WP DeltaV Grounding Options

DeltaV Whitepaper September 2010 – Page 1

DeltaV Grounding Options

DeltaV Grounding Options This document discusses the Power and Grounding changes that will be in an upcoming release of the Site Preparation and Design for DeltaV Digital Automation Systems manual (the Site Preparation manual). This information supplements the Site Preparation manual and provides additional options for design and installation of the control system ground.

Table of Contents Introduction ....................................................................................................... 2 Typical DeltaV AC Power and Grounding ........................................................ 2 AC Ground ...................................................................................................... 3 DC Ground ...................................................................................................... 5 System Power Isolation ................................................................................. 5 Floating AC Ground........................................................................................ 6 Summary ............................................................................................................ 6

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Introduction The Site Preparation manual provides recommendations for the installation of AC power distribution and system grounding that exceed those dictated by the NEC. The Site Preparation manual also provides recommendations of the installation of Isolation Transformers how they should be connected to the AC Ground. The AC power distribution and system grounding guidelines have raised questions because they exceed NEC guidelines and common installation practices and because they may result in additional installation costs. This paper discusses upcoming changes to the DeltaV Site Preparation manual intended to clarify these recommendations and to recognize an alternative solution for the grounding of signal shields. This paper should be used in conjunction with the current Site Preparation manual to help in the design of your control system’s power and grounding.

Typical DeltaV AC Power and Grounding DeltaV installation guidelines as documented in the Site Preparation Guide are based on a solidly grounded AC power distribution system. The recommended practices in the guide are based on years of proven solutions that provide excellent protection from electrical surges and transients, often found in industrial installations. The key feature of the recommended installation practice is that DeltaV connects the AC Ground to the dedicated Instrumentation Ground and as a result, recommends connecting signal shields to the AC Ground bars. This is often viewed as an unnecessary expense that conflicts with code. The following note from the manual should always be considered: Important Guidelines “This manual provides most of the information you need to prepare your site. It is believed that the guidelines and recommendations in this manual meet or exceed applicable government electrical codes and regulations. If differences occur between this manual and codes and regulations, codes and regulations take precedence. The guidelines and recommendations cover most situations. At some sites, specific peculiarities may require alternate approaches. If your site requires a different approach, contact your Emerson Process Management local business partner or field sales office (LBP/FSO).” It can be argued that the Site Preparation manual does not conflict with the electrical code, but it certainly goes beyond the code when it comes to handling EM noise and the grounding of signal shields. As a result, Emerson has re-evaluated the recommended installation practices to help customers and engineering consultants optimize the installation costs by aligning these practices with accepted industry practice.



AC Ground The intent of the AC ground bar as defined in the NEC Handbook (NFPA 70) in particular with respect to article 250, is generally to provide a connection for equipment grounding conductor(s) when AC power is brought into a metal enclosure. A bonded connector must be provided between the metal enclosure and the AC ground Bar. The AC ground bar must be connected back to the equipment grounding terminal bar located at the power distribution source. All metal equipment casings, panels, DIN rails and so on in a cabinet must be grounded appropriately in order to mitigate a short circuit of any AC feed and ensure a good ground path back to the source. Some grounding diagrams shown in the Site Preparation manual show the AC ground bar connected to the DeltaV instrumentation grid, and also connected back to the equipment grounding terminal bar. The purpose of this additional connection to the DeltaV instrumentation ground is to improve the noise dissipation capability of the AC ground by providing a low impedance, unique direct path to ground (true earth) for the higher frequency noise. This makes the DeltaV AC ground suitable for shield noise dissipation, isolating this noise from the DC ground reference. However, this additional connection adds cost to the site installation and is not required for purposes of proper AC grounding. According to the ISA grounding scheme and customer accepted practice, the AC ground should not be connected to the instrument ground.

