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Monitoring 'grounds' in DC power bus
2

Monitoring 'grounds' in DC power bus

Monitoring 'grounds' in DC power bus

(OP)
Monitoring 'grounds' in a DC power bus.

I'm trying to figure out how to SCADA-ize an antique chart recorder in a steel mill (from a distance, I've not been there yet to look and see) that indicates "grounds" on a DC bus system used for powering cranes.

My issue is how to transducer-ize the existing system to signals useable with conventional low level DC analog inputs (4-20mA/0-1/5/10Vdc).

By the mill's explanation, there's a DC bus with a rail at -125Vdc with respect to ground, a rail at +125Vdc with respect to ground, 250Vdc between the rails. They use the word 'ground' for both DC zero reference and the fault condition that is monitored.

There are two 'ground' fault indicators
- indicator light bulbs which are 'grounded' on one side (he's not sure where they're connected to on the other side) which do not glow under normal conditions, but glow more brightly as the fault voltage/current increases.

- a vertical trend paper chart recorder, whose scale is -125Vdc on the left, +125Vdc on the right with zero in the center. A non-fault condition trends a line at zero in the center of the chart; a 'ground' condition drives the pen to either side of the zero at a magnitude proportional to the fault.



The mill says there's only 3 wires coming into the recorder, -125Vdc, 'ground' and +125Vdc. They claim to have no documentation for the recorder or its wiring, other than an observation that there 'are a couple of resistors' in the recorder's input circuit (no clue whether the resistors are a divider or an IR drop).

The explanation is that a 'ground' drops one rail's voltage and increases the other's with the shift from nominal shown as a deviation from zero. Their example is the negative rail drops from nominal -125V to -115V, while the positive rail increases from 125V to 135V, with the pen moving to 10V on the chart.

If that's the case, then commercial isolated signal conditioner modules, 200Vdc input/4-20mA output should do the trick. Putting a signal conditioning module on one rail, another on the other rail and subtracting the difference will show the difference in the rail potential.

But the indicating light bulbs leads me to believe that the recorder might be monitoring the difference between the DC reference zero and an earth ground point, like the recorder's chassis.

I'll be going out to look at it, but a DC power bus is outside my experience, so I'm looking any advice what others do for monitoring 'grounds' on a DC power bus.

RE: Monitoring 'grounds' in DC power bus

"modules, 200Vdc input"

If one rail is hard grounded due to fault, then might the other rail might rise to +/- 250 volts (125 x 2) relative to ground? If so, make sure your selected module can handle 250 input, plus whatever margin you think is appropriate.

RE: Monitoring 'grounds' in DC power bus

Rail supply lines are intentionally not grounded. Lamps are 250V. One from each bus to physical "ground", thus 3 wires. Normally you see 125V across each one. During a "ground" event, voltage will be unbalanced. Note- the lamps are a low impedance and help hold the voltage balance. The voltage sensors will probably work, but their high impedance input will cause more noise than visible in that trace. 125V battery supplies in (older) switchgear are often monitored by lamps. Makes it easy to trace circuits when you have + & - sources. But, without the lamp load, they bounce all over the place as loading changes. There are many ground detection relays on the market, not sure if any will put out 4-20ma.

When the first ground occurs, no problem, just go find it. At second ground you blow fuses.

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