light flicker in an MRI room

[mshields]

We've got a problem with lights flickering in an MRI room when the MRI is ramped up; i.e. when a patient is being scanned. The lights are designed for the application, the conduit with all power and signal is shielded with the exception of a 24V circuit which actually uses PVC. NOW, this is I'm requiring to be changed out as it is a) not in accordance with our spec, b) the lighting manufacturer's recommendations and while it has no bearing on the flicker issue it happens to be not in conformance with NEC 517.13 requiring redundant grounding inclusive of metallic conduit to patient care area's.

The cost of changing out this conduit is going to be substantial insofar as they need to shut down the magnet; apparently a very expensive proposition. The aggravation of the client is even more severe.

My concern, is that the PVC conduit was actually installed as per the MRI manufacturer's service tech who is stating that he's seen it done this way in countless applications and is sure it's not the problem. My thought is to first try wrapping the conduit in copper tape, so as to test the notion that the PVC conduit is in fact the culprit.

Bearing in mind that armed with this same copper tape, the contractor has plugged up all other potential sources of leakage, my question is; is there anything else any of you would recommend our doing to diagnose this problem.


Thanks,

Mike

 

[GoldDigger]

Before taking the whole exercise of copper tape wrapping as valid, you need to realize that copper tape will shield against electric field variations only. It will not do any good for problems caused by induced voltage from varying stray magnetic fields.
It will have the same effect a putting the wires in aluminum or stainless metal raceway, but will not give the same magnetic shielding that ferrous metal raceway would. And even that is limited compared to the effect of using tightly twisted pairs for all supply and return wires of a circuit.

PS: Since MRI is non-invasive, are you sure that the MRI room is properly categorized as a patient care area?
PPS: Since the problems occur as the magnet is being ramped up and down in field strength, the most likely candidate for electromagnetic interference is the slowly varying magnetic field. See the first paragraph again.
The other possibility would be noise generated by the power supply for the magnet. But although that may vary with the value of the current it is producing, is should not be in any way related to the changing field. If they can stop the ramp at any point and the flickering continues, then look at the power supply. If the flickering only happens as the magnet current changes, it is a magnetic induction effect.

A sufficiently strong magnetic field can cause problems with maintaining an arc in long linear fluorescent or other gas discharge lamps. If the lights are dimmable, look hard at the control circuitry.
PPPS: Good luck in a highly specialized area where just general knowledge and intuition from regular electrical experience are often not good enough.

 

[steve66]

Have you seen the lights flicker? I would want to see it myself before making any drastic changes.

I'd also suspect a loose lamp, or a bad lamp, or something like that. That's one reason to see it - is it all the lights or just one or two?

I'd also have the contractor measure the voltage on the input and output of the DC power supply while the MRI is ramping up, just to make sure the voltage isn't dropping from some reason.

 

[GoldDigger]

I'd also suspect a loose lamp, or a bad lamp, or something like that. That's one reason to see it - is it all the lights or just one or two?

Good point. As the magnet ramps up there will be changing mechanical forces on all ferrous metal in and around the room.

 

[Shoe]

PS: Since MRI is non-invasive, are you sure that the MRI room is properly categorized as a patient care area?

Note that the MRI may not be a "critical care area" (i.e. invasive procedures) but it most certainly is a "patient care area" that requires redundant ground path.

 

[Microwatt]

MRI lights

MRI lights

I would first have to ask is the 24V system in PVC considered a branch cicuit? If not, would it have to comply with 517.13? Next, what type of lighting is it? I've seen a manufacturer of an MRI machine who is also a huge manufacturer of lamps recommend a competators lamp in their MRI rooms to avoid issues. This application was incandescent lamps.

 

[lechner]

I'd also have the contractor measure the voltage on the input and output of the DC power supply while the MRI is ramping up, just to make sure the voltage isn't dropping from some reason.

I'm with steve66. The magnet draws a lot of power. I too would check the line voltage under load.