3ph isolating transformer, Neutral to Earth voltage

[Martin]

I am installing final equipment in a new electrical installation and our equipment is fed via a local 3-phase 1:1 isolating transformer (Input is wired Delta and output is wired Star). Our equipment is all single phase and split across the 3 phases. It comprises many switch mode PSUs.

I have measured up to 50V between Neutral and Earth. This varies depending on load. This is obviously a concern, but since it is fed from an isolating transformer that is not referenced to earth, is this a problem?

We have questioned the main electrical contractor that is providing this supply, and they are now proposing to bond the Neutral star point on the secondary of the isolating transformer to the rebar in the floor. Currently the Neutral star point is NOT bonded to earth.

However I can't see why they don't just bond the Neutral to the supply Earth, on the isolating transformer secondary, to basically make it a TN-S supply. I didn't really understand their explanation for not doing this, but it was along the lines of preventing noise on the main earth / negating the isolation.

Can anyone else explain what I am missing here?

 

[Deleted member 26818]

To start with, they will not be bonding the Neutral star point, they will just be earthing the Neutral.
Yes they could connect instead to the supply Earth, it would give a lower resistance for Earth faults.
Either option means that you will no longer have an isolating transformer.

What RCD protection is there?

 

[Martin]

All final circuits are RCD protected.

I understand that the transformer would no longer provide safety isolation if connecting N to E, but would it still provide noise filtering? This is the main purpose of this isolating transformer.

 

[Deleted member 26818]

The fact that you are picking up 50V between Neutral and Earth, and no RCDs are tripping, suggests that the voltage is either from the supply side, or there is a live to Earth fault on a non RCD protected circuit.
Then again it could be induced voltage or a problem with your meter.
Connecting the Neutral to the supply Earth would negate any noise reducing benefits using an isolating transformer would provide.
Your transformer would be a waste of money.

 

[marconi]

What type of voltmeter are you using to measure this voltage difference? And to which earth are you connecting your meter- the supply Earth or terra firma? What type of main supply does the installation have - TNCS or TNS or TT? Is equipotential bonding in place yet and what is connected together? When you say 'All final circuits are RCD protected' we assume you mean those fed from the secondary are as well.

Last is the transformer actually a special isolating transformer with a Faraday or electrostatic Shield?

 

[marconi]

I forgot to include in #5 - what do the manufacturer's installation instructions for the iso tran say about how to wire it in for particular situations like yours? (I suspect you do have a shielded isolating transformer which provides filtering of noise currents and attenuation of voltage transients).

 

[davesparks]

All final circuits are RCD protected.

On the outgoing side of an isolating transformer?

 

[Martin]

What type of voltmeter are you using to measure this voltage difference? And to which earth are you connecting your meter- the supply Earth or terra firma? What type of main supply does the installation have - TNCS or TNS or TT? Is equipotential bonding in place yet and what is connected together? When you say 'All final circuits are RCD protected' we assume you mean those fed from the secondary are as well.

Last is the transformer actually a special isolating transformer with a Faraday or electrostatic Shield?

Using a Fluke 117 True RMS meter. Same readings from different meters too.
Connecting meter to the supply earth.
Not sure what the supply is. I know that it's delta into the iso trans.
Equipotential bonding should be in place, as the site is nearly finished and energised, but we are only using final circuits, so can't be sure.
Isolating transformer feeds, final DB that has RCBO's on all circuits.
The transformer is in a metal earthed box, with mesh over the vents. I don't know the details of the iso trans, as we didn't install it.
We have several rooms with the same arrangement and same voltages between N-E. Most are around 20V with no equipment connected.

 

[marconi]

I am installing final equipment in a new electrical installation and our equipment is fed via a local 3-phase 1:1 isolating transformer (Input is wired Delta and output is wired Star). Our equipment is all single phase and split across the 3 phases. It comprises many switch mode PSUs. The IsoTran is being used to provide galvanic isolation between the supply mains and electrical installation lines and neutral for the purposes of EMI control, voltage transient attenuation and noise current filtering, especially high frequency currents as might be produced by SMPUs. I suspect the isotran is a shielded Faraday cage type. See: What is Faraday shield - https://canadatransformers.com/faraday-shield/
I have measured up to 50V between Neutral and Earth. This varies depending on load. This is obviously a concern, but since it is fed from an isolating transformer that is not referenced to earth, is this a problem? The voltage you measure with respect to the MET is an artefact of the floating secondary windings which you have said are not earth referenced at the moment. The voltage is dependent on loads connected, stray capacitances and insulation resistance and is the vector sum of 3 unbalanced phase components. See:

Basic Electrical Engineering - https://books.google.co.uk/books?id=NamQz0aZMukC&pg=PA200&lpg=PA200&dq=what+is+the+potential+of+an+isolated+star+point&source=bl&ots=aZVi_eU8q8&sig=HEPtS7aNZZl__SM4QTh8ILziC1I&hl=en&sa=X&ved=0ahUKEwjPl_C768jZAhVhKsAKHQa3CnwQ6AEINjAF#v=onepage&q=what%20is%20the%20potential%20of%20an%20isolated%20star%20point&f=false

We have questioned the main electrical contractor that is providing this supply, and they are now proposing to bond the Neutral star point on the secondary of the isolating transformer to the rebar in the floor. Currently the Neutral star point is NOT bonded to earth. I am very surprised they are only now proposing to bond the secondary star point. Sounds like an omission to me. The usual supply for a data centre is an isolated star wound secondary derived TNS with the earthed N locally derived by connecting to the local earth mass close to the data equipment. eg: the rebar of of the floor. The N must be earthed for the ADS and residual current detectors on the secondary side wiring installation to do there jobs. And the data equipment uses the secondary earth MET for its cpc returns.


However I can't see why they don't just bond the Neutral to the supply Earth, on the isolating transformer secondary, to basically make it a TN-S supply. I didn't really understand their explanation for not doing this, but it was along the lines of preventing noise on the main earth / negating the isolation. The purpose of the isotran (with shielding and Faraday cage) is to separate the secondary side wiring installation from the noise voltage/currents which are present on the primary side. Connecting the input N to the output N and to the MET would reintroduce the galvanic connection with all the problems that would arise - differential and common mode noise, voltage transients, varying voltage gradients along N (with respect to Earth and terra firma) from other loads on the primary side and feedback of SMPS noise into the mains, high neutral voltage transients from 4 pole switching/disconnections/cable faults of 3 phase 4 wire.

Can anyone else explain what I am missing here? A rather galloped response I'm afraid but I hope you get the gist. Take a look at these two references:

1. Choosing the right earthing system, Bernard Jover - Schneider Electric - https://blog.schneider-electric.com/energy-regulations/2014/01/03/choosing-right-earthing-system/

2. http://www.apc.com/salestools/NRAN-7NB2FG/NRAN-7NB2FG_R0_EN.pdf

For the second reference it is 5/26 onwards.

Hope this help you a little. Others I am sure will be able to clarify and add.