Analog line protection

A panel specification I am working with calls for lightning/surge protection and fused terminal blocks on all PLC analog lines. What is the better design practice? Placing the fused terminal block in between the analog modules and the surge device or the surge device between the analog module and the fused terminals? The consulting engineers PIDs are not specific.
This is for a water treatment plant in Florida. The analog input lines will connect to field mounted differential pressure transmitters measuring flow through orifice plates. The analog output lines will connect to modulating valves.

Seems to me it could get costly if you use protectors on each channel from analog modules not to mention that most analog devices work on 1-5, 1-10, 0-20ms or 4-20ma so if isolated what surges are you protecting them from? Come to think of what do you use?

I can see using the fused terminals per channel but I think I would just surge protect the panel itself and/or isolate the power for the analog (maybe with something like a UPS).

Seems to me it could get costly if you use protectors on each channel from analog modules not to mention that most analog devices work on 1-5, 1-10, 0-20ms or 4-20ma so if isolated what surges are you protecting them from? Come to think of what do you use?

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I think Godfey's issue is that he has field devices that are really in the field. His field devices could be in close proximity to a lighning strike. In that case the voltage surge would enter the panel directly through the analog line as opposed to through the machine supply. Please correct me if this is an incorrect assumption.

I don't know that is matters what order the devices go in. The analog outputs should be current limited so even if the surge arrestor fails shorted you won't cook the output. However, if I had to pick I would put the fuse between the analog I/O and the surge arrestors. This is based on personal preference more than anything and has no design backing to it.

I am assuming you are using analog I/O that has a reasonable amount of inherent common mode isolation. If not it might get a little tough to find surge arrestors that will catch a surge but not shunt your actual signal to ground. Also, you may not be able you proviide direct differential mode protection. I don't think a zener diode is robust enough the handle the surge energy. An I think the reverse breakdown of an avalanche diode is too high to protect you in differential mode.

I'm certainly not an expert in surge arresting so if someone contradicts this I would go with the other suggestion.

Keith

This is for protection from frequent lightning strikes in Florida. The field devices are 100-200 feet from the cabinet. Yes they are costly at $60.00 x 40 pieces but there was money in the estimate to cover it. I am using differential inputs and outputs. The lightning protection is the DIN rail mounted DL 180 series from Citel. http://www.wici.com/products/model/citel.htm They feature 3 element gas tubes with solid state electronics. The response time is < 1 ns.

Ok that makes sense, not being use to having devices "in the field" I wasn't thinking about those conditions.

BTW I don't think it matters either which way you do it but in a situation like this I agree that I would probably do the same as was stated...put the surge between the fuse and device. I can not say why but it seems to be the logical place, if it fails (shorts) then fuse should blow and you would know its defective. Just a thought.

Water and wastewater treatment facilities are very prone to surges and lightning strikes. I normally put the fuse between the field device and the surge arrester, but I'm not sure it makes much difference. If you look at the diagrams many surge protecttors area actually in parallel with the load anyway, and all rely on fast repsonse dumping the surge to ground to protect the expensive stuff. The response time of most surge protectors is usually much faster than a circuit breaker or fuse.

Phoenix contact and others make some protection devices built into terminal strips, and some even have a disconnect or fuse built in.