There are some differing opinions among my peers regarding high resistance grounding of low voltage (480) delta systems. Nearly 15 years ago our plant upgraded several ungrounded delta systems by installing ground detectors. The detectors were GE, Seimans and Powercon. All of these detectors used a WYE transformer and power resistor to derive a neutral. I contend that this subjected our system to section 250.36 of the NEC. Others argue that since the "instrument" was only installed to provide detection of the ground, the system is still "Ungrounded". I am looking for more written word to support either case.
High Impedence Grounding of Delta System
- Thread starter G Man
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Re: High Impedence Grounding of Delta System
So small instrument transformers were added for ground fault detection, and the secondary of these transformers was grounded? If that is correct, your 480V system is still ungrounded. The secondary of the added transformers has no effect on the grounding of the primary.
Steve
So small instrument transformers were added for ground fault detection, and the secondary of these transformers was grounded? If that is correct, your 480V system is still ungrounded. The secondary of the added transformers has no effect on the grounding of the primary.
Steve
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: High Impedence Grounding of Delta System
There were several transformer methods used for develeoping an artificial neutral. One method was to connect the primary of a transformer into a grounded wye and use a 120V delta connected secondary for the detection circuit. Another method used a similar wye connected primary but the secondary delta was not connected to any load.
These methods did not change the source into a "wye" therefore no system neutral was created which meant that (B) could not apply so these installations were not subject to 250.36.
There were several transformer methods used for develeoping an artificial neutral. One method was to connect the primary of a transformer into a grounded wye and use a 120V delta connected secondary for the detection circuit. Another method used a similar wye connected primary but the secondary delta was not connected to any load.
These methods did not change the source into a "wye" therefore no system neutral was created which meant that (B) could not apply so these installations were not subject to 250.36.
Re: High Impedence Grounding of Delta System
Thanks for the quick response. I probably need to provide a bit more information. The switchboard sold with the manufacturers label as a high resistance ground detection system, contains a wye-delta transformer whose secondary is open and whose primary is connected to each phase of the ungrounded delta supply. The primary of the wye is grounded through a resistor that supposed to be set to the charging current of the system. I assumed that this provided high resistance grounding as described in the green book for existing delta's and followed 250.36 of the NEC "where a neutral is not available..." However, I may be reading too much into this.
Thanks for the quick response. I probably need to provide a bit more information. The switchboard sold with the manufacturers label as a high resistance ground detection system, contains a wye-delta transformer whose secondary is open and whose primary is connected to each phase of the ungrounded delta supply. The primary of the wye is grounded through a resistor that supposed to be set to the charging current of the system. I assumed that this provided high resistance grounding as described in the green book for existing delta's and followed 250.36 of the NEC "where a neutral is not available..." However, I may be reading too much into this.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: High Impedence Grounding of Delta System
What you are describing is similar to an artifcial neutral. It's intent was to provide a high resistance (usually it included a pulsing system) ground refence for trouble shooting an ungrounded system. It was not intended to act as a ground fault return path to the source. If your system source does not include a neutral point then 250.36 does not apply unless the transformer was installed as a "grounding" transformer (like a zig-zag unit).
What you are describing is similar to an artifcial neutral. It's intent was to provide a high resistance (usually it included a pulsing system) ground refence for trouble shooting an ungrounded system. It was not intended to act as a ground fault return path to the source. If your system source does not include a neutral point then 250.36 does not apply unless the transformer was installed as a "grounding" transformer (like a zig-zag unit).
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