Protecting Honeywell ST3000 Field Transmitters Against Lightning Induced Damage - Pressure, Level, Flow Measurement in All Industries

Protecting Honeywell ST3000 Field Transmitters Against Lightning Induced Damage - Pressure, level, flow measurement in all industries

Problem: How to protect field Lightning Protection Solutions for transmitters from damage induced by ST3000Transmitters: lightning or power surges. Honeywell’s ST3000 transmitters can Lightning is a very fast, high electrical be supplied with a ‘Lightning Protection’ energy transfer between earth and the sky. The option, or more correctly Surge Protector. lightning strike seeks out the lowest impedance Basically, this consists of adding surge path and normally strikes tall trees, power protectors at the entry point of the 2-wire pylons or lightning rods on buildings or power connection, which is normally passive, structures. The likelihood of a direct strike on but acts quickly to prevent excessive energy filed mounted transmitters is normally very from being passed onto the electronic low, but if the strike to ground is in the components. The ‘Lightning Protection” proximity of the transmitter wiring, then the option is part of the terminal block. The high electrical field strengths involved are Lighting Protection (LP) option terminal likely to induce high electrical surges in the block is clearly identifiable by its RED color. wiring. Equipment at both ends of the wiring When this option is ordered, the components will be exposed to the lightning induced surge. in the figure below are added to the transmitter:

Description of the Lightning Lightning Strike From Phenomena: Field Wiring 22 Ohms 300 v T2 Lightning strikes can vary widely in 4 - 20 mA Loop To ST3000 total energy contained and in duration. When 22 Ohms this is converted to an induced strike to local Note: Simpliufied Schematic Diagram transmitter 2-wires, it changes into a lower energy but increases in duration. There is no The spark gap tube T1 will fire and ‘typical’ lightning strike but data suggests a conduct to ground any voltage appearing ‘typical’ induced strike as having a peak across the input terminals in excess of 300 amplitude of 800 Volts, a rise time of 15 to volts. The time the tube takes to fire is less ∝ 600 ∝Sec and a decay time of 40 to 3000 than 1 Sec. For the short time before the tube fires, the standard surge protector ∝Sec as shown: Transob (T2) limits the voltage to which the Volt main electronics are exposed. The two resistors ensure that the Transorb (T2) is not 800 ‘blasted’ by excessive energy during this initial time. 400

0 0 500 uSec 'Typical' Induced Strike

- 1 - The rated specifications of the surge The resistance of the ground point protection cater to the wide variety of forms should be 1Ω or less in order to make the that the induced surge can appear as. It lightning protection effective. The Lightning covers two different wave forms A and B, Protection (LP) option is included in the each with two different peak currents various safety approvals/certificates that amplitudes and number of repeated strikes apply to the transmitter. that it is required to withstand. The diagram below shows some Other Equipment typical waveforms that correspond to Impulse It should be sated that separate surge Discharge Current and the number of protection of the receiving instruments is repeated strikes: required if they also are prone to damage from electrical surges. Several suppliers (e.g. MTL) mp mp Waveform A Waveform B have suitable protection devices, packaged like safety barriers, which can be installed at the Decay to 50% of Peak Decay to 50% of Peak cable termination area for the receiving instruments. In addition, the Total Plant 0 0 0100010 uSec 0208 uSec Solution (TPS) have Field Termination Impulse Discharge Current (Amp) Number of Operations Impulse Discharge Current (Amp) Number of Operations 200 300 10,000 1 3,000 10 Assemblies (FTA) that has built-in surge protectors for transmitters operating in DE Installation Considerations: mode. The detailed installation of the Lightning Protection Option is covered in the Upgrading the ST3000 to Include ST3000 Installation Guide. In the event that Lightning Protection an optional Smart Indicating meter is used in combination with the ST3000 transmitter, the If the original transmitter did not surge protector also gives protection to the include the Lightning Protection (LP) option meter. and it is later considered desirable to add it, the It is very important that an external protection can be easily added on. A ground wire is connected between the conversion kit is available for all ST3000 series transmitter housing and a secure local 900 and 100e transmitters. This kit includes the grounding point. This connection should be mounting screws, cover and terminal block. as short and as direct as possible to ensure a The part number for this kit is : 51197425-002. low impedance discharge path.

Description Model ST3000 Transmitter with integral Lightening STD924-E1G-00000-1C,LP,MB Protection option, mounting bracket, FM approval

Lighting Protection Kit 51197425-002

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