Ratio, polarity and magnetics
The secondary circuit of an instrument transformer is grounded at one chosen location to hold it at a safe potential and to give protection a stable reference. This check confirms that the ground exists, that it is at the intended point, and that no second, unintended ground has crept into the wiring.
A secondary grounding check verifies how the secondary circuit of a current transformer or voltage transformer is tied to ground. The accepted practice is to ground each secondary circuit at a single, deliberately chosen point, usually at the first point of application such as the terminal block of the transformer or the switchgear compartment, according to the engineer's drawings. The check has three parts: confirming that the intended ground connection is present, solid and tight; confirming that no other connection to ground exists elsewhere in the secondary wiring, the relays, the meters or the test switches; and confirming that the grounding point matches the design. It applies to current transformers, voltage transformers and coupling-capacitor voltage transformers in switchgear and substations, and the same discipline of verifying ground connections is part of the testing of surge protective devices, whose grounding path carries large transient currents. It is a continuity and isolation test of wiring, not a measurement of a transformer property.
Grounding the secondary serves safety and function. An ungrounded secondary can float to a dangerous potential if insulation between primary and secondary fails or if capacitive coupling raises it, which puts people and instruments at risk. A secondary grounded at two or more points is just as bad in a different way, because the ground path is a parallel conductor and differences in ground potential during a fault drive current through the secondary wiring. That stray current can distort what a relay sees, operate a differential scheme falsely, damage a shield, or burn a conductor. In current circuits, a second ground can also bypass part of the burden and change the transformer's behavior. Finding a missing or extra ground at commissioning is routine, and it is among the more common wiring errors on new installations and after relay replacements, when jumpers and test links are changed.
The secondary of an instrument transformer is electrically separate from the primary, so its potential relative to earth is undefined until a ground is applied. A single ground fixes that potential without creating a loop. A second ground in a different location creates a closed loop through the ground system, and the ground grid is never at one potential during a fault: current flowing toward a fault produces voltage differences between points on the grid, and those differences push current through any loop that connects two of them. The risk is therefore proportional to how far apart the two grounds are and how large the fault current is. Verifying a single ground can thus be reduced to two measurements: that the intended point has a good low-resistance connection to the ground system, and that the rest of the circuit is isolated from ground except through that point, so that no alternative path exists.
The circuit is de-energized, isolated and grounded under the owner's safety procedures. The secondary circuit is disconnected from the intended ground point, at the grounding link or terminal, so that the circuit is deliberately left floating. With the intended ground removed, insulation between the secondary wiring and ground is measured, or continuity to ground is checked, from several points in the circuit, including relay terminals, meter terminals and test switches; the circuit should show isolation from ground, which proves that no unintended ground exists elsewhere. The intended ground link is then restored and its continuity to the station ground is verified with a low-resistance ohmmeter or a continuity tester, and the connection is checked for tightness and condition. The location of the ground is compared against the drawing. For voltage circuits, the neutral or the designated leg is the ground reference, and for current circuits a single ground is applied at the point defined by the design. Instruments used for isolation measurement are of a type and voltage that the connected solid-state devices can tolerate; sensitive devices are disconnected or isolated for that portion of the work.
The record states which secondary circuit was checked, the location of the intended grounding point, and the result of the isolation or continuity check with the ground removed. It also notes the result of the continuity check of the intended ground connection to the station ground and the condition of that connection. The instrument and its calibration status, the date, and any temporary isolation of devices are noted, along with the as-found condition, for example an extra ground that was found and removed, or a missing link that was installed. Where a drawing was consulted, the reference is mentioned. The record shows that the circuit was left with a single ground in its service condition.
The finding is a matter of arrangement and not of a numeric limit, and this page gives no value. A satisfactory result is a secondary circuit that is isolated from ground everywhere except at the single intended point, with a solid, low-resistance connection at that point, positioned where the design places it. Any additional path to ground is a defect to be located and removed, since it may be hidden in a relay input, a test switch, a shield or a damaged conductor. Any absence of the intended ground is also a defect. Where a requirement for a minimum isolation level applies, the governing standard and the manufacturer's instructions for the connected devices determine it. ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 call for confirmation that the secondary circuit is grounded at a single point for current and voltage transformers, and the surge arrester sections address grounding connections; refer to your licensed copies for the requirements.
At acceptance, the check is done on every secondary circuit as part of the instrument transformer sequence, and it is repeated whenever the secondary wiring is modified. In maintenance, it is repeated after relay or meter replacement, after the secondary wiring has been disturbed, and when unexplained relay behavior points toward a stray current. Current transformer circuits that feed differential schemes need particular care, because the single ground is usually placed at a defined point common to the whole zone and an additional ground at a remote location is a known cause of misoperation. Voltage transformer secondaries may be grounded at the neutral, or at a leg in a corner-grounded system, depending on the design. Where shielded cable is used, the shield is grounded according to the design, which can differ from the circuit ground. Coupling-capacitor voltage transformers add the grounding of the drain coil and carrier equipment, which has its own requirements.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
One ground fixes the circuit at a safe potential without making a loop. Two grounds form a loop through the station ground, and voltage differences during a fault drive current around it, which can distort relay inputs, trip a differential scheme falsely or overheat the wiring.
The only way to prove there is no second ground is to leave the circuit floating and test for isolation from ground. With the intended ground still connected, every reading would show continuity and the check could not distinguish the intended path from an unintended one.
At the point the engineer specifies, which is often the first point of application, such as the transformer terminal block or the switchgear compartment, and common to the whole protection zone in differential schemes. The drawings and the relevant standard govern, and the check confirms the ground is where the design puts it.
It should not be. A floating secondary can reach a hazardous potential if primary insulation fails or capacitive coupling raises it, endangering people and equipment. An open current transformer secondary is the more extreme case, because it can produce very high voltage across the open terminals.
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