Isolation XFMR Grounding

[Mike Howell]

I'm working up instructions for an MRI installation. The package from the MRI equipment supplier calls for the installation of an isolation transformer for convenience outlets, lighting, etc. View attachment 15723

The guidance states "THE ELECTRICAL GROUND FOR ANY AUXILIARY EQUIPMENT USED BY THE OWNER MUST BE ISOLATED FROM THE ELECTRICAL GROUND FOR THE MANUFACTURER'S EQUIPMENT. CONNECTION OF THE GROUND TO BUILDING STEEL, FOR EXAMPLE, TO THE MANUFACTURER'S EQUIPMENT GROUND IS NOT PERMITTED AND WILL CAUSE A GROUND LOOP. IF A GROUND LOOP DOES EXIST THE ELECTRICAL CONTRACTOR WILL HAVE TO INSTALL AN ISOLATION TRANSFORMER AS DESCRIBED HERE."

The sketch (hopefully attached) shows what appears to be equipment grounding conductors (bonding conductors) connecting filter enclosures, RF shield, and a power distribution panel enclosure on the secondary side of the isolation transformer to the enclosure of the isolation transformer. It does not show a connection of the isolation transformer enclosure to the building grounding electrode system. Only 120V single phase loads are supplied by the isolation transformer.

Question 1: Is the secondary neutral required to be connected to the building grounding electrode system?
Question 2: Is the isolation transformer enclosure required to be connected to the building grounding electrode system? Question 3: Is the secondary neutral required to be bonded to the isolation transformer enclosure?
Question 4: How does an isolation transformer remedy a ground loop? I would think the RF shield around the room is effectively bonded to building steel (and therefore to the building grounding electrode system), so if an equipment grounding conductor connects the RF shield around the room to the isolation transformer enclosure that would pretty much connect all the 'grounds'. Phase and neutral currents may be isolated by the isolation transformer, but the grounds?

I would like clear advice as to what to tell the electricians to do regarding connecting 'grounds'.

 

[cuba_pete]

SDS/single point ground

SDS/single point ground

In my experience (not in the medical sphere), if the equipment is requiring a separately derived service then the transformer would be bonded/grounded according to the NEC. It seems odd that the manufacturer would require an SDS for everything except their equipment. They don't mandate anything except how their equipment should be connected.

Is the "Siemens Equipment Ground" identified on another drawing? Your attachment looks like a notional drawing for an SDS en-suite.

Regardless of all of this, a Single Point Ground sounds desirable for the Siemens equipment. The EGC must still provide a fault path and not an un-safe separate grounding scheme.

Remember, there is no such thing in the NEC as an "isolated ground", literally meaning EGC's attached to physically isolated ground rods for equipment. There is no point to that junk.

There are exceptions, however, for single-point grounding methods which utilize "isolated grounding circuits" re:250.96(B), and "isolated ground receptacles" re: 250.146(D). All bonding and grounding methods must still meet the intent of the NEC and meet safety and protection standards:

250.146(D) Exception, so as to terminate within the same building or structure directly at an equipment grounding conductor terminal of the applicable derived system or service.

That being said, there are other rules for grounding and bonding, for example, in patient areas.