Protection
The voltage balance relay, device 60, compares two voltage sources and operates when they stop agreeing, which signals a blown fuse or lost potential. Testing injects balanced and unbalanced voltage to prove the operate point, the delay and the blocking outputs.
Device 60 is the voltage balance relay, and it is mostly a supervisory element. It compares two sets of voltage, typically two voltage transformer secondaries on the same bus, or a transformer and a second reference, and operates when the two stop agreeing. A blown potential fuse, an open secondary, a tripped miniature breaker or a failed voltage transformer makes one set collapse or unbalance while the primary system is healthy. The relay detects that disagreement and acts as a loss-of-potential detector. Its output usually blocks the protective functions that depend on voltage, such as distance, undervoltage, directional and synchronizing elements, or raises an alarm or transfers control to a standby source. The same device number also covers current balance schemes in some applications. The test confirms the operate point, the delay and the output of each supervised function.
Relays that depend on voltage see a lost potential as a fault: an undervoltage relay sees a collapse, a distance relay sees an apparent short circuit and a directional element can lose its reference. Without supervision, a blown fuse can trip a breaker, transfer a motor bus or drop a load for no real reason. The opposite error is just as serious, because a supervision relay that cannot operate lets a lost potential go unnoticed until a fault arrives and the protection misbehaves. The element normally sits silent for years, so an inoperative one is not obvious. Testing proves that the comparison actually responds to an unbalance and that the blocking contacts act on the circuits they were wired to supervise.
The relay forms the difference between the two voltage sets, phase by phase or as a combined quantity. When both are healthy and equal, the difference is nearly zero and the element is restrained. When one set drops or unbalances, the difference grows, and the element operates once it passes a threshold. In an electromechanical balance relay, the two voltages drive opposing torques on a balance beam or a disk, and the contact closes when one side overpowers the other. In digital relays, the two sets are measured separately and compared in software, often with the ability to require that the voltage on the healthy side is present while the other is not, so that a true bus outage is not mistaken for a blown fuse. A short time delay prevents operation during switching or fault transients.
The relay is isolated from both voltage transformer secondaries so nothing can back-feed a live system, and its outputs are blocked or the links opened so that the blocking and transfer actions do not run on live equipment. A relay test set supplies balanced voltage to both inputs, with equal magnitudes and the correct phase sequence, and the relay is confirmed to remain at rest. One input is then reduced, or one phase of it is removed, in steps, to find the point at which the element operates, and the voltage is returned to normal to find the reset. The delay is measured by switching abruptly from balanced to unbalanced and timing the contact closure. The test is repeated for each phase of each input, and for each polarity of the disagreement, because the two sides of the comparison can behave differently. The output contacts and the blocking, alarm or transfer circuits are then proven, links are restored and the settings are left as found.
The record states the relay type and settings, the voltage applied to each input, the unbalance at which the element operated for each phase and input, the reset point, the measured delay and the response of each output. It notes the test connection and phase sequence, the instrument and its calibration status, which links or blocks were applied and restored, and the as-found and as-left settings.
The operate point is compared with the setting from the protection study, allowing the tolerance published for the relay, and the two inputs are compared with each other, since the comparison should respond equally on both sides. Phase-to-phase differences within one input point to a faulty element or a connection error. The delay is judged against the set value and the reset against the manufacturer's data, and results are compared with earlier tests on the same relay. ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 include the voltage balance relay among the protective relay tests. For the tolerances and values, refer to your own licensed copy of those standards and to the relay instruction book.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
It sees that the two voltage sets no longer agree and operates, usually blocking voltage-dependent relays, raising an alarm or switching to a standby source. This prevents a blown fuse from being misread as a real fault.
The comparison can behave differently depending on which side collapses and which phase is lost. Each combination is proven so a real loss of potential is caught whichever source or phase it affects.
The number is also used for current balance, but in loss-of-potential supervision it compares voltage sets. The relay instruction book states which quantity the particular device compares.
Many relays also check that the healthy source still has voltage. If both sources vanish together, it is a real outage and not a single-source failure, and the supervision does not operate.
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