Estimating EMI in Wiring Harnesses - RF Cafe Forums

 Post subject: Estimating EMI in Wiring Harnesses

Posted: Thu Jan 26, 2006 6:08 pm 

Information is needed to finalize a cable assembly design.

The current design consists of a coil cord and two stainless steel connectors. The EMI shielding consists of Sn/Fe/Cu mesh tape wrapped around the wire bundle with a 50% overlap and terminated to each connector with a stainless steel clamp. Uncoiled length of cable is about 2 meters. Normal braiding is not possible due to the need for coiling.

This cable needs to provide at least 40dB attenuation to a 400MHz test frequency.

Also needs to meet MIL-STD-461: CE, CS, RE, and RS at values ranging from 10KHz to 18GHz.

It is acceptable to qualify by design. This avoids the need to do expensive swept frequency tests and expedites the whole process.

Normally a shielded cable can qualify by design if measurements with a micro-ohmmeter show a very low resistance from shell-to-shell and across the faying surfaces at each end.

The requirements for this cable are <2.5 milliohms from each shell to the overall shield on the other side of the clamp. The cable design meets this limit.

Shell-to-shell values are a problem. Actual measurements with a micro-ohmmeter give a value between 1.25 and 1.35 ohms shell-to-shell. Braided cable would typically have a value in the milliohm range depending on weight of shield per meter and overall length. Because of this difference it may not be possible to qualify by design based on provable milliohm measurements.

My questions are:
01 Is the 1.25 to 1.35 ohm range too high for mesh tape? If so why, and how can we get to a milliohm range?
02 If a milliohm range is not necessary to use DC resistance tests as proof of compliance then what data is available to support such a position?
03 Would aluminized mylar foil help?

EMI testing hasn't been required at our level of manufacture for many years. Usually the odd job that requires it goes to an outsource test vendor but time constraints favor a faster solution.

Any help is greatly appreciated.

 Post subject:

Posted: Mon Jan 30, 2006 8:57 am 

Greetings TXH1138:

The >1 ohm value does sound high for an uncoiled (assumed straight?) length of just about any metal regardless of composition. Does your resistance measurement vary as the coil is extended and retracted, or if it is twisted in an extreme way?

I would suspect the contacts between the braid and the SS connectors. Can you access braid before the shells to verify that the actual braid resistance is in the <1 ohm range?

Does the resistance increase over time, indicating a corrosive barrier forming as with an anodization due to dissimilar metal contacts? In fact, if you reverse the leads on your micro ohmmeter, does the resistance value change? You be experiencing a galvanic current that is causing your meter to read erroneously high.

Well, that's my 2¢ worth.

_________________
- Kirt Blattenberger
RF Cafe Progenitor & Webmaster

 Post subject: Estimating EMI in Wiring Harnesses

Posted: Tue Jan 31, 2006 2:25 pm 

Thanks Kirt!

I'll try that with the micro-ohmmeter.

Measurements of the mesh tape show much variance when flexed. The mesh to shell faying surfaces are rock solid. That's the only good news.

It's not the end of the project if we can't qualify with milliohm measurements. It's just that now there's a need to do compliance testing for conducted and radiated emissions at the cable level.

Such testing was more common up to the early 1990's. Personal experiece showed that, after the qual test phase, it was always a waste of time and money to run EMI on a cable. They always passed. Better to test the whole system then make tweaks during LRIP. Manufacturing processes are so much better these days and understanding of shielding, bonding, and grounding too.

Only thing left is to establish the protocol and qual the design.