PVC Conduit in Healthcare Occupancies with Linear Accelerators

Status
Not open for further replies.
M

maxpwr

Member
Have any of you dealt with Varian Linear Accelerators? As with many X-ray/Radiology pieces of equipment, there is a 208V connection from a shunt-trip breaker to a power distribution unit and then several conduits from that location to the actual Linac and to the control room, etc.

Some conduits contain low voltage signal cables only but others contain the X-ray or 208V power conductors, all of which are provided by the Linac installer.

The owner is requesting that we use PVC conduit in the slab for all low voltage and 208V (480V in some rooms). Given that this is a patient care area and I have power coming from a critical branch transfer switch, I feel that Article 517 would require metal conduits for a redundant ground path and the fact that it is critical power.

However, the cables and conduits don't really bond to a junction box or even the Linac itself. They generally just terminate in a pit below the equipment.

Am I missing something or could PVC conduit be used for such installations? I know I haven't covered all bases here in my question but I've always specified GRS conduit for X-ray installations previously for these reasons. The Owner is now asking me if the conduit pathway isn't continuous anyway, why use metal conduit?

Thanks.
 
maxpwr said:
However, the cables and conduits don't really bond to a junction box or even the Linac itself. They generally just terminate in a pit below the equipment.
Click to expand...

The 208V coming through the conduit to the pit...
is in a listed cordset etc being sleeved by the conduit?
or is it individual conductors (THHN etc)?
and how do they 'terminate' in that pit?
 
All references are from the 2005 NEC:(My AHJ has not adopted the 2008 yet)

First, in a patient care areas 517.13 (A) governs the use of metal conduits only for all branch circuits. (first sentence)

Second, the conduits can not "teminate in the pit", check 300.15 (A-M) the last sentence should do.

Third, 517.13 (A) last sentence requires the conduit to meet the requirements of 250.118 qualifing as an equipment grounding return path. (Although I hate the word-this is where they get "redundant grounding" from)

Hope this helps with your owner.

Kevin
Electrical Inspector/Former E.C.
 
I know I'm not exactly explaining it correctly but the installation guide shows three options. Two include a metal junction box and one is just a pit below the equipment. Generally 12"x24"x10" or so. They say check with local authorities on which is required. The cables are all pre-manufactured cables and generally not individual THHN conductors.

From this pit, 3-4" conduits go to the control room (low voltage) and 2-4" conduits go to the modulator cabinet, one of which contains 208V. I've seen several of these installations after the fact and the cables just "emerge" from the conduit and up into the linear accelerator equipment above. There is no continuous metal pathway because the vendor provides cables that plug into the front of the cabinet. They even have an installation scenario where they recommend a trough to enter the pit below the equipment.

Since my first post, I have the Owner convinced that metal is the way to go for any 208V carrying conduits, PVC for low voltage signal wiring, and he is in agreement, but his thinking is something like:

"Well, we could just lay these cables on the floor but since the connection point is on the bottom of the equipment, we are providing a pathway into the bottom and it happens to be conduit. Why can't we use PVC for everything since we really don't 'need' conduit anyway."

I've specified all metal conduits before and am going to keep that installation.

Thanks for the input.
 
KevinVost said:
Second, the conduits can not "teminate in the pit", check 300.15 (A-M) the last sentence should do.

Kevin
Electrical Inspector/Former E.C.
Click to expand...

I think 300.15 B or C allow this. The equipment sits directly over the pit, and I think the wires terminate in the equipment. I'm pretty sure the equipment has removable covers to access the wiring.

I agree that the requirment for metal conduit only applies to branch circuits. (I'm not sure why you referenced 250.118 after saying 517.13 doesn't apply).

Anyhow, I think some of these conduits may be for equipment wiring, and the NEC may not apply. I would suggest that the original poster consult with his AHJ, and the varian project manager. If neither of them have a problem with PVC, I don't see an issue with it.
 
I have installed one of these varian linac machines and the pit was formed out of concrete with the medical floor troughs and associated wiring harnesses some of which were pvc all came to the pit under the table. There are tons and tons of ground wires pulled every which way so I dont think grounding is an issue and the cabling is a listed part of the equipment not a branch circuit.
 
Status
Not open for further replies.
Top