Conventional Electric MeteringLesson 3 of 5FOUNDATION LESSON
CT/PT ratios, polarity and error mechanisms
Instrument transformers extend measurement to currents or voltages unsuitable for direct connection.
- READING TIME
- 15 minutes
- OBJECTIVES
- 3 learning objectives
- SELF-CHECK
- 3 check questions
- INTENDED FOR
- Meter-shop, field, engineering, procurement and revenue-assurance teams
Educational lesson. Confirm project-specific requirements before application.
01LEARNING OBJECTIVES
After this lesson, you should be able to:
- 01Explain ratio, polarity and burden effects
- 02Calculate a basic measurement multiplier
- 03Recognize wiring patterns that distort power and energy
02EXPLANATION
What is physically and operationally happening?
Instrument transformers extend measurement to currents or voltages unsuitable for direct connection. A current transformer reproduces primary current at a defined ratio; a potential transformer reproduces voltage. The meter record must apply the correct combined ratio, while polarity and phase association preserve the direction and relationship of voltage and current.
Burden and accuracy application matter because connected wiring and devices affect transformer performance. Open-circuiting a CT secondary can also create dangerous voltage and must be prevented through approved procedures and equipment. These are engineering and safety matters, not merely data-entry concerns.
- 01
Primary quantity
Actual line current and voltage exist at the service.
A / V primary - 02
Instrument transformer
CT/PT ratio and polarity produce scaled secondary quantities.
A / V secondary - 03
Meter element
Voltage and current phase relationships are measured and accumulated.
W / Wh secondary - 04
Multiplier
Approved ratios and constants convert the register to primary energy.
Wh primary
Illustrative CT ratio and polarity chain
Magnitude and direction must remain consistent from primary conductor through the billing multiplier.
01 · ON SITEPrimary current P1→P2
02 · ON SITECT magnetic coupling
03 · ON SITESecondary S1→S2
04 · ON SITETest block
05 · ON SITEMeter current coil
06 · RECORDConfigured ratio
07 · BILLINGBilling result
04OPTIONAL ENGINEERING DEPTHEngineering reasoning for this learning pathThe same design relationship, checks and evidence apply across every lesson in this path.
- Preserve voltage/current phase relationship
- Use the exact form, class and service configuration
- Reconcile physical asset, meter record and billing record
- 01Draw and verify the actual measurement circuit
- 02Check CT/PT ratio, polarity, burden and multiplier
- 03Test the end-to-end revenue chain, not only the meter
One phase contributes negative power
Reversed CT polarity or phase mismatch
Inspect wiring and compare phasors
Stable percentage billing error
Incorrect ratio or multiplier
Recalculate from installed configuration
- Approved service and wiring schedule
- Traceable test and phasor record
- Serial, seal, configuration and billing reconciliation
05WORKED EXAMPLE
Combined multiplier
A metering point uses 400:5 CTs and 13,800:120 PTs.
- CT ratio = 400 / 5 = 80
- PT ratio = 13,800 / 120 = 115
- Combined multiplier = 80 × 115 = 9,200
- A 10 kWh secondary register difference represents 92,000 kWh primary
06COMMON FAILURE OR MISCONCEPTION
“If the CT ratio matches, the measurement chain is qualified.”
Polarity, phase association, burden, accuracy, wiring, PT ratio, meter constant and records must also be verified.
Check what you can explain without looking back.
Choose an answer and report your confidence. The confidence signal is stored only until you submit this page.
A 600:5 CT has a ratio of:
Why does CT polarity matter?
Why should a live CT secondary not be opened?
08SOURCE NOTES
Sources for further study.
International references support further study. They do not replace Philippine laws, regulations, utility requirements or project specifications.
- 01IEC 62052-11:2020 Metering Equipment Requirements and TestsInternational Electrotechnical Commission
Scope of general requirements, tests and test conditions for electricity metering equipment.
Open source ↗ - 02Advanced Metering in Smart Distribution GridsNational Institute of Standards and Technology
Measurement accuracy and modern grid conditions.
Open source ↗