IT earthing system: FOR or AGAINST?

Home Electrical Engineering Forum General Discussion IT earthing system: FOR or AGAINST?

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  • #10598
    Laurent
    Participant

    The IT earthing system, when used properly and in the relevant electrical installations, provides numerous advantages, despite the misconceptions held against it. The main advantage is that it increases the availability of the installation. It helps reduce the risk of fires and explosions. It increases the lifespan of the electrical equipment. It also facilitates preventive maintenance and at the same time optimizes the overall maintenance costs, which means that the initial extra-costs are paid back.
      

    With IT earthing system, the power supply has no point connected with earth (source: Wikipedia)

    The IT earthing system allows you to increase the availability of your installation

    This is the first and main benefit of the IT earthing system. With the TN system, an insulation fault corresponds to a short circuit, so the protection device will trip (thus part of the installation is stopped), and the fault current is very high. In comparison, with the IT earthing system, in presence of an insulation fault the current cannot loop via the transformer’s neutral, so the first fault current value remains very low.But to take full benefit of it, the IT earthing system should be used effectively. Generally, an electrical network completely using the IT earthing system might be difficult to manage, especially while locating the first faults.

    The optimum economic arrangement, which also allows easier maintenance, often consists of a TN system with islands of IT systems for circuits that really require this availability:
    – Continuous process,
    – Safety circuits,
    – Some lighting circuits ….

    The initial additional cost is quickly recovered, especially when you compare to the operating losses during the shutdown, which may be very significant.
    Also, manufacturers now provide offers to implement IT system much more easily, and also devices that enable automatic location of faults.

    The IT earthing system allows you to reduce the risk of fire or explosions

    Many fires are started by high local over-heating or by an electric arc due to an insulation fault. The higher the fault current, the greater is the risk: a contact between a live conductor and a metal part may, particularly in sensitive areas, start a fire when the fault current exceeds 500 mA.For the risk of explosion it is the same approach. This risk is particularly present in mines, silos, oil rigs and chemical industry.
     

    The IT earthing system allows you to increase the lifespan of your equipment

    Due to the very limited fault currents in IT systems, the electrical equipment is subjected to less stress, so its life duration is increased.A high fault current can result in significant damages to the installation, or to the loads, which would therefore require more cost and time for repairs.
    The gain is difficult to estimate, but it is certain that the aging of the electrical installation is largely related to these phenomena.

    The IT earthing system allows you to optimize the maintenance

    Even in a non-continuous process, the tripping of a circuit breaker requires the intervention of the maintenance department.
    With the IT earthing system, the first fault does not cause any tripping.
    It is therefore possible to optimize the maintenance, as there is no need to have maintenance people on site at all times: at the beginning of each day the maintenance department can check for the presence of a fault.
    Protection devices will only trip if a second fault occurs before the first fault is cleared, which means that after a first fault, an IT system behaves like a TN system.Additional costs of the IT system (transformer, insulation monitoring devices) are quickly paid back by these direct savings in maintenance and productivity.

    The IT system facilitates your preventive maintenance

    The IT system allows for good visibility on the electrical network. Even when there is no fault, monitoring the insulation resistance enables to detect the degradation of the insulation of a circuit or a device, so it becomes possible to act before an actual fault occurs. Like other measurements, the insulation resistance is a good indicator to follow the electrical installation condition. 

    And what about you?
    Are you FOR or AGAINST the IT earthing system?

    #12412
    Anonymous
    Guest

    I work in the maintenance team of an industrial site, which is using IT earthing system on most of the electrical installation. My experience is that this IT earthing scheme is difficult to maintain, because whenever there is a fault, it takes us a very long time to locate this fault. We are using clamp-on meters which are difficult to use when you have to clamp on several rigid cables. As there are many circuits to check the fault localization may take us several hours. I find that aspect a major drawback of the IT earthing system.

    #12287
    Anonymous
    Guest

    I think IT has it’s applications. Previously I have used an IT system on a materials handing plant where downtime was costly. On detection of one motor fault the system could continue to run until a second fault was detected (with the faulted motor being replaced when system demand was not high). It is more difficult to implement and we had to use earth fault monitoring and Petersen coils. While I would use it (and can see benefits) on a special installation like this one, I would generally prefer a TN or TT system on more common installations.

    #12288
    Spir Georges GHALI
    Participant

    Certainly, I'm for The ” IT Earthing System “, especially in some places like :

    – Hospitals ” Operation Rooms ” as we can't accept the tripping of electricity once an earth fault occurs.

    – In the areas where the explosion's risk is high.

    – In some applications where the continuity of service is very important.

     

    By the way, the fault's location management is not easy, but if we install at the beginning the necessary products of ” IMD ” it will be more easier, because if we have many outputs after the Isolation Transformer, we can use the ” IMD ” equipments that's composed of ” one Injection equipment ” and ” Detectors equipments with Torroids ” where the number of them is suitable for the outputs' number, so, this combination can detect and locate the output where the fault occurs.

    #12286
    Laurent
    Participant

    @ Chris480, @ Spir Georges GHALI: yes several manufacturers propose some fault localization systems. Generally, to have a balance between initial investment costs and fault localtion costs, the torroids are installed from the beginning, but only on the main circuits. These torroids are connected to the fault detection modules which enable to know instantly on which main circuit the fault is located. Then maintenance people just have to focus on the secondary circuits of that zone, and use clamp meters to find the exact location of the fault, which is faster and easier as secondary circuits have smaller cables. Of course, it is always possible to install more torroids and fault detection modules, to make the fault detection even faster.

