Current Balance Relay (Device 46) Testing: Phase Unbalance and Open Phase

What it is

A device 46 current balance relay compares the three phase currents of a circuit and operates when they differ too much from one another. It is most often applied to motors and to feeders serving motors, where a lost phase, a blown fuse, a failed contact or a utility disturbance leaves the machine running on unequal currents. Some relays judge the ratio between the smallest and largest phase current, others compare each phase with the average, and others look at the difference against the highest phase. The unit may be a small electromechanical or solid-state module, or one of the functions inside a motor protection relay. Field testing uses a three-phase current source whose channels are controlled independently, so unequal currents can be created on purpose, together with a timer on the output contact.

Why it is done

A motor running on unbalanced currents heats unevenly. The extra current is not distributed equally between phases, and the unbalanced component flows through the rotor at a high frequency, producing losses that ordinary overload protection may not recognize as quickly. A single-phasing condition is the severe case: the motor keeps running on two phases, overheats and often burns out. A current balance element recognizes the abnormal pattern and separates the motor, or warns operators, before damage is done. If its sensitivity or delay has drifted, or if a current transformer is wired incorrectly, the element can fail to see a real lost phase or can trip on harmless variations. Since the condition is rare, only a deliberate test proves the element.

The principle

The relay measures the magnitude of each phase current and forms a measure of unbalance from them. One method divides the smallest phase current by the largest, which equals unity for a perfectly balanced load and falls toward zero as a phase is lost. Another subtracts the average from each phase and expresses the largest deviation as a part of the average. Both are indifferent to the absolute loading and respond only to the pattern. A minimum-current threshold usually keeps the element from operating when the load is so light that small absolute differences look large in proportion, and a delay prevents operation on brief starting transients. Because this element looks at magnitudes only, it differs from the negative-sequence element, which also considers the phase angles of the currents.

How it is performed

With the relay isolated and its trip output blocked, the technician records the identity and the settings for unbalance level, delay and the threshold that enables the element. The three-phase current test set is connected to the relay current inputs so that each channel feeds one phase, with the output contact timed by the set. The three currents are first set equal at a healthy load level, and the relay should be quiet. One phase is then reduced slowly, in small steps, until the output operates, which gives the pickup expressed in the relay's own unbalance measure, and the process is repeated for each phase so that every input is proven. The reset is found by moving the current back. To measure the delay, the set steps suddenly from balanced to a chosen unbalance and records the interval to operation. A single-phase condition is also applied to prove the response to an open phase. The enabling threshold is verified by repeating a test at a lower load. If possible, the contact is proven through to the trip circuit.

What is recorded

The record states the relay identity and the settings under test, the injected currents for each case, the unbalance found at pickup for each phase, the reset and the timed delay. It notes whether the enabling threshold and the open-phase response were verified, the test set and its calibration status, and the as-found and as-left settings. Adjustments made and the confirmation that the output reached its circuit are also noted.

How results are judged

Results are compared with the settings from the protection study, because the acceptable unbalance depends on the motor and the supply. The shapes of the criteria are a pickup that agrees with the setting on every phase inside the manufacturer's stated accuracy, a delay that matches the setting, correct enabling and blocking at low current, a clear response to an open phase and a clean contact. Maintenance results are also trended against earlier tests. The scope of relay testing is described in ANSI/NETA ATS-2025 for acceptance and ANSI/NETA MTS-2023 for maintenance. For tolerances and clause detail, consult your own licensed copy of those standards and the manufacturer's data.