When working with current transformers (CTs) and potential transformers (PTs) in metering setups, calculating the burden accurately is critical to ensuring precise measurements. In this post, we’ll break down how to calculate burden, why it matters, and how to avoid common mistakes that could compromise your metering systems. By the end, you’ll have a solid understanding of CT and PT burden calculations, along with valuable resources to dive deeper into related topics.
What Is Burden?
In the context of CTs and PTs, burden refers to the load imposed on the transformer’s secondary circuit, expressed in ohms or volt-amperes (VA). This load includes:
- The impedance of connected wires.
- The input impedance of connected devices, such as meters or protective relays.
- The impedance of any connection in the circuit, like test switches and meter base terminals.
For accurate metering, the total burden must stay within the transformer’s rated burden capacity. Exceeding this capacity leads to measurement errors, which can impact billing, energy analysis, and operational reliability.
Why Burden Calculations Matter
Accurate burden calculations ensure that CTs and PTs operate within their design specifications, preserving their accuracy. Mismatched burden can cause:
- Reduced Accuracy: Overloading a CT or PT results in distorted measurements, compromising billing and operational insights.
- Safety Risks: Incorrect burden can overheat transformers, creating potential hazards.
- Financial Losses: Errors in metering can lead to incorrect billing, affecting both utilities and customers.
- Premature Failures: Overburdened CTs and especially PTs can lead to premature failures leading to lost revenue and costly repairs.
How to Calculate Burden for CTs
Calculating the burden on a CT involves summing up the impedances in the CT’s secondary circuit. Here are the steps:
- Identify Components in the Circuit: These include the wiring resistance, the meter input impedance, and any additional connected devices.
- Measure or Look Up Impedance Values:
- For wiring, calculate resistance using Ohm’s Law:where ρ is resistivity, L is wire length, and A is wire cross-sectional area.
- For meters and devices, refer to the manufacturer’s specifications.
- Sum the Impedances: Add up the resistance and reactance components to find the total burden.
Example: A CT has a secondary wire resistance of 0.5 Ω, a meter impedance of 0.3 Ω, and a relay impedance of 0.2 Ω. The total burden is: 1 Ω. To know whether this is an acceptable number or not, consult the nameplate.
For detailed examples of how CT meters are used, see what other are finding out at our CT Meters page.
How to Calculate Burden for PTs
The process for PTs is similar, but PT burden is typically expressed in VA. Use the formula:
- Determine Secondary Voltage (V): This is the PT’s rated secondary voltage (e.g., 120V).
- Calculate Total Resistance (R): Sum the impedances in the PT’s secondary circuit.
- Calculate Burden in VA: Plug values into the formula above.
Example: A PT with a secondary voltage of 120V and a total resistance of 600 Ω has a burden of:
- V²/R = W
- 120²/600 = 24VA
Explore more examples on our CTs and PTs page.
Tools to Simplify Calculations
For those frequently working with metering setups, tools such as burden calculators or spreadsheets can save time and reduce errors. Our guide to CT and PT Calculations includes helpful templates and examples.
Testing and Verification
Once you’ve calculated the burden, it’s important to test the setup to ensure compliance with the CT’s or PT’s rated specifications. Use appropriate testing equipment to measure the actual burden. For details on testing polyphase systems, visit our article on Performing Load Checks on Transformer-Rated Meters.
Common Pitfalls and How to Avoid Them
- Ignoring Wire Length: Underestimating the impact of long wire runs can lead to errors. Always include wire resistance in your calculations.
- Relying on Outdated Data: Ensure all device specifications are up-to-date.
- Skipping Testing: Calculations alone aren’t enough. Always verify the burden with real-world tests.
Conclusion
Calculating burden on CTs and PTs is a fundamental skill for metering professionals. By accurately calculating and verifying the burden, you ensure reliable, safe, and efficient metering setups. For more insights and practical examples, explore related topics on LearnMetering.com:
