Figure 1. Bus Network Savings

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1 New Field Wiring Architecture Reduces Hazardous Area Wiring Costs By 65% Combining Bus Networks with Tray Cabling And Nonincendive Wiring Concepts by Wally Stommes President, StoneL Dramatic savings may be realized over conventional explosion proof point-to-point wiring (i.e., hard conduit) in hazardous Class I, Division 2 areas by combining bus networking with tray cabling and nonincendive protection concepts. By optimizing protection concepts on a bus network, users can realize greater installation flexibility with a less expensive bulky infrastructure and improved safety. This, in combination with greater field device functionality, such as predictive diagnostics, offers a compelling case for installing this new wiring architecture. Why Bus Networking? Based on hundreds of installations and thousands of bus networked field devices, it has been proven that bus architectures will cut installation costs up to 40% (see Figure 1). The installation capital cost savings vary for each application, with the primary determinants being network type, number of devices installed per segment, segment length and hazardous area wiring requirements. Figure 1. Bus Network Savings For example, a simple, discrete network, such as AS-Interface in a hazardous Class I, Division 2 area, produces a savings of $1,450 per installed node compared to conventional point-to-point wiring (see Figure 2). When using a higher-level protocol, such as Foundation Fieldbus (FF) H1, the up-front capital cost savings may be lower for a similar network but, nonetheless, still quite impressive. 1

2 Figure 2. Network Economic Analysis Conventional AS-I FF * Computer I/O; Master/Gateway $ 70 $ 50 $ 160 Conduit, Cable Tray, Wiring & Fittings $ 1,600 $ 350 $ 380 Valve Monitor/VCT with Pneumatic Valve $ 420 $ 590 $ 900 Switched Protected Drop Connector NA $ 170 $ 170 Installation & Commissioning Labor $ 800 $ 350 $ 350 Power Supply $ 50 $ 30 $ 60 Total Installed Cost $ 2,940 $ 1,540 $ 2,020 * FOUNDATION Fieldbus is not directly comparable. This system may be more cost effective when combining analog and discrete field instruments on the same segment. Functionality for FOUNDATION Fieldbus devices is also significantly greater, offering increased diagnostic and operational capabilities. In typical process installations, the cost savings have been so compelling that most new installations with 50 or more field instruments are now taking advantage of bus networking architecture. However, substantial savings can be realized with as few as 10 field instruments installed on a bus network. Optimized Wiring Safety Concepts. The National Electrical Code (NEC) allows for alternative protection concepts, which make it possible to eliminate hard conduit in hazardous areas to save an additional 25% without sacrificing, and perhaps even improving, safety. These wiring concepts are an ideal complement to bus networking, but they apply whether or not a bus network is implemented. Figure 3. Hazard Protection Savings 2

3 According to the code, two different wiring protection concepts may be used in hazardous Class I, Division 2 environments (gasses present under abnormal conditions) to further cut costs, improve reliability and enhance safety: 1. Tray cabling for the primary bus network trunk. 2. Nonincendive field wiring and related apparatus for drop connectors and field instruments. Tray Cabling. Tray cabling systems are specifically addressed in the National Electrical Code (Articles 725 to 727), but have not been used extensively for instrumentation. Most recently (NEC 2002), this area has been expanded to provide even more opportunities to use tray cabling for instrumentation in Class I and II, Division 2 areas. The general guidelines for using this concept are: 1.) Utilize Power Limited Tray Cable (PLTC) or Instrument Tray Cable (ITC), 2.) run the cable in cable trays or provide mechanical support for cable runs of up to 50 feet (15 meters) outside of trays, and 3.) limit the power going into the cabling to 100 watts for PLTC and three to five amps at a maximum voltage of 150 V for ITC. ITC and PLTC may be run in separate trays or in the same trays as higher power circuits, provided they are separated by either a mechanical barrier or two inches of space. When ITC and PLTC are run outside trays, mechanical struts or messenger wire must support it or it must be run inside liquidtight flexible conduit. Power limitations, when considering bus network segment requirements, are rather forgiving. Power must be limited to 100 watts and be supplied by a Class 2 approved power supply for PLTC. That translates to at least three amps for an AS-Interface network operating at 30 volts and four amps for a DeviceNet network operating at 24 volts. This is enough power to generally cover the requirements of these protocol segments. Foundation Fieldbus H1 and Profibus PA have maximum segment voltages of 32 VDC and typical currents up to 0.5 amps; so this concept is more than adequate to cover all applications with these protocols. ITC power limitations are even more liberal with current levels up to five amps (20 gauge or larger wire) and a maximum voltage of 150 volts. Since there may be some confusion over current limiting capabilities for power supplies (current clamping characteristics), a safe bet is to use a Class 2 approved power supply that, as described earlier, will keep current and voltage levels legal for both PLTC and ITC applications. 3

