Over Voltage Surge Protection on FF Trunks

[spragujl]

Last week we had our first ever FF wiring block failure (Relcom Megablock) which tripped the crude unit at our Jeddah Refinery. The feed flow control to the crude unit was on the failed segment. Failure analysis showed the Megablock failed due to over-voltage surge on the trunk which blew the internal surge protector and shorted the FF + / - trunk conductors. This incident was not caused by lightning, and we don't typically install external lightning/surge protection on either our conventional or FF field wiring here in Saudi - since we've never really had lightning or surge problems. But we are now questioning that logic.

So, my question to the FF community is: Do any of you install lighting/surge protection on your FF field wiring? Do you install it because of frequent lightning strikes - or as a standard practice? What are the downsides of using surge protection - in terms of cost, maintenance, complexity?

Thanks for any feedback - Jim Sprague, Aramco

[rezabejd]

Jim,

We have lightning, but have no surge protection on the segments. Most devices have "transient protection" option specified. We do not use Mega-blocks, but we do use the "pluggable" over-current protection on many (but not all) spurs . . .

Maybe we need to be thinking harder about the degree to which sophisticated termination appliances add additional failure modes.

[Peter]

Jim,

We have lightning, but have no surge protection on the segments. Most devices have "transient protection" option specified. We do not use Mega-blocks, but we do use the "pluggable" over-current protection on many (but not all) spurs . . .

Maybe we need to be thinking harder about the degree to which sophisticated termination appliances add additional failure modes.

It is neccessary to consider the benefits against risks of failure. If, for example, a low-cost Relcom Megablock is used without the Spurguard option, one spur short could pull down the whole segment. Spurguards limit the current per spur and so prevent this from happening. In lightning prone areas, surge protection should be seriously considered. In the northern parts of Australia, surge protection is considered mandatory by most users. Many other users are starting to see that there are other sources of potentially damaging transients - electric motors, welding, switching noises etc. Effects are usually cumulative, resulting in inexplicable failures. Independent research in Europe attributes 28% of device failure to transient voltages. This is preventable by careful selection and installation of surge protection devices.

Peter Inns, MTL Australia

[Mike ONeill]

Most fieldbus wiring blocks incorporate some measure of spur short-circuit protection and usually something to prevent inadvertant damage on initial connection - we use transient suppression diodes, other use MOVs. In all cases, these are not 'serious' surge protection networks, which are multi-stage combinations of Gas Discharge Tubes (deals with the high current), Transient Suppression Diodes (tries to clamp the let-through voltage while the GDT wakes up) and some sort of series impedance (generates the voltage to get the GDT to fire). Surge suppression products are available as external accessories from most suppliers.
Of course, surge protection doesn't cover accidental mains connection (yes, it does happen) and they will ALL fail on continuous over-loads from whatever source.
If the ultimate protection is required, there is a configuration available from MooreHawke which allows duplicated Trunks to be connected to a field device coupler, and these trunks can each be wired through a series fuse using a regular fuse terminal block. This fuse will 'blow' if that trunk is exposed to a continuous overload or accidental conection to a high power source, BUT the internal TVS will protect the wiring block electronics and the other trunk will continue to operate normally, so the segment will not shutdown. This is a unique feature and recommended for the highest possible security on a critical segment.
This fault-tolerant segment design is only available in FF; yet another reason why FF is the worlds favorite fieldbus.