Ideally, with two identical feeds and no other factors, the load should be split relatively evenly between Main Breakers A and B. However, in reality, a perfectly even split is rarely achieved and several factors can contribute to load imbalances.Here's a breakdown of why a perfect split is unlikely and what factors to consider

Factors Contributing to Load Imbalance:
Impedance Differences

The biggest reason for unequal load sharing is differences in impedance. Even small differences in the following will impact current flow:

Cable Lengths

Feed A cables are shorter than Feed B cables.

Cable Size

Even slightly different conductor sizes between the feeds.

Termination Quality

Poor terminations (loose connections, corrosion) on one feed increase resistance.

Network Protector Characteristics

Differences in the network protector transformer impedance.

Transformer impedance

If feed A and B come from different transformers, differences in transformer impedance affect the load split.

Main Breaker Impedance

Slight manufacturing variations can cause differences in impedance.

Load Distribution within the Switchgear



Feeder Placement

If feeders drawing larger loads are connected closer to one main breaker than the other, that breaker will carry more load. For instance, if a large motor control center is fed from a section of the switchgear closer to Main Breaker A, it will draw more current through that breaker.

Phase Balance on Individual Loads

Even a seemingly balanced three-phase load can have slight imbalances, which can affect the overall load on each main breaker.

Network Protector Settings and Operation



Network Protector Operation

Network protectors are designed to prevent reverse power flow. They don't necessarily force equal load sharing. If one protector is more sensitive or has different settings, it might carry more or less load.

Asymmetrical Load Characteristics



Non-Linear Loads

Loads like variable frequency drives (VFDs), UPS systems, and electronic equipment draw non-sinusoidal currents (harmonics). These harmonics can circulate and cause imbalances.

Meter Accuracy

Meter tolerances on the main breakers. The meter showing you the load may not be 100% accurate.

Power Factor Differences

If loads connected to one side of the bus have a significantly different power factor than loads on the other side, this can affect the current distribution.

Why Exact Load Sharing Isn't Always Critical (But Should Be Monitored)



Design Margin

Switchgear and its components are designed with safety factors and overhead. A moderate imbalance is usually within acceptable operating limits.

Redundancy

The purpose of two feeds is redundancy. If one feed fails, the other can carry the entire load (up to its capacity).

What to Do


Verify Meter Accuracy

Use a calibrated meter to measure the current on each phase of each main breaker to confirm the accuracy of the installed meters.2.

Inspect Connections

Visually inspect all connections (cables to breakers, breakers to bus, etc.) for tightness, corrosion, and signs of overheating. Use a thermal imaging camera to identify hot spots that indicate poor connections.3.

Measure Impedance

If the imbalance is significant and concerning, consider impedance testing of the feeds (cables, network protectors, and main breakers) to identify the source of the difference. This typically requires specialized equipment and expertise.4.

Load Balancing

If possible and practical, try to redistribute loads within the switchgear to achieve a more even balance. This may involve moving feeders or adjusting the sequence of connected loads.5.

Monitor Over Time

Even if the imbalance is currently acceptable, continue to monitor the load on each main breaker. A gradually increasing imbalance can indicate a developing problem (e.g., deteriorating connection).6.

Consult with a Professional

If you're unsure about how to diagnose or correct a load imbalance, consult with a qualified electrical engineer or switchgear service technician. They have the expertise and equipment to perform thorough testing and analysis.

In summary: While you expect a relatively even split, it's rarely perfect. Understand the potential causes of imbalance, monitor the load, and investigate if the imbalance is significant or worsening. Early detection and correction of problems can prevent equipment failure and ensure reliable power distribution. Flag for review