[Dale]

Recently we have carried out the periodic inspection in a manufacturing plant. This plant has a 6.6kv network supplying 3 remote buildings each with transformers. We found that the Zs readings throughout the installation in one of the buildings were high compared to previous inspection reports.
On speaking to the Site Engineer he informed me that they had a fault on the 6.6kv feeder cable to that particular building. The cable had blown but along side the MV cable was a supplementary earth which he feels may not of been repaired.
This then made me think that the earth and neutral were not strapped on the LV side of the transformer.
On further iinspection the neutral earth link had been left in the open position on the live side of the main switchgear.
My question is is there anty reason for this or is it an oversight from the electrical contractor who installed the original supply?
Looking forward to your comments.

Topic

[Hamid]

1-How did you read the mentioned Zs? 2-The vector group of mentioned transformer is Dyn11 probably, is it correct? 3-If yes, the supplementary earth conductor of MV cable is not a part of primary earthing system; because the primary delta connection of same transformers are ungrounded ordinary and it has used as cable shielding probably. 4-The secondary earthing system selection (TNS, TN-CS,) could not affect the primary impedance. 5-The electrical loss connection due to electrodynamics short circuit forces and some earth path born occurring due to sever short circuit is possible and these phenomenon can be caused that Zs increasing.

[Dale]

1.Various Zs readings were taken throughtout the installation were taken a loop impedance tester and they were all high compared to the previous recorder results. 2. The vector group of the the transformer is Dyn 11 3. yes but maybe the earth passed through the transformer housing down the supplementary earth back to the 6.6kv switchgear and was strapped down to neutral at another transformer 4. So if that is the case then it is an oversight on the electrical contractor and because they had a reasonable earth loop reading they never thgought to strap it down to neutral 5. therefore we are correct to strap down on the LV side and not have the supplementary earth repaired

[Hamid]

1-It is clear, when a system is designed as a TNC grounding system, the separation of neutral and ground bars may increase the impedance of system ground loop; because in TNC system the neutral path is a part of grounding system. 2- Good grounding path of sufficiently low impedance ensures fast clearing of faults. A fault remaining in the system for long may cause several problems including those of power system stability. Faster clearing thus improves overall reliability. It also ensures safety. A ground fault in equipment causes the metallic enclosure potential to rise above the

[Author]

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