4 Tray cable wiring may be used directly with nonincendive equipment in any Division 2 area with unrestricted voltage and current levels, provided the nonincendive equipment ratings are not exceeded. "Nonincendive equipment" encompasses devices containing circuits that have all arcing contained within hermetically sealed capsules or with nonarcing circuits that do not have sufficient thermal energy to create ignition. This type of device is designed to prevent ignition of gases in their most easily ignitable concentration under normal operating conditions. Guidelines for wiring in cable trays are listed in Table 1. Tray cabling may be used all the way down to the individual field instruments through the wiring drop connectors, provided the guidelines are followed. However, a preferred method is to use nonincendive field wiring for lower current instrument drops. Table 1. Tray Cabling with Nonincendive Equipment Networking Guidelines (Class I, Division 2) Field Enclosures General-purpose enclosures are suitable for use with nonincendive components (nonincendive equipment). Conduit seals are not required for nonincendive equipment. Enclosures may be opened without deenergizing circuits. However, wiring may not be manipulated while circuits are energized. Field Wiring Instrument Tray Cable (ITC) must be limited to 150 volts and 5 amps for 20-gauge wire or larger or 3 amps for 22 gauge. ITC applications must have power supply with overcurrent protection that does not exceed current limits for respective wire gauges (Class 2 approved power supply is recommended). Power Limited Tray Cable (PLTC) must be limited to 100 watts and 60 VDC (e.g., VDC or VDC). PLTC applications must have a Class 2 approved power supply. PLTC and ITC used in cable tray systems must have separation of at least 2 in. (5 cm) or a mechanical barrier from higher power circuits. PLTC and ITC may be used as open wiring between cable tray and instruments for up to 50 ft. (15 m) and must be mechanically protected by angles, struts, channels or other mechanical means, or supported by messenger wire. PLTC and ITC may be used as open wiring between cable tray and instruments for up to 50 ft. (15 m) where cable complies with metal clad (MC) and is identified for such use. PLTC and ITC may also be used in explosion proof conduit, in liquidtight and other explosion proof approved systems. 4

5 Table 1. (continued) Connections Approved explosion proof fittings Quick connectors are suitable provided all of the following conditions are met: 1. Power is removed from the circuit before plugging or unplugging. 2. Current does not exceed 3 120VAC. 3. Cord is listed for hard usage with receptacle and plug of the locking and grounding type. 4. A label is attached to the receptacle warning against unplugging while energized. PLTC and ITC may be connected using compression type fittings installed in a manner to avoid tensile stress at the termination points. Flexible cord and quick connectors are suitable provided all conditions are met as described above for Class I Division 2 locations. Field Instrument Components Nonincendive components consist of: 1. Current interrupting contacts that are hermetically sealed against external gases. 2. Non-arcing (e.g., solid state) where the maximum operating surface temperature does not exceed 80% of the ignition temperature of the exposed gases. Nonincendive Field Wiring. By providing for the restriction of energy in the circuit, "nonincendive field wiring with associated apparatus" makes it possible to use standard wiring to further reduce costs. This protective concept is defined as a system that will not create ignition of the gases in their most easily ignitable concentration under normal operating conditions. Normal operation for nonincendive field wiring is defined as maximum voltage under open circuit conditions and maximum current under short circuit conditions. (A word of caution is in order here. Based on discussions with practitioners in the field, there is a great deal of confusion over the terms nonincendive equipment and nonincendive field wiring. Both have to do with protection concepts in Class I & II, Division 2 areas. The former is a type of device that can be used without limitation as long as the ratings are not exceeded. The latter is a wiring concept with specific overall circuit parameters that must be adhered to in order to assure field-wiring safety.) Nonincendive field wiring and associated apparatus are treated much the same as intrinsically safe circuits with single fault application because gases are not normally present in the Division 2 environment. In practice, both current and voltage levels must be limited to acceptable levels for particular gas groups under single fault conditions by a voltage and current limited barrier. Energy storage levels in the circuit also must be limited to prevent ignition from cabling and field instrumentation. In bus networking applications, a protected drop connector typically acts as the barrier. 5

