Extending a Shielded Cable into an AP

B

blazin912

#1
I am updating a design to extend a shielded cable into an AP. The shield ties to earth through y-caps.

The applied part is a plastic enclosure with 3.3VDC voltage internal. To extend the shield we are looking at coating the inside of the enclosure but will have limited clearance from the secondary voltage to the "earthed" shield.

Trying to determine if this is an issue, or as I've seen elsewhere this shield connection is not a safety issue but rather EMC related and therefore does not need to meet the MOPP requirement of 0.8mm.

Any thoughts?
 
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C

cdewitt

#3
Questions -
Is the coating tied to the cable shield through this Y cap or is this a direct connection? If it?s direct and the cable becomes hot then the coating is also.
Is the coating accessible to the user or patient on the outside of the enclosure or is there at least creepage/clearance distance to someone touching it? If there is at least creepage/clearance and it becomes hot it may not matter.
If someone can touch it, how much leakage current is there?
Do you just need to meet touch current or is the enclosure an applied part in which case you need to meet BF or CF currents?
Have you done the risk management process from IEC60601-1 3rd edition, 4.6 ME EQUIPMENT or ME SYSTEM parts that contact the PATIENT?

C DeWitt
 
B

blazin912

#4
Coating will be connected to Earth by Y Caps
Coating not on exterior
Type BF applied part

We passed originally without coating, by foil method.

This is an EMI mitigation so I'm trying to determine if it is acceptable.

If the coating is connected to signal reference ground, does that eliminate the creepage/clearance requirement?

Stackup:

Wall thickness (plastic): 0.5mm
Coating: 0.025mm
Air from coating to PCB copper: ~0.317 - 0.495mm

Your thoughts?
 
C

cdewitt

#5
If you connect the coating to earth (PE) through Y caps then if there is an ESD discharge to the coating the charge will have no path to drain this charge back to earth. The coating may not be on the outside but ESD can still discharge to it at the seams. To prevent this either the seam needs to be sealed or there needs to be a large enough gap between the edge of the seam and the edge of the coating to prevent discharge. Roughly 1mm/1kV so to prevent discharge at 15kV the coating needs to be 15mm back from the edge of the seam on each side (30mm). This doesn?t work well as an EMC shield so something needs to cover the gap.
A related problem is creepage and air clearance at the seam. Even if you can?t touch the coating directly it still needs to be far enough from the seam to meet creepage otherwise it needs to meet BF leakage.
Is the shielded cable connected to any infrastructure or other equipment like an ethernet or USB cable? What we do is to connect the coating to PE but connect the cable shield to the coating with Y caps. That way the coating has an ESD discharge path to earth but is isolated from the cable so if the cable becomes hot the coating is not. This is not a perfect shielding solution, it?s a tradeoff between safety and EMC but it works fairly well.

C DeWitt
 
B

blazin912

#6
Hmm, well I think were in the opposite situation:

Coating is on the remote housing, meaning that the only way to bring anything to it is through the cable.

So:

Housing Coating -- Cable -- Y Cap -- Earth

Housing is floating without the cable connection, this is a custom connection point, directly to electronic assembly. Take that cable out and the floating electronic assembly is isolated by > 1000mm of clearance. :lol:
 
C

cdewitt

#7
OK, different systems require different solutions. You might think about paralleling the Y cap with a string of large value resistors to provide a discharge path for ESD.

C DeWitt
 
C

cdewitt

#9
Assuming this is a high frequency signal and/or the shield is needed for immunity, bypass it to the coating on the case with the Y cap as you said but carry the shield through to the PCB. Do not use a pigtail to connect the shield at either the coating or the PCB, this adds inductance and makes the shield less effective. The Y cap will be sort of a pig tail but keep the path length between the shield and the coating as short as possible. Possibly use multiple caps to connect around the opening the cable comes in through. Make sure the jacket covering the cable extends into the enclosure at each end by the creepage distance or more. Check the leakage at the applied part by wraping it in foil and measuring leakage to ground.

C DeWitt
 
B

blazin912

#10
I guess what I'm lacking understanding of is when Creepage and Clearance is required and between which items?

ie Clearance between what and Working voltage of <17V DC is 0.8mm

Operator? Patient? Mains? Earth?

I understand how we measure leakage, but trying to understand if one of these is a definite no, ie Shielding with Safety GND with very little clearance versus shielding with a highly isolated (CF rated) reference ground

Thanks for your help
 
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