No power to Mark V card

U

Thread Starter

Ubong Akpabio

please i carried a loop check on some of devices to be specific my GT air inlet clogging filter transmitter. i noticed there was no power at my marshaling kiosk and my mark v cabinet. after troubleshooting i noticed that even the associated pressure switch did not have power at their terminal point on the Mark V board, but when disconnected from the board the was power on those points in the card. yes there is voltage supply from my dc source and my split voltage is okay. how come i do not have voltages at this point on my Mark v card?
 
We have 3 pressure switches, 63TF-1, 63TF-2A, 63TF-2B, not a transmitter. You state that you have voltage on the Mark5 TB with the leads lifted but not when connected. A short could do that but would also give a ground alarm. If this is commissioning a new install, I would check that the Mark5 card jumpers are set properly as per the application manual and make sure the cables are correct, point to point. Is this a 4-20ma transmitter, or a pressure switch, what is the point name?
 
A "short" or even a ground of a 4-20 mA input will not be reported by the Mark V as a 125 VDC Battery Ground--or as a change in supply split voltage. That's because the (nominal) 24 VDC supply produced by the Mark V power supplies is isolated from the 125 VDC battery through transformers and power conversion equipment on the power supply cards.

Also, the <b>ONLY</b> ground monitoring on the Mark V is done on the 125 VDC supply, not any of the lower voltages used in the cores, or for the excitation of analog inputs--including the AC excitation of LVDTs. Also, for the majority of low-voltage power supplies in the Mark V (+/-5 VDC, +/-15 VDC, etc.) the negative leg of the supply is already grounded at the Mark V power supply card (PCOM, Power Commoon, or DCOM, Digital Common). So, a ground on the negative leg of low DC voltages can't--and won't--be detected or annunciated. And, a ground on the positive leg can result in low voltage (if a high resistance ground) or no voltage (a low resistance ground) and even, in worst cases, a blown fuse.

Shorts are different from grounds, though commonly lumped in with them during discussions. Shorts will almost always result in blown fuses somewhere.

I would suggest if you don't have voltage when the wiring is connected to the Mark V there is a high resistance somewhere in the circuit--or the transmitter is NOT one originally provided by the turbine packager. The Mark V is not capable of working with all milliamp transmitters present in the marketplace. I have seen some HART-compliant transmitters not work at all with the Mark V, and appear to drag the loop voltage down, though not to zero VDC (in my experience)--but I suppose it could certainly happen.

As for the switches, unless the Mark V is using 24 VDC for discreet input voltage wetting/interrogation and one leg of the 24 VDC is grounded (which I don't think would work with a Mark V), I would have to suspect there is a wiring problem somewhere. Loose terminations, or something like that.

Lastly, I hate to sound like a broken record on this issue but the ribbon cable connectors (male and female) are subject to corrosion, even in well-maintained control room environments. Conductive grease should be applied (judiciously--because too much is just as bad as not enough!) to every connector at least every once every two years or so. It's not something that can be done once and then forgotten. At a minimum, during a mechanical maintenance outage, the Mark V should be powered down and every ribbon cable unplugged from it's connector and the pins checked for corrosion, and periodically a thin film of conductive grease should be applied.

If the environment in the control room where the Mark V is located is humid and the air conditioning is not maintained at the proper temperature, this can greatly accelerate the corrosion of the connectors. And, if it's also dusty, that's the worst combination. (Proper temperature control <b>DOES NOT</b> mean keeping the compartment temperature as low as possible, almost uninhabitable for humans. Because if it's humid outside the compartment then every time the doors are opened moisture WILL condense on cold surfaces, including electronics, and if there is dust on the surfaces, well, that's the best recipe for failed electronic cards (after overcurrent and overtemperature).)

So, the likely culprit is loose connections, or corroded cable connectors, or improper devices being substituted. As suggested, a point-to-point check of the wiring from each device to the Mark V terminal board would be best in each case.

Please write back to let us know how you resolve the issue!
 
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