A CT-operated energy bill that a clerk has multiplied twice
A small flour mill is supplied at 415 V, three-phase, four-wire, through three 200/5 A ring-type current transformers of class 1, connected to a three-phase four-wire static energy meter. In June the meter's kWh register advanced by 1240 kWh. The utility's billing clerk applied a multiplier of 40 to that figure and billed 49,600 units at ₹7.20 per unit, producing a demand of ₹3,57,120. The owner says the mill runs about 6 hours a day for 26 days at an average load of about 8 kW, and refuses to pay. The clerk's position is that 'the CT ratio is 40, so the meter reading must be multiplied by 40'. You are asked to inspect the installation. A clamp meter on the CT primaries reads 22 A, 21 A and 23 A; the corresponding secondary currents read 0.55 A, 0.52 A and 0.57 A. The meter nameplate states 3 × 240 V, 5 A, and its display is marked 'kWh'.
1. Decide whether the correct billed energy is 1240 kWh or 49,600 kWh, and justify the decision with an independent load estimate.
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CT ratio first. 200/5 = 40. That part of the clerk's statement is true; the mistake is in assuming the meter has not already accounted for it.
Independent load estimate. 8 kW average × 6 h × 26 days = 1248 kWh. The meter's register advanced by 1240 kWh. The agreement is within 1 %, which is far inside the accuracy of the estimate.
The decisive test. If the meter were secondary-referenced, its own register would have advanced by 1240 / 40 = 31 kWh, which over 156 operating hours implies an average load of about 0.2 kW - less than a single ceiling fan plus a light. A flour mill with a 15 kW connected load cannot run a month on that.
Conclusion. The static meter has been programmed with the CT ratio of 40 internally, so its kWh display is already primary energy. Multiplying again inflates the bill forty-fold. The correct billing is 1240 units, which at ₹7.20 is about ₹8,928 plus fixed charges and electricity duty.
How to prove it on site, not by argument. Note the meter's make, serial number and program/CT setting (usually visible in the display menu or on the test certificate), take a photograph of the display, and read it again after a known interval while logging the load. If the program setting cannot be read, request the utility's own reference-standard check. Never simply refuse to pay - pay the undisputed amount and dispute the rest in writing, or the connection can be disconnected.
2. Use the clamp readings to assess the health of the three CTs, and explain what a reversed or open CT secondary would do to the bill.
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Ratio check. 0.55 / 22 = 0.025; 0.52 / 21 ≈ 0.0248; 0.57 / 23 ≈ 0.0248. All three correspond to a 5/200 ratio, so each CT is transforming correctly.
Balance check. The primaries are 22, 21 and 23 A - within about 9 % of each other, which is normal for a lightly loaded mill with single-phase lighting on one phase. There is no missing phase and no shorted secondary.
What reversed polarity would look like. If one CT secondary were connected the wrong way round, that phase's current would subtract instead of add, and the meter would under-register - the consumer would be billed too little. That is a real and common metering fault, which is why the clamps were taken.
What an open secondary would look like. A broken or loose secondary terminal reduces that phase contribution and under-registers; the consumer benefits and the utility loses.
Safety point that must be stated. Never open-circuit a CT secondary while primary current is flowing. The CT is a current source, so an open secondary drives the voltage to a dangerous level, saturates the core, and can fail the insulation and endanger the person working. Short the secondary with a proper shorting terminal or test block before disturbing any secondary connection, and only then work on the circuit.
Class and burden. Class 1 CTs carry a ±1 % ratio error at rated burden. Practically all of the 40× error in this bill came from the billing arithmetic, not from the CTs.
3. Outline the practical steps you would take before the bill is revised, and the safety and commercial advice you would give the owner.
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Evidence gathering. Record the meter reading, the meter serial number and the CT nameplate details with photographs. Obtain the meter's test certificate or the utility's record of the CT/PT ratio programmed into it. Log the load for a representative day with a clamp meter at 30-minute intervals to build an independent energy figure.
Verification of the meter. Ask the utility for a check of the meter against a reference standard at site, and confirm the meter constant (pulses per kWh) so the register advance can be cross-checked. If the meter has been mis-programmed, the correction should date from the last verified reading, not from the date of complaint.
Documentation and payment. Make the dispute in writing, quoting the readings and the calculation, and pay the undisputed ₹8,928 immediately so that the connection is not cut off while the dispute is pending. Keep the receipt.
Safety advice. Do not let anyone open the CT secondary circuit or reseal the meter; that is the utility's sealed equipment, and tampering attracts penalties and can be dangerous. Report the loose or damaged items rather than repairing them.
Commercial advice. The meter seals, the CT ratio and the programmed multiplier should be verified whenever a meter is replaced or a CT is changed, and the owner should keep a monthly log of readings and production hours. A simple spreadsheet of units against running hours would have caught this in the first month instead of at ₹3.5 lakh.