[tag]

Hi all,
In a private dwelling in Spain, I have a situation where the feed from the street comes in as a dual 136Vac feed (2 phases) which are combined at the primary switchboard to give a 240Vac supply to the household.
The result of this is that you have a "floating" earth, where your measurements between N & L is 240Vac, yet between each N or L with Earth is 136Vac.
This is due to the old Spanish supplies, which are still going through the transition/upgrade to the current standard of 400/230Vac (3phase/single phase) setup.

Although for your standard domestics this installation is acceptable, for more sensitive devices, as well as any PAT testing, this causes issues.

To correct the setup, I intend to put inline an isolating transformer to achieve 2 targets.
Rectify the feed to the sub-board with a true L-N-E installation where N-E are bounded.
And to separate out the RCP of the 2 boards (so that a leakage in the "rectified" zone will not trip the main RCP at the residence primary board.

My request here is, that someone cast a second eye on the setup to confirm my thoughts on the wiring.

The primary board has the 2 phases wired as an N-L combination, with earth unbound. An RCP across the N-L line.
The primary winding on the transformer is connected to each 136Vac phase, with the secondary winding thus giving 240Vac between N-L.
On the second winding of the transformer, the intention is to bind N to E and feed the sub-board
and to have an RCP on the sub-board for leakage protection of this zone.

Any comments? Any hidden issues that I am missing?

Thank you,
Tim.

 

[splatz]

If I follow, the problem is some sensitive equipment doesn't function correctly unless one of the lines on the supply are grounded; so what you want to do is use an isolation transformer with 240V from one phase of a 240/136 wye source on the primary side, and 230V on the secondary side, with one of the lines grounded on the secondary side. The secondary side will be used to supply the sensitive equipment.

As long as the grounding and bonding are done correctly on both the primary side and the secondary side, I think it all makes sense. With an isolation transformer, there's no electrical connection between the two sides, so a ground fault (current leaking to ground) on the secondary side could not be detected by the RCP on the primary side. So yes, you'd need separate RCP to protect the secondary side.

However, not having worked with this setup, I might be missing something. Some of the people participating here work on setting up industrial equipment made for foreign electrical systems, maybe they'll see something I don't.

 

[tag]

Thanks, @splatz

To help the query, please see the drawing attached. Am I missing something?

Note; currently, any PAT testing done with test equipment connected before the transformer fails.