Lightning strikes damaging AI modules

[RCC1]

I haven't seen it since 24 VDC is generally not considered dangerous and isolation is usually desired, but my experience is somewhat limited. Would a ground connection to the 24 VDC common help or hurt in this case?

I would check you schematics, back 20 years ago the 30 volt rail on the DCS systems were made from a -15 and + 15 volt power supplies. The center was a common lead.

 

[EC Dan]

Quick update on this: talked with Phoenix Contact again and their response was as follows:

After speaking with one of our lead marketing engineers about your situation, we’d recommend trying surge protection device - TTC-6-1X2-F-M-24DC-UT-I – 2906764, at least on your analog input channels. This product has a lower voltage threshold and should therefore clamp sooner on transient voltages. This ought to be helpful given that your analog channels are particularly sensitive. If you still have input channels fail with this device, it’s possible you have some sort of disturbance on your ground circuit.

PHOENIX CONTACT | Surge protection device - TTC-6-1X2-F-M-24DC-UT-I - 2906764

In regards to the status indicator on these surge protectors, I was informed that it more likely to be triggered by an overload event rather than a true surge event. You can see the written response I received below.

The TTC-6-2X1-F-M-24DC-UT-I (2906767) SPD is equipped with an "overload" status indicator that operates Independently of the FMRS set. An overload status indicator alerts equipment users when the suppressor is damaged by a sustained input power source voltage that exceeds its maximum operation voltage value.

If, for example, 120 VAC was mistakenly applied to the TTC-6-2X1-F-M-24DC-UT-I that operates on 24 VDC, it would be inappropriately driven into continuous current conduction and likely suffer damage. Its overload protection circuits would engage and electrically remove the suppressor from the circuit as it transitioned to its end of life in a controlled and safe manner.

Basically I was recommended the wrong part back in 2019, which is more designed to work with two digital signals with a common reference rather than a 4-20 mA loop signal. The difference is the TVS diode links the + and - of the 4-20 mA loop rather than linking both signal circuits to the common reference:



Wish I had done much due diligence back then instead of investing thousands on the wrong part Regardless, they did work for a while. I will cycle these new ones in over time. Also surprised to find out the visual indicator doesn't actually pop on a lightning arrest but rather a misapplied voltage.