Ratio, polarity and magnetics
A polarity test confirms the direction in which a transformer winding, a battery string or a circuit is connected relative to its markings. A reversed instrument transformer makes relays and meters misread direction, and a reversed battery or string can damage equipment, so polarity is proven before energizing.
A polarity test establishes which terminals of a device share the same instantaneous direction, and confirms that the device is connected the way the drawings and markings say it should be. On a current or voltage transformer, polarity marks identify the primary and secondary terminals that are positive at the same instant, so that a current entering the marked primary terminal leaves the marked secondary terminal. On direct-current systems, such as battery strings and solar arrays, polarity is the positive and negative orientation of each string, bus and connection. On rotating machines the field winding polarity, and the order of the poles, matter for correct excitation. The test is applied to instrument transformers in switchgear and at other equipment, to battery systems, to motors and generators with field windings, to relays whose operation depends on the direction of current or voltage, and to photovoltaic circuits. It is simple, quick and basic, and it is one of the tests that most often uncovers a wrong connection.
Direction information is what lets protection and metering distinguish one situation from another. Directional relays, differential relays, power and energy meters, and synchronizing and summation circuits all depend on the instrument transformer polarity being right. A current transformer connected backward makes a directional relay operate for a fault in the wrong direction or fail to operate for one in the right direction, makes a differential scheme see load as an internal fault, and makes a meter register negative or reduced power. In a summation system, where the currents of several current transformers are added, one reversed unit subtracts instead of adding and produces a false result. In direct-current systems, reversed polarity can destroy electronics, blow fuses, short a string, or make a battery charger or inverter fail. Because a reversed connection is easy to make and hard to see, the test is made before the first energization and repeated after any rewiring.
Polarity is a convention of direction, not a magnitude. In a transformer, the primary and secondary windings are wound on the same core, and at any instant a voltage induced in one produces a voltage in the other whose direction depends on the winding sense. Polarity marks identify the terminals that go positive together, and the markings define the relationship between primary and secondary quantities. The test checks the markings against the physical reality of the winding. A common method applies a brief, small direct current pulse to the primary and observes the direction of the deflection on a sensitive meter connected to the secondary: a deflection in a given sense confirms the marked relationship. Other methods compare the phase of a primary and a secondary alternating quantity, or use a dedicated polarity checker. For direct-current systems, the same idea is applied with a meter that shows the sign of the voltage at each terminal, which simply identifies positive and negative.
The circuit is de-energized, isolated and grounded under the owner's safety procedures. For a current transformer, the secondary is connected to a sensitive analog or digital instrument that shows sign, with the other devices in the circuit disconnected or accounted for. A small, brief direct current from a battery is applied across the primary, momentarily, with the secondary circuit closed and never opened under current. The direction of the kick on the instrument is noted at make and at break and compared with the polarity markings. A tester that injects a pulse and shows the polarity directly can also be used. For a voltage transformer, a similar pulse is applied on the primary, or an alternating comparison is made with the secondary voltage against a reference. For the connection of the whole circuit, the polarity of each current transformer in a summation or differential arrangement is checked at the terminal block against the drawing. On battery systems, the voltage and sign of each string, each cell connection and each bus is measured with a calibrated meter before connecting charger or load. On solar strings, open-circuit voltage and its polarity are measured at each string before combining. Direct-current field windings are checked with the machine stationary and the field briefly excited at a low current, using a compass or a polarity meter to confirm that the poles alternate in sequence.
The record shows which device or circuit was checked, the terminals compared, and the method used. It holds the observed polarity at each point, such as the direction of the deflection or the sign of the measured voltage, and whether it agreed with the markings and the drawings. For a battery or string, the measured voltage and its sign for each string or connection are noted. The instrument and its calibration status, the date and the conditions are included, along with the as-found state, for example a transformer found reversed and the correction made. Where several current transformers were involved in a scheme, their results are recorded as a group so that the scheme as a whole can be reviewed.
The outcome is binary and relative to the markings and the design: the observed polarity either agrees with the marking, the drawing and the intended connection, or it does not, and this page gives no numerical limit. A mismatch is a defect to be corrected, either by a change to the wiring or, where the marking is wrong, by a correction of the documentation, and the test is then repeated. In a protection or metering scheme, polarity is also confirmed in the context of other checks, such as ratio and burden, and by current injection through the whole circuit, which would reveal a reversed unit as a wrong phase or sign. In direct-current systems, the sign and the voltage level are compared with the string or battery design. The markings and the methods are described in IEEE C57.13-2016 and IEEE C57.13.1-2017, while ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 call for polarity tests across the equipment classes listed; refer to your licensed copies for the requirements.
At acceptance, polarity is verified on every instrument transformer and at every device whose function depends on direction, and it is the first check after installation. In maintenance, it is repeated after replacement of a transformer or relay, after rewiring, and whenever protection operated unexpectedly. On current transformers the test is done with the pulse method or a polarity tester, and where several units feed a summation or differential scheme the polarity of the group matters. On voltage transformers and coupling-capacitor units, the primary and secondary marks are compared in the same way. Battery energy storage and station batteries are checked on every string and bus, and solar arrays on every string and combiner. Motors and generators with fields are checked for pole sequence. Relays that are directional are checked as part of the relay settings and function tests.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
It identifies the primary and secondary terminals that are positive at the same instant. Current entering the marked primary terminal leaves the marked secondary terminal in the same direction. Relays and meters rely on that relationship to tell the direction of power or fault current.
A directional relay can trip for faults in the wrong direction or fail to trip for faults in the right one, a differential scheme can see load as an internal fault, and a meter can read reversed or reduced power. In a summation circuit, a reversed unit subtracts instead of adds.
A brief, small direct current is applied to the primary while a sensitive instrument on the secondary shows the direction of the deflection, at make and at break. The direction is compared with the markings. Purpose-built testers show the polarity directly. The secondary is never opened during the test.
A reversed string or connection can damage chargers, inverters and electronics, blow fuses or create a short circuit. Measuring the sign and the voltage of each string before connecting it to the bus confirms that every connection is made as designed.
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