CT-Operated Meter Reading 720 Units Against a 45 kW Load: Is the Meter Fast or the Billing Wrong?
A 45 kW three-phase industrial consumer is supplied at 415 V through a CT-operated static meter with 100/5 A CTs connected star-star. The meter's configuration menu shows CT ratio 100/5 and PT ratio 1. For the billing month (26 working days, two shifts of about 12 hours a day) the meter register advances by 720 kWh, and the bill is prepared for 720 units. The consumer's own sub-meter, installed by his electrician after the main switch, records about 14 100 kWh for the same period. The test-terminal seal is intact on two phases but missing on the third. The consumer disputes the bill and claims the meter is running fast.
1. Compute the multiplication factor, the energy that should have been billed, and compare it with the consumer's sub-meter. What is the actual defect?
Read the worked answer
MF = CT ratio ÷ meter rated ratio = (100/5) ÷ (5/5) = 20.
Energy to be billed = register advance × MF = 720 × 20 = 14 400 kWh.
Cross-check against load: 45 kW × 12 h × 26 days = 14 040 kWh at 100 % utilisation - the same order as 14 400 kWh, which is entirely plausible once small lighting and fan loads and a little operation above nameplate are allowed for.
Cross-check against the consumer's sub-meter: 14 100 kWh is 98 % of 14 400 kWh - within the combined error of two meters.
Conclusion: the CTs, the meter and the consumer's load all agree. The meter is not fast. The defect is in billing: the multiplication factor of 20 was not applied, so 14 400 units were billed as 720 - an under-billing of 13 680 kWh. Nothing in the data indicates theft; the missing seal is a procedural issue, not a revenue issue on these figures.
2. Given the missing seal on one phase, what site checks would you carry out before concluding that no tampering occurred?
Read the worked answer
- Visual: inspect CT primary links, CT secondary terminal box, meter terminal cover and the meter itself for shunts, jumpers, a shorting link left across a CT secondary, or a broken/reversed secondary connection.
- Current: measure secondary current in all three phases with a low-range tong tester and compare with the primary current; each phase must show about 100/5 and the three must be consistent with the load. A shorted CT reads low or zero; an open CT reads zero (and is also a dangerous high voltage).
- Configuration: read CT ratio, PT ratio, meter constant from the menu and compare with the nameplate, the CT nameplate and the consumer master record.
- Voltage: confirm all three phases present at the meter and the phase sequence correct.
- Accuracy: carry out an on-load check with a portable reference standard; if a dispute persists, remove the meter under a sealed cover with a panchnama for laboratory test.
- History: compare the last 6–12 months' consumption and the sub-meter for any step change.
- Seal register: record the missing seal, establish who was responsible for sealing, take photographs and a joint statement. The missing seal must be documented, but on these figures no revenue was lost through it.
3. State the correct accounting and follow-up action, and the system fix that would prevent a recurrence.
Read the worked answer
- Bill correction: issue a supplementary bill for 14 400 − 720 = 13 680 kWh at the applicable tariff, showing the MF on the bill, after written notice and an opportunity for the consumer to be heard. Recheck the maximum demand record too: if the MD register is at secondary values, the billed MD must also be multiplied by 20.
- Do not book it as theft. The metering was accurate; the error was the utility's. Close the dispute with a copy of the site test report and the meter accuracy result.
- Database audit: list every CT-operated connection where billed units equal register units, or where MF is set to 1, and correct both the consumer master and past bills. The same omission probably affects other consumers on similar CT sets.
- Control fix: enter CT ratio, MF and meter constant in the billing system from the test report; add a validation rule that blocks billing when MF = 1 on a CT connection, and when billed units do not equal register × MF; train billing staff on multipliers.
- Seal: reseal the third phase with a new seal and record the number in the seal register.