Figure 1 - ISA recommended ground scheme

SHIELDS AND POWER SUPPLY 0 V DC TERMINATED TO EE DeltaV Whitepaper


September 2010 – Page 4

Without this additional connection to the dedicated low-Z instrument ground, the AC ground is no longer a suitable location for connection signal shields. As shown in the ISA grounding scheme, signal shields are therefore connected to the DC Ground for a low impedance direct path to ground. The following diagram shows a typical Site Preparation manual diagram with the AC ground connection to the DeltaV instrumentation ground removed. NEUTRAL SHOT WITH GROUND

ENCLOSURE GROUNDED

POWER SUPPLY GND TERMINATED TO PE ISOLATION IS MUST

EE

DIN RAIL GROUNDED

IE

Figure 2 - Modified DeltaV Grounding scheme *EE: ELECTRICAL EARTH / POWER EARTH *IE: INSTRUMENTATION EARTH / ELECTRONICS EARTH Notice that in figure 2, the shield bars are connected to the DC Gnd Bus bars, along with the DC Common of the 24 VDC Bulk power supplies. The Site Preperation manual and DeltaV installation manuals will provide more details on the DeltaV equipment installation. The point here is that the DC ground system is completely isolated from the AC ground and used isolated bus bars all the way to the DeltaV instrumentation ground.



To avoid further confusion on this topic and to help customers manage the cost of their installations, the Site Preperation manual will be updated to remove the specific instructions to connect the AC ground bars to the DeltaV instrumentation ground. To support this, the manual will instruct users to connect DeltaV shield bars and all control signal shields to the DC ground reference. The update will effectively remove specific DeltaV requirements to the AC power and grounding system, allowing this installation to be governed solely by the appropriate local electrical code.

DC Ground With respect to the DeltaV instrument ground and DC ground reference bars, there is little change to be made to the Site Preparation manual other than to indicate that shield bars and signal shields should be connected directly to the DC ground reference. It is always important to note that the quality of the DC ground reference has direct impact on the availability of any control system, not just DeltaV systems.

System Power Isolation Isolation transformers are frequently referred to in the Site Preparation manual. Are they really needed? After all, the control system components are powered by AC/DC bulk power supplies that provide 1500 VDC of isolation from the AC power. When connecting the AC ground to the dedicated instrument ground, it is extremely important to isolate the AC ground system from undue noise and transients eminating from other process equipment, such as variable speed drives, welders, commutated motors, and other major noise culprits. The isolation tranformer protects the quality of the instrument ground and is highly recommended. But if the AC ground is not connected to the instrument ground, is the isolation tranformer still needed? There are other AC powered devices in the control system such as workstation computers, servers and network communication gear, which are sensitive to dirty AC power and grounding. It is important to protect these devices from transient noise eminating from heavy process equipment. The isolation transformer is an effective solution to dirty AC power and can help ensure reliable operation of these devices. It is much less expensive to install an isolation transformer during site preparation than to add it during plant start up or after a period of troubled operation. The decision to install the transformer must be made after careful consideration of the the site-specific conditions. It is considered by many to be a sound investment. The need for an Isolation transformer is a site-specific issue and is dependent on many factors. It is likely more difficult to justify not installing this device, since it is a good insurance measure to achieving stable operation of electronic devices. Ultimately, it is a decision for the electrical engineering team and not a requirement for the system,



Floating AC Ground Removing the connection of AC ground to the dedicated DeltaV instrument ground also changes the way in which the AC power distribution system must be grounded. The AC power and grounding system can now be designed with floating or high-resistive grounds, in accordance with applicable electrical code. The DeltaV DC power must remain solidly grounded as detailed in the Site Preparation manual. The controller and I/O components are certified based on the prescribed grounding of the equipment. Note that DeltaV AC discrete I/O products are tested and certified for use with solidly grounded AC systems and should not be used on a floating or high-resistive ground. Isolated AC channels are permitted. The Bulk Power supplies are able to provide up to 1500 VDC isolation from the AC power, and they must be installed per the manufacturers’ instructions. The end result is that the AC Power and Grouding is entirely governed by the applicable codes and regulations and is independent of the DeltaV DC power requirements.

Summary The primary change coming to the Site Preparation manual is the location of signal shield connection. By shifting the shields to the DeltaV DC ground reference and removing the connection for the AC ground bar to the DeltaV instrumentation ground, the conflict with accepted industry practice is eliminated and AC power and grounding systems can be designed strictly per applicable code. These changes have the potential to reduce the cost of the power and grounding installation by simplifying the AC ground requirements. The new version of the guide will make this much clearer.

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WP DeltaV Grounding Options

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