6 Most nonincendive field wiring is suitable to carry up to 0.25 amps at bus network voltages of 30 VDC, making this protective concept ideal for use on a single bus drop. Since it is open and short circuit protected, there is no concern of electric shock at these voltage and current levels. In order to implement a nonincendive field wiring network drop, three requirements must be met: 1.) The drop connector must be voltage and current limited. (It also will need to be nonincendive equipment rated to connect to the tray cabled trunk network.) 2.) The field instrument connected to the drop connector must be approved as a nonincendive field wiring apparatus. (An Intrinsically Safe rated device will also qualify.) Be careful not to mistake a nonincendive equipment rating with a nonincendive field wiring apparatus rating. 3.) Parameters for the nonincendive field wiring drop connector and the associated apparatus must match (see Table 2, Field Instrument Components). Table 2. Tray Cabling with Nonincendive Wiring Drops and Associated Apparatus Installation Guidelines (Class I, Division 2) Field Enclosures Trunk: Nonincendive equipment with general-purpose enclosures. Drop leg: General-purpose equipment may be used. Field Wiring Trunk: Utilize PLTC or ITC concepts as described in Table 1. - Power supply must be suitable for PLTC and ITC concepts (see Table 1). Drop leg: Utilize nonincendive field wiring concepts that allow general-purpose wiring. Cable capacitance and inductance is negligible for 20-gauge wire and larger with lengths under 984 feet (300 m). Connections Trunk: Utilize standard connections suitable for PLTC and ITC (see Table 1). - Quick connectors may be used on trunk with limitations as specified in Table 1 for Class I and II, Division 2 locations. Drop leg: Current limiting and voltage-protected device must supply drop, which coordinates with maximum possible associated apparatus operating voltage. Short circuit current at maximum possible operating voltage must not be capable of igniting gas or dusts in the atmosphere. - Quick connectors may be used for nonincendive drops without limitation. 6

7 Table 2 (continued) Field Instrument Components Drop connectors must be rated as nonincendive equipment and have drop leg voltage and current limiting capability. Drop connector and field instrument (associated apparatus) parameters must coordinate as follows: Drop Connector Field Instrument Voc < Vmax Isc < Imax Field instrument capacitance and inductance must be limited. At 38VDC, allowable capacitance and inductance is as follows: Ci < 738 nf Li < mA Intrinsically safe apparatus may be used as nonincendive wiring apparatus. As a practical matter, since drop lengths are limited on bus networks, cable capacitance and inductance do not need to be considered. In Summary. Dramatic savings are now being realized by installing bus networked wiring architectures. In order to realize additional savings, installers should evaluate protection concepts used in their process environment. Tray cabling with nonincendive equipment suitable for a Division 2 hazardous area is simple to install, flexible, and the protection concepts related to ITC and PLTC are easy to understand and apply. When using this type of wiring architecture with a bus network, another 25% installation savings may typically result (see Figure 4). Figure 4. Total Savings Up To 65% 7

8 For further safety improvements and cost savings in Division 2 areas, consider the use of nonincendive field wiring and related apparatus for instrument drops from the bus network. Although this concept requires a more sophisticated understanding of code practice, the benefits far outweigh the investment in personnel training. Instrument suppliers are now providing equipment suited specifically for this bus networking protection concept. # # # For a more detailed description of these protection concepts with bus networks, contact StoneL toll free at for a free 2003 FieldLink Bus Networking Reference Guide. 8