Variable Threshold Neutral Isolator
Since 1983, Dairyland has offered the Neutral Isolator as the preferred solution for addressing "stray voltage" on farms. Power utilities who wish to isolate their primary neutral from the customer's secondary neutral can prevent undesirable primary neutral-to-earth voltage (NEV) from affecting the customer system. Where utilities have concerns about NEV effects on dairy or other livestock operations, the Neutral Isolator offers the most effective and safe solution for separating the two systems under normal conditions, yet solidly re-bonding the two ground grids under lightning or AC fault conditions.
Note: The Dairyland model VTNI is not authorized for use in solving nuisance shock to persons at swimming pools or similar human health situations where structure to ground voltage is present. Instead, proper bonding and grounding techniques need to be applied to solve such problems.
Features and Characteristics
- Solid-state, fail-safe design
- Adequate voltage threshold of 45V to avoid nuisance switching
- Variable threshold design blocks transients
- Meets NESC 97D2 for isolation
- High impedance to AC and DC current when in the blocking mode
Note: Due to the installation process for these devices, VTNI units are sold only to power utility companies, or their distributors. Please contact us if you have questions in this regard.
- threshold voltage
- 45V peak
(Higher for transients - see technical literature for values)
- lightning surge current
- 30kA crest (8 x 20 µs waveform)
- environmental rating
- NEMA 4X
|AC Fault Current (amperes-rms) 60 Hz|
|Model||1 cycle||3 cycles||10 cycles||30 cycles|
Stray Voltage Mitigation
The Dairyland VT/NI, used for blocking neutral-to-earth voltage on a utilityâ€™s primary neutral from the customerâ€™s secondary neutral, has significant advantages over other technologies, including metal oxide varistors. These include:
The VT/NI operates at set thresholds that are above normal neutral-to-earth values, but low enough to safely re-bond the primary and secondary grounded neutrals during an AC fault or lightning event. The product differentiates between 60Hz events and higher frequency transients, to present an appropriate voltage blocking level. Metal oxide varistors have a stated voltage breakdown (threshold) level that is usually in the range of several hundred volts, for devices that have adequate lightning surge current capability. As the lightning surge current rating goes up, the threshold associated with that rating also increases. This can result in breakdown levels that are quite high â€“ in the range of 300-700V â€“ in order to have adequate surge capability. The VT/NI has a high lightning surge capability with a relatively low and safe threshold.
AC Fault Ratings:
The VT/NI has been fully tested at power laboratories to verify the product ratings and performance, and is rated for stated AC fault current magnitudes and durations. These ratings represent values in excess of the typical utility distribution system available fault current. Since the isolation device is connected between the primary and secondary neutrals, AC fault current can flow in this bond if the distribution transformer were to suffer a primary to secondary winding fault. Metal oxide varistors are not rated for AC fault current, only lightning surge current, and will fail under AC fault conditions. The typical failure mode is open circuit, with the varistor case rupturing due to the fault energy. In such a case, no further overvoltage protection is offered by the failed product. In contrast, the VT/NI will fail shorted if exposed to current/time beyond the product capability, providing continuing overvoltage protection.
Having been verified in power laboratories, the VT/NI will provide fail-safe operation when installed in distribution systems. Test units were subjected to various current/time combinations to determine the product ratings, and to test the products to failure. All failures occur in the shorted mode, thereby re-bonding the primary and secondary grounding systems for safety. Further tests to values well in excess of the failure level on previously failed units resulted in the product remaining as a dead short. The failure mode of metal oxide varistors is predominantly as an open circuit, affording no further protection.
Note: For other VTNI models or quantities, contact Dairyland for more information.