A potential transformer (PT), also known as a voltage transformer (VT), is an instrument transformer designed to step down high system voltages to a safe, standardized low voltage—typically 120V—for metering and protective relaying. In a real high-voltage installation, a PT changes an unmanageable primary voltage (like 13,800V) into a proportional, galvanically isolated secondary voltage that standard 120V-rated digital multimeters, revenue meters, and microprocessor relays can safely read. Beginners and even some junior technicians commonly confuse PTs with current transformers (CTs) or standard step-down control transformers; however, while a CT steps down current and must never be open-circuited, a PT steps down voltage and must never be short-circuited.
Standard Potential Transformer Ratios and Accuracy Classes
Unlike distribution transformers that are optimized for maximum power transfer and efficiency, potential transformers are engineered for extreme measurement precision. The governing standard in North America is IEEE C57.13 (Standard Requirements for Instrument Transformers), which dictates strict accuracy classes and thermal burden limits. The accuracy class defines the maximum permissible percentage error in voltage magnitude and phase angle under a specified burden.
| Primary Voltage (Nominal) | Secondary Voltage | Turns Ratio | Accuracy Class | Typical Application |
|---|---|---|---|---|
| 12,470V | 120V | 103.9 : 1 | 0.3 | Utility Revenue Metering (Billing) |
| 13,800V | 120V | 115 : 1 | 0.6 | Protective Relaying (ANSI 27/59) |
| 24,940V | 120V | 207.8 : 1 | 0.3 | Substation Transmission Metering |
| 4,160V | 120V | 34.7 : 1 | 1.2 | Industrial Motor Protection / Sync-Check |
| 34,500V | 115V | 300 : 1 | 0.6 | High-Voltage Transmission Relaying |
Reading the Accuracy Class: A class 0.3 PT guarantees that the secondary voltage will be within 0.3% of the true proportional primary voltage at the rated burden. This tight tolerance is legally required for utility revenue metering because a 1% error on a 10MW industrial feed translates to massive billing discrepancies over a year. Class 0.6 or 1.2 is acceptable for protective relaying, where the relay only needs to know if the voltage has crossed a trip threshold (e.g., dropping below 80% nominal) rather than measuring exact watt-hours.
Worked Numeric Example: Calculating Primary Voltage and Burden
Let’s look at a real-world scenario on a 13.8kV distribution feeder. You are commissioning a new microprocessor relay and need to verify the PT secondary wiring and burden.
- PT Nameplate: 13,800V / 120V, 200VA, 60Hz, Class 0.6, Thermal Rating Factor 1.5
- Secondary Meter Reading: 116.2V AC
- Wiring: 500 feet of #12 AWG solid copper (round-trip 1000 ft)
- Relay Burden: 10VA
1. Finding the True Primary Voltage
First, calculate the exact ratio: 13,800 / 120 = 115.
Multiply the secondary reading by the ratio: 116.2V × 115 = 13,363V.
The actual primary line-to-ground voltage is 13,363V. Because the nominal is 13,800V, the system is currently running about 3.1% low, which is well within standard utility tolerances.
2. Verifying the Burden Limit
If you overload a PT (exceed its VA rating), the core saturates, the voltage drops, and you lose your 0.6 accuracy class. We must calculate the total burden in Volt-Amps (VA).
- Relay Burden: 10 VA
- Wire Resistance: #12 AWG copper is roughly 1.588 ohms per 1000 ft at 20°C. For a 500 ft run (1000 ft round trip), resistance
R = 1.588 Ω. - Secondary Current: Assuming nominal 120V,
I = V / Z. But it's easier to calculate wire VA loss directly using the nominal current. If the total connected load is roughly 50VA,I = 50VA / 120V = 0.416A. - Wire VA Loss:
I² × R = (0.416)² × 1.588 = 0.275 VA.
Total Burden: 10 VA (relay) + 0.275 VA (wire) = 10.275 VA.
Because 10.275 VA is vastly lower than the PT’s 200VA nameplate rating, the PT will easily maintain its 0.6 accuracy class. If you were using an older electromechanical relay drawing 150VA, you would need to upgrade the wire to #10 AWG to reduce line losses and keep the total burden under 200VA.
Where You Meet Potential Transformers in Practice
You will rarely see a PT on a standard residential or commercial branch circuit. They are strictly medium-voltage (MV) and high-voltage (HV) components. Here is where they show up in the field:
Utility Revenue Metering Cabinets
On any commercial building with a primary service (e.g., 12.47kV directly entering the building), the utility will install a metering cabinet containing both CTs and PTs. The PTs step the 12.47kV down to 120V so the utility’s revenue meter can calculate kWh. These PTs are sealed by the utility; breaking the seal is a criminal offense in most jurisdictions.
Protective Relaying (ANSI 27 and 59)
In industrial switchgear and solar farm substations, PTs feed voltage data to protective relays. An ANSI 27 (Undervoltage) relay uses the PT to detect a grid brownout and trip the main breaker to prevent large motors from stalling and overheating. An ANSI 59 (Overvoltage) relay uses the PT to detect a blown neutral or ferroresonance condition that could destroy facility equipment.
Generator and Inverter Synchronizing
Before closing a breaker to connect a backup diesel generator or a battery energy storage system (BESS) to the grid, a sync-check relay (ANSI 25) uses PTs on both the bus side and the generator side. The relay compares the voltage magnitude, phase angle, and frequency of both PT secondaries. If the PTs show the two sources are out of phase, the relay blocks the breaker from closing, preventing a catastrophic out-of-phase synchronization event.
Potential Transformer FAQ and Field Troubleshooting
Can I use a PT as a control transformer to power contactors?
No. This is a dangerous and common mistake. A control transformer is designed to handle high inrush currents (like pulling in a heavy contactor coil) and typically ranges from 150VA to 2000VA. A PT is designed for low, steady-state instrument burdens (typically 50VA to 500VA). If you use a PT to energize a contactor, the massive inrush current will instantly saturate the PT core, drop the voltage to near zero, overheat the windings, and likely cause a catastrophic failure or fire.
What happens if I accidentally short-circuit the secondary of a PT?
A PT acts as a stiff voltage source. If you short the secondary terminals, Ohm’s law dictates that current will spike toward infinity (I = V / 0). This will instantly vaporize the secondary windings, explode the epoxy casing, and destroy the PT. This is why PT secondaries are always protected by high-interrupting-capacity (HRC) fuses or miniature circuit breakers, whereas CT secondaries must never be fused (because an open CT circuit generates lethal high voltage).
Why are my PT secondary fuses blowing repeatedly?
If you are replacing blown PT fuses on a medium-voltage switchgear lineup, do not just install a larger fuse. Repeated blowing indicates one of three faults:
- Overburden: Someone added a new meter or transducer to the secondary circuit, pushing the total VA load past the PT’s thermal rating.
- Ground Fault on Secondary: A nicked wire in the relay cabinet is shorting the 120V secondary to the grounded cabinet chassis.
- Ferroresonance: Common on ungrounded delta systems, where the non-linear inductance of the PT interacts with system capacitance, causing massive current spikes. This requires installing secondary damping resistors or using specialized ferroresonant-suppression PTs.
For deeper troubleshooting and testing procedures on instrument transformers in the field, refer to the NETA Acceptance Testing Specifications (ATS), which outlines the exact insulation resistance and ratio test thresholds required before energizing medium-voltage PTs.
