Brian: The grounding/bonding/shielding for all the MRI's we have been involved in is ALL within the MRI Mfg specifications, and is always stringient. The RF shielding is a single point ground on the outside of the RF room.
You probably are talking about RF filters, wich in our case are supplied by a RF contractor that is specified by the MRI Mfg.
We have installed Siemens and GE units, they are basically the same; very stringent specs.
I have inspected three MRIs in my area. I learned as I inspected and can say the last one was completely in compliance with the Code. Biggest problem is that the MRI techs will insist that wiring be done in PVC because of the magnetic field. They ignore Section 517.13 that requires METAL conduit. They also think that individual conductors can be run from the ends of the PVC to the RF filters without being in conduit because the RFs are not supplied with any kind of connector for a raceway. The last one I approved was done in ALUMINUM conduit with AL. boxes and fittings. The filters had threaded ends. Alum. flex over six feet does not comply. This applies only to the wiring for the lights, recpts. etc. The feeder to the machine is allowed to be in PVC, because it is not a Branch Circuit. The MRI Techs said they had never seen one in conduit before and they were installing them all over the country. This is a patient care area. The panels in the room were bonded with a copper mesh (for degaussing ?) and bonded to the building steel outside the room. The metal raceways also created a bond when fastened to the walls. Remember NO PVC. 517.13. Make sure the electrician, the inspector and the suppier of the MRI equipment TALK to each other before it becomes an expensive change or a non-complying installation. Alan--Inspector. I believe Stainless Steel is also acceptable in a magnetic field but, that needs to be checked out. Good Luck.
[This message has been edited by Alan Nadon (edited 08-20-2005).]
Alan, Aluminum, Copper, or Steel, can all be used for the shielding envelope.
The wiring method can be EMT,IMC,RMC,HCFC or aluminum within the walls, but is typically aluminum flex or MC above the ceiling.
There MUST, be a dielectric separation from the metallic raceways inside the room and the grounded raceways outside the room, we accomplish this with carflex from the filters to junction boxes for the building wiring servicing the room.
The single point ground bonds everything back together and to the buildings earth system at the RF filter location.
The redundant grounding per 517.13 will become common at the single point grounding provision.
In reality, it is a moot point because I have never heard of any patient being attached to electrical equipment while inside the room, much less the magnet.
The receptacles are many times outside the patient vicinity as defined in NFPA 99 and NEC article 517, this in itself changes the rules for the space as far as the room meeting other requirements of both codes.
I have even been involved in some MRI's with no receptacles in the magnet room at all, house keeping used receptacles immediately outside the space.
In the event of a Code Blue situation, the patient would be removed from the magnet room where the respondents would treat the patient, so there would be no need for receptacles in this space for that issue.