High voltage conductor design/materials question

[lurker]

Hi out there,

I have been working on an issue for a while now and I think I have reached my wits end here... I hope someone out there can at least point me in the right direction. Let me see if I can explain this:

I have an issue which involves a cable that consists of two legs joined together zip-cord style (a little webbing in between holds them together). The cross section would look like " OO " essentially. Outer jacket is Santoprene TPR (rubber).

IEC 60601-2-4, Section 10, clause 56.101, subclause b. applies here.

We have a requirement on this product that the cable must withstand 10,000 cycles of flex, through 90 degrees each side of vertical (180 degrees total), 30 cycles per minute, a 5 N weight is suspended on the loose end of the cable, 300 mm from the axis of rotation. After 5000 cycles, the cable is turned 90 degrees and flexed another 5000 cycles in this new orientation (total is 10,000 cycles).

The cable will pass the "easy" bend, meaning a bend along the wide direction of the double cable, but when turned 90 degrees and the bend is in the against the wide direction of the cable, we are getting failures of the high voltage wires inside. The cable tends to fold over in this "hard" direction.

Any ideas on what can be done to help the strain relief/cable pass? Any proven strain relief geometries/designs out there for this double-cable configuration? Any ideas on whether changing the materials or cable configuration would help?

Right now the high voltage conductor (which fails) is made of Tinned Cadmium Bronze/Copper (C16200, "soft" or annealed) and is #22 AWG (26/36) construction. We need this cable to carry signal as well as deliver energy (defibrillation). Also, this product can be subject to steam autoclave at 270 deg F... While the jacket and insulation may protect it, our experience is that over time there is some penetration of the steam to the conductor.

Also, I should mention in regards to the strain relief that little holes or ribs are out of the question for the strain relief because of the contamination issues in a hospital operating room environment...

What can we do to help this cable pass the IEC flex test?

Thanks in advance for any help that can be provided.

- lurker

 

[nigelt]

Thank you for your reply to my more general question on strain relief. My problem has temporarily 'gone away', but I'll follow it up in my free time!!
Your issue is tricky.
At the risk of being accused of preaching to the choir, it occurs to me that your answer must come from the strain relief grommet. Assuming that the cable config is fixed, (for all the usual reasons!) I think you have to design the overmoulding in such a way as to grow the bend radius. At the same time restrain the tendency of the cord to buckle. (Do you find the failure in 'upper' or the 'lower' conductor?).
Perhaps a tapered cylindrical section which will thus be thicker on each of the 'flat' sides. You'll probably need 2 to 2.25" from the case flange to do the job. From what I've observed you'll be in the Shore A 50-60 range. As you'll be getting pretty thin wall near the tail end, consider a revolved semi-circle right at the end to prevent tearing. The tool has to split longitudinally either way.
I'm sure none of this will meet the expectations of the ID Dept of course!
I'd suggest getting some samples from McM-Carr or SpaeNaur and fiddling about with them to see what really happens!
You may have tried all this I know, but these are just some thoughts.
Cheers, NT

"The ideal client is one possessed of great good sense and perfect judgement;
that is to say, one who agrees entirely with the designer at all times and in every respect."
Fenwick Williams, Naval Architect.

 

[lurker]

Hi NT,

Thanks for the reply. Your problem may have gone away temporarily but that might be because your product is on the edge between pass and fail... Best to do some standard testing on it.

There is some difficulty here as if I bend the cable this way:
^
|
OO
|
v

I have no problem.

Bending it the other way: <- OO ->

That is the issue. In this direction, the cable has a 9 ft. natural bend radius. Yeah, Nine Feet. It's not even 9 feet long!

So what happens? It folds over (buckles I guess you can say). The only thing I have been able to do here is to resign myself to the fact that it will fold, and try to "control" the folding so it is spread over a longer distance, making it a more gradual transition from the 'oo' to the '8' (perpendicular) configuration.

My other option at this moment is to split the cable up before it enters the connector, so I will make two single cables go in with their own individual strain reliefs...

- lurker

 

[Tofflemire]

Hi Lurker,

Can you make the zip cord sprial around the cables to increase the conductor lengths...

Tofflemire

 

[lurker]

Hi Tofflemire,

Thanks for the response. I'm not sure what you mean, might be because I didn't describe my configuration clearly for you... The conductors are inside the zip cord. The metal strands have a 2 inch lay within their own foam FEP insulation, but the insulation/conductor unit as a whole does not have any spiral/twist/lay within the zip cord itself. I think if you are suggesting that we spiral the insulated metal strands within the zip cord, then your idea has some merit

- lurker

 

[Tofflemire]

Hi Lurker,

Yes, the more spirals you can get into cables/design the better. I know that if the process gets more difficult to put the spiral in, not to mention labor/process intensive but the spirals gives higher reliability.


Tofflemire