Analog Ammeter Low Reading

One of the possibilities it that the mechanical load is very low, and the motor is drawing very little current; it's difficult to see from here.

So?

Is the pump running? If yes, there could be a fault with the ammeter near the pump, faulty meter or loose connection to the CT. (De-energize before handling CT circuitry).

If pump is not running, there could be a short in the wire.

What happens when the original equipment is swapped back in?

I'm asking this question to try to understand what you're talking about (I'm unfamiliar with the field).

"The scale of reading is 150 each 50 divided into 8 scales, the reading is below the first step"

So each step is 6.25 Amps.

You're reading just below 6.25 Amps; and, you should be reading 1.85 Amps; how does that rate as low reading?

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AC or DC?

What does 1 VA (Watt?) mean in this context?

One might expect a difference between the "perceived power" and the "real power" if the motor is lightly-loaded. On AC, a lightly-loaded motor behaves more like a coil of wire with plenty of inductance than a convector heater that has a negligible amount of inductance. This introduces a phase lag between the voltage wave and the current wave.

Were the two measurements seen at site of those two different things, then that would explain the difference in readings.

However, given no response to the question over the actual mechanical load on the motor, this is all just supposition...

An old adage in instrumentation circles:

"Never put two measuring instruments on a process. They'll never agree, and you won't know which one to trust." _{- Anon.}

Were the instruments ever calibrated since leaving the factory?

Do the instruments actually measure the same thing? An alternate path (leakage) might be measured by one meter but not the other. The frequency response of one meter might not read non-linear harmonic components.

Friend Mrlaron,

I suppose the meter in MCC and that in the field are not on the same CT. This I say, because had both been on the same CT, both meters would have read the same.

it seems to me that the CT 100:5A, 1VA CT is not proper for supplying an instrument that is 500m away (It is OK if the meter were in the MCC itself).

I think the CT is getting saturated due to burden of 500m cable and with 5A rated secondary being much above the rated burden 1VA. This is the reason for very low reading in the field ammeter.

I suggest you to look at the possibility of wiring 4-20mA output from the meter in MCC to that in the field to overcome the problem.

The wire size between the CT and the meter is to light a gauge, you may need up to #10AWG for that length, maybe #8AWG. Also, your meter is normally a 5 amp full scale movement, so your meter would normally be 0-100A scale for a 100/5 CT.

Confucius say:Man with 2 watches never know what time it is.