Transcript
Application Note
Surge Protection
FOUNDATION™ Fieldbus
FOUNDATION™ Fieldbus network segments can be protected from surges caused by near-lightning strikes by: • Using shielded Fieldbus cable • Putting the cable into conduit or using grounded armored cable • Using surge protectors • Using transient voltage suppressors • Using Fieldbus devices that are built to the Fieldbus standard Relcom's FCS (Fieldbus Connection System) wiring blocks incorporate surge protectors (in the isolated terminator block, FCS-TI) and transient voltage suppressors (in all terminator, spur, and expander blocks) to provide this protection.
Background Lightning is a problem in electronic process control systems because many processes are outdoors and in areas where lightning is frequent. Lightning is especially problematic for Fieldbus networks because, by definition, Fieldbus is out in the field. While there is no remedy for direct lightning strikes, because the energies involved are too high, measures can be taken to minimize the effects of near-lightning strikes on Fieldbus networks. Lightning is a short burst of very intense voltage and current. Even when a direct lightning strike is shunted to ground by a lightning rod, it induces high voltages and currents in metal components near the hit. This also applies to the metal in the conductors and shield of Fieldbus cable. The purpose of lightning surge protection in Fieldbus wiring is to minimize the induced voltages and currents so that they do not damage Fieldbus equipment. Simply stated, voltages are transferred from one metal object to another by capacitance and currents are transferred from one metal object to another by mutual inductance. This transfer is not 100% efficient, but results in substantial voltages and current nonetheless. For example, if the lightning strike itself produces 100,000 Amperes in the lightning rod, the induced currents in nearby grounded metal objects may be several thousand Amperes. Similarly, if a shielded Fieldbus cable is in a conduit or a cable tray, the induced current in the conduit of several thousand Amperes induces a current in the shield of perhaps several hundred Amperes. While it is beyond the scope of this application note to precisely characterize the exact inductive and capacitative coupling mechanisms, the point is that the effects of near-lightning hits can be progressively minimized.
Solutions One way to minimize the effects of near lightning, already pointed out above, is to put shielded Fieldbus cable into conduit or to use armored cable where the armor is well grounded. This minimizes the current induced into the Fieldbus cable shield. The shield of the Fieldbus cable is grounded, but according to the standard, in only one place. This single-point shield grounding is good because it helps to eliminate current loops in the shield that introduce noise in the information carried on the twisted pair wires inside the shield. Normally, the shield is grounded on the control room side of the network. This means that a near-lightning strike in the field could produce a large current in the shield that needs to travel to the control room to be grounded. If the network is long, this result in a large voltage on the shield. The voltage on the shield, in turn, capacitively couples the large voltage onto the twisted pair cables inside. A way to minimize the large voltage surge is to use a surge suppressor.
Shield
Twisted-pair Wire Surge Suppressor
The surge suppressor (also referred to as a surge protector or gas discharge tube) is a small (1-cm diameter) gas-filled tube with electrodes at each end. When connected between the cable shield and ground, the surge suppressor functions in the following way: When the voltage across the electrodes is below a given value, in this case 75 Volts, the gas in the tube does not conduct. Under normal circumstances this means that the shield of the cable is not connected to ground and there is no ground loop. In case of a near-lightning strike, where the voltage on the shield exceeds 75 Volts, the gas in the surge suppressor ionizes, becomes conductive, and forms a very low resistance path to ground. Gas discharge tubes are rated to 10,000 Amperes and can handle the induced currents. Once the nearlightning surge is over, the tube turns off, becomes very high impedance again, and is ready for the next surge.
2221 Yew Street Forest Grove, Oregon 97116 USA (503) 357-5607 or (800) 382-3765 Fax (503) 357-0491 www.relcominc.com v1.02 99.09.17
Solutions (continued)
Summary
The next step in progressive surge protection is to minimize the voltage that can be induced on the twisted-pair cable inside the shield. There is capacitance between the shield and each of the wires in the twisted pair simply due to the way cable is constructed. A voltage change on the shield will induce a voltage on the wires. Normally, the shieldto-wire capacitances for the two wires are nearly equal so that the voltage on both of the wires in the twisted pair will be the same. This is called common mode voltage. We know from the discussion above that the common mode voltage is not going to be much greater than 75 Volts because of the surge suppressor. The Fieldbus standard requires that the common mode immunity of Fieldbus devices should be several hundred Volts so there should be no problems with devices that meet the standard's requirements (another good reason to choose field devices that conform to the standard).
Although Fieldbus network segments cannot be protected from damage or failure due to direct lightning strikes, they can be progressively protected from surges caused by near-lightning strikes by:
If, however, the shield-to-wire capacitances are not equal, the surge voltage on the shield induces a different voltage on each wire. This is called differential-mode voltage. The Fieldbus standard requires that devices should be immune to up to 35 Volts of differential voltage. However, the twisted pair wires themselves can carry up to 32 Volts to power the devices. This means that another 4 Volts of near-lightning induced voltage could damage a device. This can be prevented by using a Transient Voltage Suppressor between the two wires of the twisted pair. These devices are specifically made for transient suppression. Normally, they do not conduct, but when a given voltage is exceeded, they can absorb large amounts of power. The following diagram illustrates the use of a transient voltage suppressor.
• Using shielded Fieldbus cable • Putting the cable into conduit or using grounded armored cable • Using surge protectors • Using transient voltage suppressors • Using Fieldbus devices that are built to the Fieldbus standard Relcom's FCS wiring blocks offer both integrated surge protection and transient voltage suppression. Specifically, the Isolated Terminator, FCS-TI, has both the Surge Protector and Transient Voltage Suppressor built in. For effective surge suppression, the surge ground stud on the FCS-TI block needs to be connected to a solid ground. All Spur and Expander blocks, FCS-S and FCS-E, have built-in Transient Voltage Suppressors. Information about specific Relcom products can be found at the Relcom Inc. web site, www.relcominc.com.
Shield-to-Wire Capacitance Differential voltage Twisted-pair Wire Shield Transient Voltage Suppressor Surge Suppressor
Common-mode voltage
2221 Yew Street Forest Grove, Oregon 97116 USA (503) 357-5607 or (800) 382-3765 Fax (503) 357-0491 www.relcominc.com v1.02 99.09.17
