A medium voltage potential transformer (PT) is a precision step-down instrument transformer that isolates and scales high distribution voltages—typically 4.16 kV to 34.5 kV—down to a standardized, safe secondary voltage (usually 120V or 115V) for metering and protective relays. In a real installation, it changes a lethal primary voltage into a low-voltage signal that standard 120V-rated digital multimeters, kWh meters, and microprocessor relays can safely read, while maintaining strict phase-angle and voltage-ratio accuracy. People commonly confuse it with a standard control power transformer (CPT), but while a CPT is designed to power heavy loads with loose voltage regulation, a PT is designed strictly to measure voltage with high precision.

Standard Ratios and Accuracy Classes

When specifying or replacing a medium voltage potential transformer, you must match the primary system voltage to the standard secondary tap. In North America, the secondary voltage is almost universally standardized to 120V or 115V line-to-line, allowing utility metering and protective relays to use common, off-the-shelf 120V AC inputs. The governing standard for these ratios and accuracy limits in the US is IEEE C57.13.

Common Medium Voltage PT Ratios and Applications
Primary System Voltage (kV) PT Primary Rating (V) Secondary Rating (V) Turns Ratio Typical Application
4.16 kV 4,200 120 35:1 Industrial motor control centers
13.8 kV 14,400 120 120:1 Utility distribution feeders, large campus substations
24.9 kV 24,940 120 208:1 Rural overhead distribution, padmount transformers
34.5 kV 34,500 115 300:1 Sub-transmission lines, wind farm collector systems
Accuracy Class Matters: For revenue metering, specify a PT with an accuracy class of 0.3 (meaning the ratio error will not exceed 0.3% at rated burden). For general protective relaying (like ANSI 59 overvoltage or 27 undervoltage), a class 1.2 is typically sufficient and costs significantly less.

Worked Example: Calculating Secondary Voltage and Burden Limits

Let’s look at a real-world scenario on a 13.8 kV distribution feeder. You are installing a microprocessor relay for overvoltage protection, and the switchgear contains a PT with a 14,400V:120V nameplate (a 120:1 ratio). The PT is rated for a maximum standard burden of 75 VA at 120V.

1. Calculating Secondary Voltage During a Swell
Nominal primary voltage is 13,800V. Under normal conditions, the secondary voltage is:
13,800V / 120 = 115V

If a capacitor bank switching event causes the primary voltage to swell to 14,500V, the secondary voltage becomes:
14,500V / 120 = 120.83V

Your relay sees 120.83V. If your relay's overvoltage pickup (ANSI 59) is set to 110% of nominal (126.5V), it will correctly restrain and not trip, because 120.83V is below the threshold. The PT has successfully scaled the event without saturating.

2. Checking the Burden Limit
Burden is the total load (in VA or ohms) connected to the PT secondary. Exceeding the VA rating causes the core to saturate, introducing massive ratio and phase-angle errors. For a 75 VA PT at 120V, the maximum allowable secondary current is:
I = VA / V = 75 / 120 = 0.625 A

Using Ohm’s law, the minimum allowable impedance (Z) of your connected circuit is:
Z = V / I = 120 / 0.625 = 192 Ω

Critical Rule: If the combined impedance of your relay coils, metering taps, and secondary wiring drops below 192 Ω, the PT will be forced to supply more than 75 VA. This will cause the secondary voltage to sag (ruining metering accuracy) and eventually overheat the PT windings.

Where You Meet Medium Voltage Potential Transformers in Practice

You will rarely see a medium voltage potential transformer sitting out in the open. Because the primary terminals are at lethal distribution potentials, they are integrated into specific, grounded enclosures:

  • Metal-Clad Switchgear: Inside medium-voltage breaker cubicles (like Siemens GM-SG or Eaton VCP-W), PTs are often mounted on draw-out trucks or fixed in the rear cable compartment. They feed the door-mounted multifunction relays (e.g., Schweitzer SEL-751 or Basler BE1).
  • Padmount Transformers: In commercial loop-fed systems, a small epoxy-cast PT is often mounted in the primary compartment to provide 120V control power for the SCADA RTU or motorized load-break switch, while simultaneously feeding a revenue meter.
  • Utility Metering Cabinets: For large industrial services (e.g., a 5 MVA manufacturing plant), the utility will install a dedicated metering cabinet containing both current transformers (CTs) and PTs to step down the 12.47 kV or 34.5 kV service for the utility's billing meter.

For a deeper dive into how these instrument transformers interface with digital relays, the All About Circuits guide on instrument transformers provides excellent foundational circuit theory.

PT vs. CPT: The Cost of Confusion

A frequent and expensive mistake on the jobsite is using a Control Power Transformer (CPT) when a Potential Transformer (PT) is required, or vice versa. While both step down medium voltage, their internal design priorities are completely different. As noted in Electrical Engineering Portal's breakdown of transformer types, confusing the two will result in either inaccurate billing or blown fuses.

Potential Transformer (PT) vs. Control Power Transformer (CPT)
Feature Potential Transformer (PT) Control Power Transformer (CPT)
Primary Purpose Precision measurement and relay input Supplying operating power to loads
Voltage Regulation Tight (maintains exact ratio up to rated burden) Loose (voltage may drop 5-10% under load)
Inrush Capability Low (designed for high-impedance metering loads) High (designed to handle contactor coil inrush)
Phase Angle Error Strictly controlled (critical for kW/kVAR metering) Not controlled or guaranteed

When to choose which: If you are wiring a kWh revenue meter, a power quality analyzer, or an ANSI 59/27 protective relay, you must use a PT. If you are powering a 120V space heater in the switchgear, a motorized disconnect switch, or a SCADA radio, use a CPT. Never wire a heavy control load to a PT secondary; the inrush current will instantly blow the secondary limiting fuses or saturate the core.

Frequently Asked Questions

Do I need to fuse the secondary side of a medium voltage PT?
Yes. IEEE and NEC-style guidance strongly recommend secondary fusing (typically 3A to 5A dual-element time-delay fuses) to protect the PT windings from short circuits in the secondary wiring. A shorted secondary will cause the PT to draw massive primary current, potentially destroying the epoxy casting and causing a medium-voltage fault.

Can I leave a PT secondary open-circuited?
Yes. Unlike Current Transformers (CTs)—which will generate lethal, insulation-destroying voltages if open-circuited under load—a PT is a parallel-connected voltage source. Leaving the secondary open simply means zero current flows, and the PT operates safely at no-load. However, you should never short-circuit a PT secondary.

Why do some PTs have two secondary windings?
Dual-secondary PTs are common in utility metering. The first winding (usually 0.3 accuracy class) is dedicated exclusively to the revenue meter and is sealed by the utility. The second winding (usually 1.2 accuracy class) feeds the customer's protective relays and SCADA telemetry, ensuring that relay testing or wiring faults do not interrupt or tamper with the billing meter circuit.