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Intrinsic barrier and wiring separation
2

Intrinsic barrier and wiring separation

Intrinsic barrier and wiring separation

(OP)
I have heard that wiring from hazardous rated area (In this case instrument wiring i.e floats from a wet well in a waste water lift station) entering PLC Panel in a control building need to be separated and wired through an intrinsic barrier into the PLC's input terminals. In our case, we have wiring entering the PLC panel and is run in a separate Panduit then into Phoenix Contact's isolation amplifier signal conditioner (2924045). Questions I have are:
Can anyone point me to Code References in Canada (Alberta)?
Is my understanding and wiring configuration correct?

RE: Intrinsic barrier and wiring separation

Except for an actual sewer plant those never make any sense to me at all. The switches are inside a sealed ball for crying out loud.


But yes, intrinsic safety requires that ENERGY be limited in current and voltage to levels incapable of actually igniting the involved atmosphere.

This is assured by a LISTED intrinsic barrier that blows internal fuses and/or simple resistors or Zeners et. al. to do the job. You need a physical barrier between the hazardous area and the non-hazardous area. That can be X feet of safe atmosphere or more typically a wall or walls. It's considered that any of the wiring on the non-hazardous side of said barriers can cause ignition whereas not the other side of barriers. This means the barriers can be anywhere in the non-hazardous area with the protected wires routed into the hazardous area.

Doing this requires some thought and care. If you have a conduit coming from the hazardous area to the non-haz area that conduit can conduct haz atmosphere out to an area where it's not expected and could be ignited by many means. Ultimately that can conduct the ignition back thru the boundary to detonate the haz atmosphere. To prevent this all the conduits leaving the haz area need to be blocked from haz atmosphere flow. There are lots of ways to do that. A common one is a can of stuff you mix water into and in an hour it's hard as a rock. You ladle that stuff into the conduits so that the entire conduit is blocked off for a few inches around the cable/wires.

Further requirements are that as soon as the haz wires leave the barriers they MUST be kept separate from non-protected wires so a fire or more likely an idiot with a power drill can't somehow short non-barriered wires to the barriered wires. This means separate conduits or a physical dividing barrier placed in a wire raceway.

With respect to sewer sumps and anal retentive specifiers they now have barriers that are specifically designed for that use and they dumb it all down to monkey-level calling them something like "Level Safety Relays" or something similar.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Intrinsic barrier and wiring separation

There have been cases where sewage lift stations became filled with gasoline or other flammable liquids. Pretty sure that NFPA 820 requires raw sewage wet wells to be treated as Class 1, Div 1. At least they used to.

RE: Intrinsic barrier and wiring separation

They still do.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Intrinsic barrier and wiring separation

Assuming I'm recalling this properly, if you have an IS circuit, you are not required to seal w/ chico (what Keith is referring to in his post), but you must use some type of compound to seal between the hazardous area and the non-hazardous area, e.g. duct seal.

Mike

RE: Intrinsic barrier and wiring separation

I just went through this same review. The supplier of our formerly preferred intrinsically safe level transmitter stopped third party listing the device for use in intrinsically safe circuits. Pulling the third party certification sheet (UL, FM, etc) was necessary in this case to determine correct handling of the sensor vent tube with respect to the hazardous location boundaries.

So for sewage lift stations NFPA 820 defines the hazard zone boundaries. So we do not need to have a process engineer do the zone boundary development engineering. Any yes we have experienced "unidentified hydrocarbon - smelled like diesel fuel" in the sanitary sewer. It happens and is a painful cleanup.

Intrinsically safe wiring practice NFPA 70 National Electric Code,Article 504 -- Intrinsically Safe Systems. If you are planning on working with intrinsically safe wiring, you really need a copy of the electric code handy, there are a bunch of rules. a summary can be found here http://www.codebookcity.com/codearticles/nec/necar... My two electrical engineers sounded like two debating lawyers for a while.

We debated the seal off issue. Our conclusion is that the intrinsically safe wiring needs listed seals. But the intrinsically safe circuit may include terminal strips inside the hazardous area. There is some flexibility as to where the junction box is.

Seals can be either NEMA 7 type with chico, or glands listed for use as hazardous area boundary seals Example of a listed gland https://www.eaton.com/us/en-us/catalog/conduit-cab.... Please be aware that Listed for IEC EX and listed for NFPA 70 national electric code sometimes occurs in the same gland (like the example), but most are not dual rated.

RE: Intrinsic barrier and wiring separation

I have designed sewerage pumping facilities, and constructed and installed pump/level systems for 40+ years. Everything but the pump POWER cables cables should be intrinsically safe.Cable seals are a pain both to install and to deal with during maintenance. IS barriers and downstream wiring needs to be isolated from non-IS wiring. I have found it best to mount a/two junction box(s) on the roof/top/cover of an exterior wet-well. These boxes should be NEMA4 SS. Bottom of the box should be 24-30 inches above the top of the wet-well exterior. The box is now located in a non-hazardous area. Wet-Well wiring can exit the bottom of the box via strain devices and extend downward into the wet-well. The entry into the well chamber can be through NON-sealed holes, conduit stubs (PVC), Rigid nipples with plastic bushing, your choice. These opening should NOT be sealed. The box or boxes contain terminals for all wires. IS wires require one enclosure and power wires require a second. Or, I have used one box with a barrier to create two areas. The junction boxes can then be connected to the control panel without any seals. If a two-wire pressure sensor with a vent tube is in the system, strip the signal wires back to expose about 12 to 20 inches of the vent tube, exit the bottom of the junction through a small hole and tie-trap a cloth (wife's stockings provide this) bug screen around and over the tube end outside the box. Conductors between the J-Box and the Control Panel can be standard conductors or STP as required. Inside the Control Panel, the IS conduits enter a designated isolated area and the IS devices installed in accordance with the manufacturers instructions. The only problem I have had has been with FLYGT pumps where the control device wiring is contained in the pump power cable. This has also been a problem when using VFD motor controllers.
Sorry about the length
Steve

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