High voltage conductor design/materials question

lurker · Aug 19, 2004

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.

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

Thanks in advance for any help that can be provided.

- lurker

MintJulep · Aug 19, 2004

I don't have access to 60601, but here are a few things to consider:

Is your test representative of probably service? Are these the cables that go to the paddles? Is it reasonable to expect that they will bend the "hard way" of their own volition? Typically a flat cable would want to twist first, so that any subsequent bending will be in the "easy direction". During your test, are you artificially constraining the cable to prevent this? Does your test attempt to bend the cable in a tighter radius then it would bend in service?

If none of that proves helpful, is a radially symetrical flavor of the cable available?

lurker · Aug 19, 2004

Hey MintJulep,

Yes, the cable is hardwired to the paddles on one end and a connector on the other end. There are two paddle handles and the cable is split for a decent length prior to entering the handles. More later.

The test is very specific and the setup is standard. I don't think we are preventing the twisting action. You are right about the cable twisting because that is what it does during the test and also is what we would see in the field. The twisting has to start somewhere, and it seems to start at the entrance to the strain relief and folds over abruptly, which is where the failure is occurring (at the entrance to the connector strain relief). In my mind the strain relief is inadequate... This is another difficult thing because we don't want ribs or small holes because of contamination issues.

It is difficult to go with a radially symmetric cable because at a certain length, the cable is split apart as it is a zip chord style (a molded Y junction prevents the cable from splitting farther) and go to two separate paddle handles. Unless the cable is jacketed until it reaches the Y junction but that would make it a huge cable...

Thanks for the help so far, I hope this clarifies it somewhat. Please let me know if you have any ideas.

- lurker

rnd2 · Aug 20, 2004

Would it be possible instead of the cable presenting as OO
it presents as OO
OO
The wires could be distributed into the four cables and this would result in a more compact cable than exists which in turn would improve bending and buckling performance.

MintJulep · Aug 20, 2004

Spring style strain relief? Could be stainless.

lurker · Aug 20, 2004

Hi rnd2 and MintJulep,

Both ideas have merit, and again, I appreciate all the help provided so far.

Changing the geometry is one of the things that have been discussed and while it is possible, there are limitations to what we can do. Mostly because of the dielectric requirements, but also because of the fact that the cable is split after a certain length to go to 2 individual handles. If the geometry were to be different up to the split, I guess that is a possibility. I will try to get this idea floated and see what happens.

Spring style strain relief is a good one but the hesitation there is the same as having lots of ribs in that location - the difficulties in cleaning out the blood, etc. that will get trapped in the little crevices... Maybe a jacket over a spring? I don't know...

Thanks guys, keep'em coming.

- lurker

EdStainless · Aug 24, 2004

Your cable split has a retainer molded over it correct? Why not a similar tapered molded reinforcement at the strain relief?
I have worked with flat cable before, it is a pain. I like the 2x2 suggestion. They could all be in one jacket and still split just fine. You would only need to connect them inside the unit and the paddles.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

lurker · Aug 24, 2004

Hi EdStainless,

There is a retainer molded over the split location, a sort of Y shaped strain relief if you will. I do not understand how that would be applied to the connector strain relief end where the flat cable meets the connector, unless you mean to split the cable again prior to it's entry into the connector body?

Thanks,

lurker

EdStainless · Aug 24, 2004

No, I just meant to verify that you were able to mold over the cable. I was thinking of a thin, tappered sleve molded at the strain relief to keep the cable from bending at a single point. Maybe even mold a light stainless spring into it in order to allow a lot of flex.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

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