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Grounding question No.13 - Neutral to earth connection
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.
Author : Dale - From: England
 
#1
Tue, April 7, 2009 - 17:37
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.
 
Author : Hamid - From: IRAN
 
#2
Wed, April 8, 2009 - 15:39
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 
Author : Dale - From: England
 
#3
Wed, April 8, 2009 - 19:21
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 ‘true’ ground potential. An improper grounding results in a higher potential and also results in delayed clearing of the fault (due to insufficient current flow). This combination is essentially unsafe because any person coming into contact with the enclosure is exposed to higher potentials for a longer duration. Therefore, substation reliability and safety must be as ‘built-in’ as possible by good grounding scheme, which in turn will ensure faster fault clearing and low enclosure potential rise.

3- Conductors must be large enough to handle any anticipated faults without fusing (melting).
Failure to use proper fault time in design calculations creates a high risk of melted conductors. Two aspects govern the choice of conductor size: the first is the fault current that will flow through the conductor and the second is the time for which it can flow. The fault current depends on the impedance of the ground fault loop. The time of current flow is decided by the setting of the protective relays/circuit-breaking devices, which will operate to clear the fault. The IEEE 80 suggests using a time of 3.0 s for the design of small substations. This time is also equal to the short-time rating of most switchgear.

4- It is very evident that the connections between conductors and the main grid and between the grid and ground rods are as important as the conductors themselves in maintaining a permanent low-resistance path to ground. The basic issues here are:

-The type of bond used for the connection of the conductor in its run, with the ground grid and with the ground rod
-The temperature limits, which a joint can withstand

The most frequently used grounding connections are mechanical pressure type (which will include bolted, compression and wedge-type construction) and exothermically welded type. Pressure-type connections produce a mechanical bond between conductor and connector by means of a tightened bolt-nut or by crimping using hydraulic or mechanical pressure. This connection either holds the conductors in place or squeezes them together, providing surface-to-surface contact with the exposed conductor strands.
On the other hand, the exothermic process fuses the conductor ends together to form a molecular bond between all strands of the conductor.
Temperature limits are stated in standards such as IEEE 80 and IEEE 837 for different types of joints based on the joint resistance normally obtainable with each type.
Exceeding these temperatures during flow of fault currents may result in damage to the joint and cause the joint resistance to increase, which will result in further overheating.
The joint will ultimately fail and result in grounding system degradation or total loss of ground reference with disastrous results.
 
Author : Hamid - From: IRAN
 
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