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Two 11Kv/0.4 TX's on a PWN from a single DNO POC on an industrial site. What demarcation should be considered for earthing, EMC and circulating neutral currents where phase in-balance or harmonics?

Rich Mc

We have two 11Kv/0.4 Transformers configured on a PWN from a single DNO POC on an industrial site. The neutral point of each TX are commonly bonded at the MV side. Each TX has its own earth mat consisting of multiple electrodes. Both mats are electrically connected via buried bare  copper conductor between (100m apart). 

Originally the site had one TX feeding one building. Due to a recent building addition there are now two supplies effectively feeding one larger building. Both buildings are steel framed and are joined at the structures. Couldn't upgrade original TX due to increased load required.

What demarcation if any, is needed for earthing within the steel framed buildings? Will the two buildings (into one) be considered separate earthing arrangements or due to the MV side of both being bonded single earthing arrangement - 542.1.3.3 appears to contradict? 

Several other issues also spring to mind including adequacy of protective devices, circulating neutral currents and EMC. 

Many thanks in advance for any thoughts

Parents

  • The TN-C-S or TN-S grounding system, which provides protection against electric shock, is recommended for grounding steel-framed buildings. In this system, the grounding conductor and the neutral conductor are separated, and the building ground is connected to the grounding conductor and the neutral conductor to the neutral conductor. Since both transformers share a common grounding system, both buildings are considered one ground as required by 542.1.3.3. This means that all protective devices, such as fuses and circuit breakers, must be selected for the maximum load of both buildings. If a TN-C-S grounding system is used in steel-framed buildings, circulating neutral currents may occur, which can cause EMC problems. In such a case, the use of a TN-S grounding system, which prevents circulating neutral currents, is recommended. It is also important to make sure that the earthing system has a low enough earthing resistance to provide effective protection against electric shocks. Regular ground resistance measurements are recommended to monitor the quality of the grounding system.

Reply

  • The TN-C-S or TN-S grounding system, which provides protection against electric shock, is recommended for grounding steel-framed buildings. In this system, the grounding conductor and the neutral conductor are separated, and the building ground is connected to the grounding conductor and the neutral conductor to the neutral conductor. Since both transformers share a common grounding system, both buildings are considered one ground as required by 542.1.3.3. This means that all protective devices, such as fuses and circuit breakers, must be selected for the maximum load of both buildings. If a TN-C-S grounding system is used in steel-framed buildings, circulating neutral currents may occur, which can cause EMC problems. In such a case, the use of a TN-S grounding system, which prevents circulating neutral currents, is recommended. It is also important to make sure that the earthing system has a low enough earthing resistance to provide effective protection against electric shocks. Regular ground resistance measurements are recommended to monitor the quality of the grounding system.

Children
  • in reply to Scott Raymond

    Thanks Scott, I'd agree, and it is the case that both 'supplies' share a common ground/earth. The system is TN-S. 

    We should have impedance readings from then the earth mats and connecting cables were installed. We shall also check on site resistance between the two systems on the LV side which should give us a better indication of any issues. 

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