IT System Dedicated Transformer

https://en.wikipedia.org/wiki/Earthing_system

I cant see what you mean giving a general link of earthing schemes.I ask a question quite reasonable..

<...relevant EN standard...>

Please state which standard.

IEC 60364-7-710:2002-11 -

An insulation monitor is intended to detect when the resistance between an active and earth drops to below a set level, normally about 50kΩ or so. This is important on an isolated system as it will detect a first earth fault, ie one of the conductors going down to earth.

On an isolated supply, this first fault is not in itself dangerous as there is no return path via earth to receive a shock if you should come between a conductor and earth.

Where the danger occurs is that once this first fault occurs, you now have an earthed neutral supply as one active is now effectively grounded, and anyone coming between the other active and earth will receive a shock.

Where two appliances are powered from one isolated supply there is a possibility that the metal cases of both appliances have an earth fault to separate actives and faulty/no earth connection to at least one of those appliances, then touching those two appliances at once would present a shock hazard, In critical applications sufficient stray current could easily be generated to cause heart fibrillation.

Where only one appliance is connected to an isolated supply, this danger does not exist because if both active conductors contact the exposed metal then the fuse or other protective device will activate regardless of the existence of an earth connection.

Therefore an insulation monitor is unnecessary (although still desirable) where a single appliance only is powered by an isolation transformer.

Where multiple appliances are to be used from an isolated supply, they should be of the Class 2 double insulated type, and in critical applications one appliance only should ever be connected to ensure no stray currents.

Could it be that the equipment (user) will any way have insulation monitoring (for earth faults) which can serve to monitor the health of transformer (dedicated for the said equipment/load) as well!

dear Bwilko, forgive me if I speak ignorantly but if there are two fault at the same time on both appliances, it will be cleared automatically by oc devices since these appliances will have protective earthing any way. So I think even if when there is only one appliance this will have the same hazard for any person.

You will note in my post that I said that if a faulty earth connection also existed on at least one of those appliances. If this were the case, normal circuit protection devices would not be triggered and both cases would be live to separate actives.

Remember that an isolated supply has no reference to earth, and so the earth wires (by connecting the two cases together) would simply cause a short circuit between the two actives which should cause disconnection, however, if one of those earth wires is open circuit, then each appliance will have live metal to a separate active and so present a shock hazard.

If there is only one appliance with an active to earth fault, because there is no reference to, and therefore no return path to the supply for earth currents, there is no shock hazard present.

what I do not understand is in the first case since oc device does not work,there is no such a hazard as you mentioned in the latter case. I say the first case creates a hazard since if you have a fault in the second appliance this will cause you to be electrocuted till the oc works. am I right?

but in reality you are right. Since one will not see a full phase phase voltage but just a touch voltage in the second case..

Isolation transformers by themselves produces certain amount of leakage current. The amount of leakage current varies depending on the quality of how the transformer was made. Now adding or connecting load(s) to this single isolation transformer will further increase that amount of leakage current to a level that maybe harmful to cause shock hazards to anyone that may be grounded while accidentally touch or be in physical contact with any of the powered device(s). Specially if the equipment has been poorly earthed / or has an opened ground, exposed metal cabinet or enclosure.

To minimize the amount of shock hazards that will be developed across the system as more loads are connected, the inherent leakage current characteristic of individual equipment must be verified before connection, to ensure that the resulting shock hazard will not go beyond the safe threshold level.

Another approach is to have a good quality isolation transformer be dedicated for each powered device in your system! This scenario ensures that no monitoring is required or will be necessary!

IT equipment developes what is known as Non-Linear electrical loading. You need to learn about this or you may have a fire. Normally IT is covering the electronics and the cooling fans. We treat this a 75% Non-linear. At 100% Non-linear the neutral wire needs to be 1.73 times the ampacity of the hots. The service transformer should also be non-linear rated or the neutral within will fail. Our experience indicates that copper wound is much better than aluminum. Remember that there is no circuit protection on the neutral. Having a neutral wire that does not consider non-linear loading can cause a fire.