    #12413
    Anonymous
    Guest

    In my opinion that the grounding system is useful for all types of buildings because it helps in the protection of electrical and mechanical structures and thus to protect the building itself and the people who live in or use of these buildings from the dangers of exposure to fire or electric shock and all this from personal experience in this area

    #12431
    admin
    Keymaster

    If you are looking for warratie the continous service of the plant, IT Earthing system is the solution, but the main problems are:

    – Once your have the first failure, its dificult/expensive to find where is the fault.

    – The defect current is low, but the voltage arrise is high, so the life of the components will be longer by the lower temperature stress, but the life of the component will be shorter due the isolating stress. 400V will not have problem, but 690V could be.

    – Every electronical component which has a electromagnetic compatability filter (mandatory in Europe) send high frequency current to earth. If the installation has many electronic components you must set your isolating meter at a value that cannot detect a real isolating defect.

    #12441
    Anonymous
    Guest

    I think IT earthling system is not good option whenever the safety is the important factor. TN system is more reliable from the safety point of view. TN system cause the over current protection trips the circuit immediately after the first fault which is not the case in IT system as mentioned above. In my opinion another disadvantage of IT system is that if the second fault occurs at the same phase but in another place it still doesn’t trip the circuit.

    #12443
    Anonymous
    Guest

    IT system has advantages and also disadvantages. It should be only used in special appplications ….separated operation units like in hospitals, ex-zones (like military storage or production,etc) separated fom power supply TN system by a safety insulated transformer.

    The main problem is that in IT systems the voltage between phase to ground (important for SPDs) can reach up to phase voltage due to no grounded N of transformer.

    3-phase IT system with 230V 50Hz was first installed in Norway because of very bad grounding resistance (rocks only !) ….The have a lot of problems with this IT net and thanks to the availability of earthing resistance enhanced materials like GEM from ERICO International (www.erico.com) Norwegian power supply companies are now able to change to higher safety TN net (TN-C, TN-C-S) which becomes standard in Europe. TT net disappear and IT net only installed for special applications.
    Sectional IT systems with not grounded transformers are allways accompanied by RCD (human protection)and by insulation fault detection devices (www.bender-de.com),
    For SPD installed in 230V IT systems, the rated voltage (max. allowed continuous SPD operating voltage…between L-PE) must be setted to phase voltage x 1,73….230V +10% x1,73 = 440V AC rated voltage for SPD.
    In IT systems phase wires must be distanced min 0,7m to 1m from grounded metal parts and well insulated to avoid a flash over between phase wires and ground in case of a lightning surge. In IT systems a potential equalisation between all wires entering a device our function unit is very important to avoid serious damages due to lightning events.

    Using special LEUTRON SPD suitable for IT systems and installation according LEUTRON SEP (Single Entry point installation principle) like PowerPro serious and EPF (EnerPro +low-pass Filter) will safe your equipment against damages caused by lightning surges and HF-harmonics.

    #12438

    In an IT network, the distribution system has no connection to earth at all, or it has only a high impedance connection. In such systems, an insulation monitoring device is used to monitor the impedance. there are system that suit this particular technology. while
    TT network
    In a TT earthing system, the protective earth connection of the consumer is provided by a local connection to earth, independent of any earth connection at the generator.

    The big advantage of the TT earthing system is the fact that it is clear of high and low frequency noises that come through the neutral wire from various electrical equipment connected to it. This is why TT has always been preferable for special applications like telecommunication sites that benefit from the interference-free earthing. Also, TT does not have the risk of a broken neutral.

    In locations where power is distributed overhead and TT is used, installation earth conductors are not at risk should any overhead distribution conductor be fractured by, say, a fallen tree or branch.

    In pre-RCD era, the TT earthing system was unattractive for general use because of its worse capability of accepting high currents in case of a live-to-PE short circuit (in comparison with TN systems). But as residual current devices mitigate this disadvantage, the TT earthing system becomes attractive for premises where all AC power circuits are RCD-protected.

    #12459
    Spir Georges GHALI
    Participant

    Dears ;

     

    As we have many types of ” Earthing System “, and each one of them has advantages & disadvantages, so, Mr. Laurent mentioned to the advantages of ” IT system ” that are :

    – Too limited value of the 1st fault's current that means long life of equipments.

    – The continuity of service : very very important in some applications like Hospitals.

    – By using the ” IMD “, we will have an idea about the insulation's level of the networks that means : optimizing of maintenance.

    – Ext…

    These advantages can't be realized by another system.

     

    Certainly this system is expensive and in some cases ( depending on realisation ) we spend some time to locate the place of the fault, but we need it in some applications. As we have, for 3 phases IT, 2 main types ” IT without distribution of Neutral  &  IT with distribution of Neutral ” ( depending on the loads' type ), so by choosing the correct one and using the correct ” IMD equipments ” ( depending on the cost ), the IT will be reliable and very usefull.

    #12512
    Anonymous
    Guest

    The IT system is more reliable. However, the downside is that fault detection becomes difficult. Hence, this system should always be used with a good earth fault monitoring system

    #12738
    Anonymous
    Guest

    I’d have to say that despite reading this article I’m still undecided

    #12740
    Spir Georges GHALI
    Participant

    Tegan Smith said:

    I'd have to say that despite reading this article I'm still undecided


    Dear ;

     

    Please let us know what's the kind of application where you want to use the ” IT “.

    #12827
    Anonymous
    Guest

    can anyone advise what would be best set up to use with a delta delta 690/480v transformer being used to supply two 15kw vsd with emc filter to ground . insulation metering versus imedance relay and ground monitoring , i take it ground monitoring equipment for a delta/star set up will not work with the former it system unless with impediance relay, or am i missing something ,this is for offshore situation . and a retro fit .

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