Lightning strike damage to flow meter

[gtwy]

Hopefully this is the right forum-I have a client (municipal) who serves domestic water to an established appartment complex. I was called to look at a magnetic flow transmitter, installed eight yrs. ago, within the complex with regard to continual lightning damage of the flowmeter electronics, according to the city electrician 1 or 2 times a yr over the last 3-4 yrs.
The meter is located in an underground vault, pvc pipe outside the vault, cast iron within the vault. The water line runs adjacent to a road running thru the complex. The electroncs for the meter are in a polycarbonate box 6' above grade. A 20a 120v ciruit is fed from a building on the other side of the street underground to the enclosure.
Examining a couple of "fried" circuit boards (their mounted mounted above grade), the damage is near the connector that feeds low v D.C. to the coils on the meter, not near, circuit wise, to the 120v input to the board. Also damage to the terminal strip at the underground portion of the meter.
I'm wondering now if the damage may not be entering from the utility power- could it somehow be propagating from the vault? There is no 120v power at the uderground portion of the meter.
The surroundings of the above-grade electroncs seemed tame enough- an 8 foot or so berm behind the installation and a couple of 15 foot or so trees within 20 feet.

 

[MikeHalloran]

What with the PC box, the above grade electronics would be more protected, just nailed to a tree.



Mike Halloran
Pembroke Pines, FL, USA

 

[gtwy]

Yes, the installation isnt perfect, they have several similiar installations in the area that have never been damaged. The equipment grounding means at the enclosure are marginal- ground rod connected to the terminal strip w/ the rest of the equipment grounds and an inadequate receptacle type surge arrestor for the 120 v supply that has never been damaged.
It may very well be that the surge is entering from the top, but looking at the meter and board, it's surprising that the surge arrestor, MOV's on the board etc haven't ever been damaged, yet there is terminal strip damage in the vault and circuit damage not near the 120v input. The meter still functions, just the electronics that they're relacing. Lighting does crazy things.

 

[Marmite]

Sounds to me like there may be more of an issue with ground fault current rather than lightning, causing a potential difference between the underground part of the meter and the electronics in the box fed from a separately grounded supply on the other side of the street.
Regards
Marmite

 

[gtwy]

The city electrician will be analyzing the grounding system at the 120 v feeder panel on the other side of the street w/ the local utility co, as well as revising the grounding system at the flowmeter. Apparently when the circuit board blows, it's always after a thunderstorm. The electrician had installed a ground rod in the vault and tied the meter/cast iron piping to it.

 

[badservo]

I agree with Marmite, sounds like a potential difference. Is the ground rod in the vault that is tied the meter/cast iron piping also connected to the ground conductor of the 120 v feeder panel on the other side of the street ? Perhaps a 120v isolation transformer at the meter control board would help.

 

[gtwy]

No, but that was one of the recommendations to fix. The ECG run with the feeder conductors (12 awg)is tied to a term. strip in the above ground enclosure. There is a ground rod connected to that strip, and a seperate rod in the vault. They installed a new rod above grade and will put in a lightning arrestor on the supply.
When you say a potential difference, are you refering to a potetial present not associated with lightning? The damage indicates a substantial energy surge- some components near the connector going to the meter blown off.

 

[badservo]

I was thinking potential with lightning. It still sounds like a possibility, connecting the vault ground to the enclosure ground might help reduce any potential durring a lightning event.