Possible Bonding/Grounding Problem, Cathodic Protection

[DM2-Inc]

We're having a problem with electrical interference on a fire alarm system.

We have a Fire Alarm Control Panel installed in a building, where the conduit is run underground to a concrete pad that's elevated above grade about 4', on concrete pillars. On the pad, we've got a stainless steel junction box that's not painted, and is mounted on galvanized, unpainted steel columns.

When we meter from the steel structure to earth surrounding the columns, we're seeing about 0.6 Volts DC. When the circuit is installed in the conduit, it causes problems with the FACP. Outside of the conduit it's fine. The conduit is fastened to the galvanized steel structure very well. In looking at the structure however we don't see any bonding straps on the steel, or any connections to a ground rod.

The electrical engineer advised that they have cathodic protection in the plant with 12 VDC. I believe that because the steel on the pad is not properly grounded and that's what's giving us fits.

A conduit for the light fixture comes up from the ground, out of a trench, and i'm told some portion of this rigid conduit is in contact with earth before it comes in contact with the pad. I haven't see where the cathodic protection is installed, if at all on this pad (outdoor pad), but if it is being used there, I can't see how it would work if there's no return path to earth.

Can someone enlighten me?

 

[Jeffro]

As I understand, you are getting the voltage on the conduit due to the fact that it is bonded to the structure being protected by the cathodic protection?
I work with cathodic protection on large water pipe lines. We maintain a vlotage of -.850 to -1.5 VDC and that seems to be the standard for underground protection (on steel). Being that its galvanized, I would think that it would not need as much (less negative). Im not sure how you are measuring the voltage or at what point. You may want to try using a referance electrode used for measuring voltages thru the soil such as http://www.mcmiller.com/IonxPortable.aspx
To be short, Im not sure exactly what the problem is, but am very interested to know. I would *think* that the voltage being applied to the structure is higher than normal and also being bonded/in direct contact with the conduit may be applying some sort of interferance problem.

 

[DM2-Inc]

We measured the voltage at a steel water pipe, attached to the structure, and that changed to some type of plastic pipe before entering the ground. The measurement was simply with a Fluke Volt meter with one probe on the pipe and the other simply shoved into the ground.

The voltage level, some 0.6 or close to that, isn't the issue but rather that it is somehow getting onto the fire alarm circuit. Imaging a platform 30' x 30' with a junction box at one end and addressable modules mounted in the box. The modules are potted. The circuit off of the skid to the control room is run down the concrete columns in rigid and then transitions to PVC. This is the addressable side of the circuit (SLC).

From the module in the junction box, the cable runs in rigid conduit to the other end of the skid where it monitors a fire water flow switch, and valve tamper switch. This is the "Non-Addressable" side of the module (IDC).

We placed one Capacitor on the (+) side of the SLC and ran it to ground, and did the same with the (-) side, but that didn't solve the problem. Considering that the technician reported that he didn't see and bonding straps on the steel structure, a ground rod, or and bonding strap on the concrete, I had my doubts that this capacitor would solve any problems since it would sink any voltage to a ground that really didn't exist.

We moved the addressable module out of the junction box at the corner of the platform, and relocated it to a GUAT fitting within 3' of the switch that the module is monitoring. The cable between the junction box, and the addressable module in the GUAT fitting is all shielded with the drain wire tied all the way back to the control panel. This solved the issue with the fire panel.

I don't think that it addresses the boding issue on the platform however. While the panel doesn't show a fault, without adequate grounding, we're likely to see this all get damaged during a lighting strike.

My question is:
1 - Wouldn't this elevated concrete platform still need to be grounded?
2 - Can cathodic protection work on this structure if there isn't any point of it connected to ground?

Any thoughts?