Relay Burden Calculator

What is Relay Burden?

Relay burden is the voltage load that a relay circuit imposes on the secondary winding of a current transformer (CT). In protection and metering systems, current transformers step down high fault currents to manageable levels. The cables connecting the CT to the relay introduce resistance, and the current flowing through that resistance creates a voltage drop -- this voltage drop is the relay burden.

Understanding relay burden is critical because every CT has a maximum rated burden. If the actual burden exceeds this rating, the CT core saturates, distorting the secondary current waveform. Distorted signals cause protection relays to misoperate, potentially leaving faults uncleared or causing nuisance trips.

The Formula

The relay burden is calculated using:

[\text{Relay Burden} = I^2 \times 2 \times D \times R]

Where:

• I is the current flowing through the relay circuit in amps.
• D is the one-way cable length in meters.
• 2 accounts for the outgoing and return conductors forming the complete loop.
• R is the cable resistance in ohms.

The factor of two is essential because current must travel through both the outgoing and return conductors. The result is expressed in volts.

Calculation Example

Suppose you have the following relay circuit parameters:

1. Cable Length (D): 40 meters
2. Cable Resistance (R): 15 ohms
3. Current (I): 2 amps

Substitute these values into the formula:

[\text{Relay Burden} = 2^2 \times 2 \times 40 \times 15]

Breaking it down step by step:

[\text{Relay Burden} = 4 \times 2 \times 40 \times 15 = 4{,}800]

The relay burden is 4,800 volts.

In practice, a burden this high would far exceed any standard CT rating, signaling that the cable resistance or length must be reduced, or a CT with a higher VA rating should be selected.

Why Relay Burden Matters

Relay burden is not just a theoretical value -- it directly affects the reliability of your protection system:

• CT saturation: When burden exceeds the CT rating, the core saturates and the secondary current no longer accurately represents the primary current. This can delay or prevent relay operation during faults.
• Cable selection: Longer cable runs and thinner conductors increase resistance and therefore burden. Engineers use the relay burden calculation to select appropriate cable sizes and plan cable routes.
• System design: Protection engineers must sum all burdens in the CT secondary circuit -- relay coils, lead wire resistance, and any connected instruments -- to ensure the total stays within the CT capability.

Reducing Relay Burden

If the calculated burden is too high, consider these strategies:

• Use larger cross-section cables to reduce per-unit-length resistance.
• Shorten cable runs by placing relay panels closer to the CT location.
• Select a CT with a higher burden rating (higher VA class).
• Use numerical relays instead of electromechanical relays, as they typically have much lower burden requirements.

By keeping relay burden within the CT rated limits, you ensure that protection relays receive accurate current signals and operate correctly when faults occur.