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Emergency Diesel Generator Set Neutral Earthing

M 90

Hello All,

I am currently working in a project where the main supply source (MV/LV transformers) neutral point is isolated from earth (Through very high resistance), hence IT earthing system is adopted for the electrical network. Beside the main source, an emergency diesel generator set is provided and connected to the same network to supply the critical loads in case of main power cut off/failure. Generally, when selecting the earthing system for generator neutral point, I follow the same system applied for the main source. But, I have read some advices which states that it's not preferable to go with IT systems with generator sets, nothing concrete, but I don't feel confident adopting such solution as I personally never experienced such system with a generator.

The question is : Is it safe to Isolate the neutral point of the generator set -following the main source earthing system-? If no, is it allowable to propose two different earthing systems within the same network, one at normal operation and the other at Emergency operation ?

Thanks in advance for your help

  • 0

    I guess the first question is why has IT been adopted for the main supply? (it's an unusual choice, especially for large systems). If it's for availability during 1st faults for example (e..g. for some life critical systems), then you many well feel obliged to maintain the same kind of resilience when running from generator.

    I don't know of any reason in principle why IT shouldn't be adopted for generators - it's close relative - separated supplies - are commonly used for small (<3kVA) portable generators.

       - Andy.

  • 0

    I don't understand why an IT earth system should be different from a generator earth system
    Can someone explain?

  • 0 in reply to Pedro Osorio

    An IT earth system isn't earthed.  There's has no connection between neutral and earth at any point.

    IT is very unusual for anything except small installations run off a portable generator, solar panels, or similar.

    (corrected)

  • 0

    An IT earth system isn't earthed. 

    The exposed-conductive-parts on an IT system are earthed (hence the "T")  - (as might the supply N - via a suitably large impedance).

    I don't understand why an IT earth system should be different from a generator earth system

    The earthing system (i.e. the connections of c.p.c.s,  exposed- and extraneous-conductive-parts etc with perhaps a local earth Electrode) is normally common to an entire installation (or at least a building) and shared by both the normal and generator supply. The system (e.g. TT, TN, IT etc) might make use of that in different ways at different times though. E.g. an installation might be TT when supplied by the grid, but become TN when running off a backup generator.

      - Andy.

  • 0

      Thanks for your reply, the IT earthing system is adopted as the transformer supplies critical loads. Hence the availability shall be ensured during 1st fault condition

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