Bottlehead Forum

General Category => Technical topics => Topic started by: azrockitman on August 25, 2013, 06:48:11 AM

Title: drain wire purpose
Post by: azrockitman on August 25, 2013, 06:48:11 AM

I'm curious what purpose drain wire serves on STP wire, when only one end is connected and the other is clipped?   I see a reference that it keeps RFI and EMI levels low.  Is the name, "drain", implying that that single connection allows RFI/EMI to "Drain away" from the connected point along that wire......

Sorry if this is a stupid question.

Thanks 

Title: Re: drain wire purpose
Post by: Grainger49 on August 25, 2013, 07:00:03 AM

It is standard to ground a shield on only one end when it is not a signal carrying conductor (like in coaxial cables).  If you ground both ends you can pick up more noise.

I don't think this has been put in Ripley's Believe It Or Not yet, but it is true.  It is the standard in industry for 4-20mA signals used in control systems.

Title: Re: drain wire purpose
Post by: Paul Joppa on August 26, 2013, 05:43:49 PM

Quote from: azrockitman on August 25, 2013, 06:48:11 AM

I'm curious what purpose drain wire serves on STP wire, when only one end is connected and the other is clipped?   I see a reference that it keeps RFI and EMI levels low.  Is the name, "drain", implying that that single connection allows RFI/EMI to "Drain away" from the connected point along that wire......

Sorry if this is a stupid question.

Thanks

You got it just fine. The cable actually has a metalized plastic shield which touches the wire, so an alternate meaning is that it drains the voltage from the shield (which, being plastic, is pretty difficult to solder to directly). But some cables with woven copper shields (which solders quite well, thank you very much) also have drain wires.

Title: Re: drain wire purpose
Post by: azrockitman on August 26, 2013, 06:12:24 PM

Thank you gentlemen.  I appreciate the insight!