How does a lightning protection device work in a transmitter? Is this device integral to the transmitter or can it be purchased separately?

 

Lightning/surge protection devices generally protect instrumentation against secondary effects of lightning -- resistive, inductive, or capacitive coupling that occurs in electronics or cabling as a result of lightning strikes nearby. These devices cannot protect from a direct lightning strike. These devices are typically referred to as "surge" protection, rather than lightning protect, since surges can occur from many other sources besides lightning (although lightning is certainly the most spectacular).

While surge devices are designed to attempt to handle the three possibly effects of surges, one size does not fit all applications -- you really need to have someone who understands surges to choose what surge protection device should go where. These devices typically use a combination of three different components to quickly divert and minimize the surge felt by the instrument. These include gas discharge tubes, metal oxide varistors, and high-speed clamping diodes.

Gas discharges tubes (GDT's) can handle very high surge currents, but are relatively slow to start and can thus let through a lot of surge before they operate. Metal oxide varistors (MOV's) can handle fairly high surge currents, but their clamping voltage rises the more surge current that passes through. High speed suppression diodes can only handle relatively small surge currents, but they do have very accurate and rapid voltage clamping performance.

While some instrumentation manufacturer's do put lightning protection in their units, it usually isn't the same quality as the protection built into separate surge devices. In addition, you don't want to have a situation where the surge exceeds the capabilities of the instrument's surge protection and causes the instrument to fail -- you are much better off financially to have the separate surge protector fail and you can then replace it for $100 or so.

Many manufacturers make these separate devices. One manufacturer I've worked with extensively is MTL. You can learn more about these by checking out www.mtl-inst.com and clicking on the "Telematic" button.

Shane Filer
Neal Systems
215-968-7577

 

Ultimately *ALL* lightning protection works as a by-pass. Lightning damage is caused when a high voltage potential passes through your device on its way to a lower voltage potential. While you cannot really "stop" this current flow, you can help provide a lower impedence (ie: better in the eyes of a surge) by-pass for it that DOESN'T include your device as "expensive fuse".

In general a transmitter is a 3-ground device - there is 1) the case/chassis, 2) the sensor interface to the process/pipe, and 3) the 4-20mA
of FieldBus link back to a remote device. A surge can occur between any 2 of these 3. Surges between #1 & #2 are prevented by creating a direct link between them external to your product and using a sensor with at least a minor amount of isolation. While the metal case & metal pipe often are indirectly connected "somewhere", you may want to do explicit bonding design by wire at each transmitter to make sure the impendence is the lowest possible.

Assuming potential at #1 = #2 because of good bonding & sensor isolation, this just leaves a surge between combined #1+#2 to #3 - a surge to/from the case/process through your transmitter to the data comm signal wire. Many vendors sell small 4-20mA or Hart or FF-H1 compatible LPU that likely bolt onto your case (as surge ground) & link to both sides of the data wire.

99.9% of the time, "surge protection" built into a product is 400-1500watt "spike" protection that by-passes small energy high-voltage spikes, but
cannot handle lightning. Common Lightning ratings are in the 20,000 Amp range for very short times.

Take a look at http://www.robustdc.com/library/PDFs/an018.pdf for some beginner's level diagrams and explanation.

Best Regards

Lynn August Linse
IA Firmware Specialist, Digi Int'l (www.digi.com)

 

> Ultimately *ALL* lightning protection works as a by-pass. ...<

please visit emersonprocess.com/rosemount. you will get your answer in white paper.













high voltage potential passes through your device on its way to a lower voltage potential. While you cannot really "stop" this current flow, you can help provide a lower impedence (ie: better in the eyes of a surge) by-pass for it that DOESN'T include your device as "expensive fuse".
>
> In general a transmitter is a 3-ground device - there is 1) the case/chassis, 2) the sensor interface to the process/pipe, and 3) the 4-20mA
> of FieldBus link back to a remote device. A surge can occur between any 2 of these 3. Surges between #1 & #2 are prevented by creating a direct link between them external to your product and using a sensor with at least a minor amount of isolation. While the metal case & metal pipe often are indirectly connected "somewhere", you may want to do explicit bonding design by wire at each transmitter to make sure the impendence is the lowest possible.
>
> Assuming potential at #1 = #2 because of good bonding & sensor isolation, this just leaves a surge between combined #1+#2 to #3 - a surge to/from the case/process through your transmitter to the data comm signal wire. Many vendors sell small 4-20mA or Hart or FF-H1 compatible LPU that likely bolt onto your case (as surge ground) & link to both sides of the data wire.
>
> 99.9% of the time, "surge protection" built into a product is 400-1500watt "spike" protection that by-passes small energy high-voltage spikes, but
> cannot handle lightning. Common Lightning ratings are in the 20,000 Amp range for very short times.
>
> Take a look at http://www.robustdc.com/library/PDFs/an018.pdf for some beginner's level diagrams and explanation.
>
> Best Regards
>
> Lynn August Linse
> IA Firmware Specialist, Digi Int'l (www.digi.com)

 

Surge/lightning protection of transmitters based on 4-20 mA/HART/Fieldbus/Profibus-PA is one of the topics covered in chapter 3 of the book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance" (buy online in hardcopy or download immediately in softcopy). If your email does not support this hyperlink feature correctly, please copy the URL and paste it into your Internet browser. Mind the line wrap, make sure to get the complete path all the way to the 3036:
http://www.isa.org/Template.cfm?Section=Shop_ISA&Template=/Ecommerce/Product
Display.cfm&ProductID=3036

Jonas Berge
==================
[email protected]
www.smar.com