices are acceptable only if the system is nonincendive.

Just some curiosity questions:

  • Does the nonincendive power source originate outside the enclosure?
  • Is the nonincendive system installed per a manufacturer's control drawing?
 
ALL of the equipment inside the cabinet is rated for C1D2 operation, with the exception of the fuse blocks and the fuse. Also, the fuse block has a 24V led circuit that is triggered when the fuse blows.

Can you define incendive as it pertains to the NEC?
 
fredericktex said:
ALL of the equipment inside the cabinet is rated for C1D2 operation,...
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New side question: If the internal equipment is rated for Division 2, why are you even concerned about nonincendive?

BTW "Class" is always designated by a Roman numeral; i.e., CI, D2, NOT C1,D2 (No big deal, of course, I understand you anyway - but if you're in a learning mode ;).
 
rbalex said:
New side question: If the internal equipment is rated for Division 2, why are you even concerned about nonincendive?

BTW "Class" is always designated by a Roman numeral; i.e., CI, D2, NOT C1,D2 (No big deal, of course, I understand you anyway - but if you're in a learning mode ;).
Click to expand...


I'm concerned about nonincendive because it seemed to be one of the exceptions in the code that would allow us to use glass fuses and non-CI/D2 fuse blocks in our instrumentation loops. I should also mention, a lot of these units have already been installed and rework is expensive and time consuming so we want to leave them alone, if we can legally do so.

I didn't know about the Roman numeral thing... Thanks :D
 
fredericktex said:
I'm concerned about nonincendive because it seemed to be one of the exceptions in the code that would allow us to use glass fuses and non-CI/D2 fuse blocks in our instrumentation loops. I should also mention, a lot of these units have already been installed and rework is expensive and time consuming so we want to leave them alone, if we can legally do so.

I didn't know about the Roman numeral thing... Thanks :D
Click to expand...
You didn't need an exception, glass enclosed fuses already meet Section 501.115(B)(1)(1).
 
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