CT meter under-recording at a flour mill: one phase missing

Expected energy: 30 kW x 8 h x 25 days = 6000 kWh, and the consumer's check meter recorded 5950 kWh, which confirms the mill genuinely consumed about 6000 kWh.

Test the hypotheses

1. Uniformly slow meter: recorded/actual = 4100/5950 = 0.69. A slow meter would under-record proportionally on all three phases, but 0.69 is not a typical calibration drift; it is very close to 2/3 = 0.667, the exact figure expected if two of three phases are being measured. If one phase is missing, expected display = 5950 x 2/3 = 3967 kWh, close to the observed 4100 kWh (the small excess is normal phase imbalance).
2. Wrong CT ratio programmed: if the meter were set to 200/5 instead of 100/5, the display would halve (about 2975 kWh). If it were left in secondary mode without the factor 20, the display would be about 300 kWh. Neither matches 4100 kWh.
3. Ratio check on live currents: 42 A primary / 20 = 2.10 A secondary, and 45 A / 20 = 2.25 A secondary. The first two displayed currents match the CT ratio exactly, which proves the meter's ratio programming is correct. So the ratio is not the problem.
4. Third phase: the clamp meter reads 40 A on the third phase, so current is flowing, but the meter displays 0.00 A. The measurement chain for that phase is broken: an open or loose CT secondary terminal (S1 or S2), a blown or open test-block link, a broken pilot wire, or a shorted CT secondary.

Conclusion: one phase's CT secondary circuit is open. A CT whose secondary is open drives its core into saturation and produces a very high voltage at the open terminals, which is both a shock and a fire/insulation hazard. The meter itself is probably healthy; the defect is in the CT circuit.

The rule that governs everything: a current transformer must never be open-circuited on the secondary while its primary is energised. The secondary tries to drive its rated current into an open circuit, the core saturates, the terminal voltage can rise to a dangerous level, insulation can break down, the CT can be permanently damaged, and anyone touching the terminals can be killed.

Procedure

1. Inform the consumer and the utility control room; plan a shutdown.
2. Isolate the primary: open the mill's incoming main switch and the utility supply; lock and tag. Prove dead with a two-pole tester.
3. If the primary genuinely cannot be shut down, the CT secondary must first be short-circuited at the test terminal block, using a proper shorting link or shorting switch, phase by phase, before any terminal is loosened.
4. With the circuit isolated, check:

• terminal tightness and burning at the meter and at the CT secondary box;
• continuity of each pilot wire, end to end;
• the test-block link position;
• CT polarity (P1 to S1) and correct routing of the phase conductors through the CTs;
• insulation resistance of the secondary wiring;
• a single-point earth on the secondary (S2), which should exist at one place only.

1. Repair or replace the damaged terminal, link or pilot wire.
2. Verify by secondary injection: inject a known current into each CT secondary (for example 2 A) and confirm the meter displays all three currents and that the power and energy registers respond on all three phases. Then verify under real load that all three phase currents appear on the display.
3. Note the meter reading before and after the work, reseal, and record the seal numbers. Any test-block short must be removed before restoring supply; leaving it in place would short the CT and stop the phase from being recorded again.

Quantifying the shortfall Unrecorded energy = 5950 - 4100 = about 1850 kWh for the month (using the consumer's check meter as the cross-check). The ratio 4100/5950 = 0.69 supports a one-phase loss rather than a billing or ratio error.

Basis for correction

• The cause is a defective CT secondary circuit, with seals intact and no evidence of tampering. This is a metering defect, not theft, so a penalty or theft provision would be wrong.
• Standard practice in such cases is to rebuild the bill for the affected period on the average consumption of the preceding three months or the same month of the previous year (confirm the exact period in the current MERC Supply Code regulations). Here the average of previous months (about 6000 kWh) and the check meter (5950 kWh) agree, so the assessment is well supported.
• Document everything: joint verification report signed by the consumer, photographs of the open/loose terminal, seal numbers, meter readings before and after, and the secondary injection results. Without this record the assessment is likely to be disputed.

The commissioning trap The replacement meter must be programmed with the same CT ratio, 100/5 = 20.

• If the new meter is left with a default ratio (say 200/5) the display will show half the true energy and the consumer will be under-billed for as long as nobody notices.
• If the new meter is supplied in secondary-kWh mode and the billing system still assumes the old meter's factor of 1, the bill will be short by a factor of 20 - a very large error.
• Before energising: check the ratio and meter constant in the meter's settings menu, print or note the configuration, run a load check comparing displayed current with the clamp-meter primary current divided by 20, and record this on the meter change report.
• Set the starting reading to the replaced meter's final reading so that the historical register in the billing system is continuous, and give the consumer a copy.