Bushing Potential Devices and Cables

A bushing potential device (also called a bushing voltage tap, potential tap, or bushing monitor) is an electrical accessory attached to a transformer or other high-voltage equipment bushing to measure the bushing’s electrical potential (voltage) and often to provide insulation monitoring. It provides a safe, low-voltage representation of the high voltage present on the conductor that passes through the bushing, allowing operators and protection/monitoring systems to monitor the bushing condition and detect developing insulation problems.

PSC has access to the complete technical records for all General Electric bushing potential devices, such as GE KA-108, ensuring the preservation of critical design aspects while providing new accessories that enhance efficiency and offer cost savings in generating plants, substations, and transmission lines. 

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Bushing Potential Devices:

Capacity or internal tap devices that provide a scaled, earth-referenced voltage from the energized bushing conductor. Ideal for relays, meters, SCADA, and condition monitoring.

  • Output options: 1–100 V AC (typical), 0–10 V, 4–20 mA, digital/fiber‑optic via active transducers
  • Accuracy: passive ±0.5–5%; active ±0.1–1%
  • Frequency: 50/60 Hz; low phase error
  • Fitment: integral taps, external/retrofit units, active electronic models

Bushing Potential Device Cables:

Shielded, low-noise cables engineered for bushing taps and monitor connections - maintain signal integrity and insulation coordination.

  • Ratings: voltage class matched, high dielectric strength, UV/oil resistant.
  • Construction: shielded/armored options, low capacitance, high EMI immunity.
  • Terminations: pre-terminated connectors, ring lugs, customizable lengths
  • Compliance: IEC/IEEE insulation and creepage/clearance considerations

How it Works

A bushing potential device (bushing voltage tap) forms a safe capacitive divider or uses an internal condenser tap to deliver a low‑voltage signal proportional to the bushing HV. Pairing the device with the correct cable ensures accuracy and noise immunity for meters, relays, and monitoring systems. Typical outputs: 1–100 V AC; active outputs: 0–10 V, 4–20 mA, digital/fiber‑optic.

Applications include power transformers, EHV/HV bushings, metering inputs, protection relays, and online condition monitoring.

  • Transformer voltage class and bushing type
  • Desired output (AC V, 0-10V, 4-20 mA, digital)
  • Relay/meter input impedance and burden
  • Cable length, shielding, termination style, environment rating

For units, choose between passive and active transducers (based on accuracy and isolation needs). Specify nominal output and frequency (50/60 Hz). Define accuracy/phase error requirements.

For cables, specify length and routing environment. Select shielding/armor level and jacket material. Select terminations (ring lugs, connectors), and color code.

Typical Output Ranges

Passive (capacitive tap direct output)

  • Low-voltage AC output proportional to HV, commonly in the following ranges:
  • 1 V to 100 V AC (most common: 1 V, 10 V, 50 V, 100 V outputs depending on tap ratio and application)
  • Example: a 400 kV bushing with a 1000:1 division would give ~400 V on the tap; smaller division ratios yield lower voltages.

Test taps often present a few volts to a few tens of volts for direct portable meter connection.

Standardized metering outputs

  • Some installations use standardized outputs to interface with relays/meters, such as 100 V or 110 V nominal tap voltages (less common than for instrument transformers but possible).

Electronic/transducer outputs

Voltage outputs:

  • 0-10 V AC or DC (for conditioned signals)
  • 0-5 V DC (less common)

Current outputs:

  • 4-20 mA (common for industrial monitoring loops)

Digital outputs:

  •  Modbus, Ethernet, IEC 61850 reporting, or proprietary protocols when integrated with electronic condition-monitoring units.

Isolated/fibre-optic outputs:

  • Optical transmitters providing scaled digital or analog representations; useful where galvanic isolation is needed or in noisy environments.