FP-1000 Lightning Protection

[bellis]

I've got a setup involving Type K thermocouple wires attached to TC-120's which are then connected to a FP-1000, which is then connected back to a computer via a RS-232 cable. The thermocouple wires are run from a weather proof housing out to a large strip of Asphalt, in which they are embeded.

This being Florida, we get large numbers of lightning strikes. Unfortunately these striks are finding there way up the thermocouple wires through the TC-120's and into the FP-1000.

The strange thing is that the TC-120's are not damaged at all by this, all the damage occurs to the FP-1000 and the attached computer. Specificly, it seems like the serial port controller chip on both ends has been fried.

After going through abo
ut 3 FP-1000's and PCI Serial cards, we got an optical isolator for the serial cable. This worked for the computer, but FP-1000 module was still damaged.

What would be the best way to prevent this? Is there some way we can ground the TC-120's/FP-1000 so these spikes have somewhere to go? Also, why would the TC-120's not recieve any damage from this, since the spike is traveling through them on the way to the FP-1000?

Thanks,
Brian Ellis

[Aaron_G]

Brian,

There are a number of theories that we can put forth regarding the causes or modes of failures that you are seeing during the lighting strikes.

Let us first start and explain why both the FP-1000 and computer are damaged. RS-232 ports by their definition share a ground as part of the wiring. There typically is no isolation on RS-232 so a high-voltage event will, in most cases, effect both devices that are communicating through RS-232. This is reinforced by the fact that the computer no longer suffers damage when an isolater is placed on the RS-232 line.

Now we have to decide why is the FP-1000 getting damaged when the FP-TC-120 is not. You ask "Why are the FP-TC-120's not being damaged when the spike is travelling through them on the way to the FP-1000?" Well, the answer is probably that the spike is not travelling through them on the way to the FP-1000. In all likelihood the lightning surge is not being transmitted through the FP-TC-120 as a surge through the FP-TC-120 that damages the network module should not leave the FP-TC-120 intact. Depending upon the type of thermocouples you are using and the method of transmission of the lighting surge along the thermocouple we can think about several possible transmission paths.

The first possibility depends upon your wiring. Are the V&C terminals on the FP-1000 wired ("cascaded") to the V&C terminals on the FP-TB-1/2/3 that the FP-TC-120 is in? If the V&C or either of the terminals is cascaded, the isolation barrier between the input signals of the FP-TC-120 is bypassed. This would allow a surge signal coming in via the thermocouple to skip the isolation barrier, and instead go straight into the power suppy of the FP-1000. A very simply solution is to simply remove the cascade wiring as the FP-TC-120 does not require an external supply for it's I/O signals.

Alternatively, if the thermocouples are shielded thermocouples, and you are wiring the shields to a case that the entire system is in, or to a ground that is linked to the power supply being used by the FP-1000, we could theorize that the power supply is picking up the surge and transmitting the surge through into the FP-1000, once again bypassing the isolation barrier that is in the FP-TC-120. Likewise, if a thermocouple is shorting against the weatherproof housing (assuming it is a non-grounded metallic box), we could see the box being electrified and possibly effecting the power supply if the power supply is grounded to the enclosure (which is not grounded).

Regards,
Aaron