EMC testing when mating cables have spare cores, what is best to do with them? - Page 1
Forum section: electronics / projects

author: TwinScroll
author_rank: Contributor
timestamp: May 04, 2021, 05:58:29 am
content: I am being lined up to work on (and get through EMC testing)a product that someone else designed (they are no longer working).I have just been over the design as a first pass and noticed several area that look risky / will likely need changes or updates.The DC power cable is using a SubConn connector (but this is not an underwater device) and is 5 Meters long.The device uses 2 x 0.5mmSQ cores but has several spare conductors present and these are not connected at either end (floating).The cable is not screened and the conductors are in not twisted pairs either.I am going to look into changing the cable to a screened twisted pair and I am hoping this is possible with the rest of the team, but if not...Q) what is considered best practise for unused conductors so they don't become unintentional antennas?ThanksTwinScroll

author: ogden
author_rank: Super Contributor
timestamp: May 04, 2021, 06:12:44 am
content: Quote from: TwinScroll on May 04, 2021, 05:58:29 amBest practice is to ground them as close to enclosure/shield of device as possible.

author: Whales
author_rank: Super Contributor
timestamp: May 04, 2021, 06:41:58 am
content: Are both ends of the cable connected to independently grounded devices? May or may not be worthwhile only grounding the spare conductors on one end to avoid ground loops. EDIT: although you might already have a ground link connected in the cable anyway.

author: ogden
author_rank: Super Contributor
timestamp: May 04, 2021, 06:59:41 am
content: Quote from: Whales on May 04, 2021, 06:41:58 amIt depends. Additional information about "devices" needed to give definitive (if any) answer. It is unknown - two interconnected devices are passing EMC tests as "system" or only one device with connected cable is tested (as usually). In short: when two devices shall have galvanic isolation then obviously do not connect their groundsIf you are concerned about mains power-induced ground loops then ground one end down to DC frequencies and ground another end using Y capacitors for EMC (RF frequencies).

author: TwinScroll
author_rank: Contributor
timestamp: May 04, 2021, 08:01:09 am
content: I should have added a bit more context, sorry.The spare conductors from the cable do not stop at the edge of the enclosure.The enclosure has a pre-made / molded mating connector so the ‘spares’ come ‘inside’ the enclosureThere are then several short stubs of wire (not currently doing anything) that are connected to a length of unscreened cable on the outside.I may have been overthinking this, but was concerned they would become a good path for EMI maybe via radiated : conducted - to find a way out of the enclosure.To answer the other question above, we are not qualifying this as a complete system, just the enclosure.Thanks for any suggestions guys

author: ogden
author_rank: Super Contributor
timestamp: May 04, 2021, 09:00:07 am
content: So initial advice stands: ground spare wires inside box as close to enclosure/shield of device as possible. Yes, it still depends because wire ends inside box can act like (loop) antennas. As EM field strengths inside box are unknown then this is all what can be said. That's why precompliance testing is good idea - to check that current EMC precautions are enough.

author: TwinScroll
author_rank: Contributor
timestamp: May 04, 2021, 10:08:30 am
content: Thanks. This enclosure is off for Pre Compliance. Will feedback on the results.