Screened Mains Cable

[ntom]

I picked up a 2m length QED Qonduit mains cable a while back which came with an item of hi-fi i bought.

This is a pretty chunky screened mains cable. At two metres I thought i'd chop it in half to make up 2 x 1m cables -much more useful for my specific requirement.

The original cable has a moulded IEC socket & a good quality MK mains plug.

Having cut the cable in half & checked the screen connection it transpires this is connected at the IEC end not the mains plug end. This seems counter intuitive to me. If indeed the screen is to drain RF, surely the direct route is to connect the mains plug end.

 

[misterdog]

Correct.

Well it said post quick reply.

 

[davidsrsb]

For shielding to work, you connect both ends. You are logically extending the casing to enclose the mains wires. Connect only one end and you are attaching a radio antenna to it

 

[Yank]

For shielding to work, you connect both ends. You are logically extending the casing to enclose the mains wires. Connect only one end and you are attaching a radio antenna to it

Connect both ends and you risk system ground loops, especially if there are multiple similar AC cords in the system.

I imagine QED's design is the result of trying to minimize returns because the new accessory makes the system worse.

 

[davidsrsb]

You solve the ground loop problem by using a 470pF 1kV capacitor in series at one end - breaks the dc and 50Hz harmonics loop while completing the path at RF

 

[Lee T]

I picked up a 2m length QED Qonduit mains cable a while back which came with an item of hi-fi i bought.

This is a pretty chunky screened mains cable. At two metres I thought i'd chop it in half to make up 2 x 1m cables -much more useful for my specific requirement.

The original cable has a moulded IEC socket & a good quality MK mains plug.

Having cut the cable in half & checked the screen connection it transpires this is connected at the IEC end not the mains plug end. This seems counter intuitive to me. If indeed the screen is to drain RF, surely the direct route is to connect the mains plug end.

Your correct, this is of no use whatsoever
I would also have thought, connecting the drain wire at both ends is of no use either, the idea is to drain noise direct to earth.

 

[davidsrsb]

Shielding a RF SYSTEM means logically putting it all inside one RF tight enclosure (aka Faraday cage). It doesn't have to be earthed at all, fortunately for aircraft.
In practice this means multiple boxes, interconnected by coaxial cables, which connect shield to box at each end

 

[Julf]

Shielding a RF SYSTEM means logically putting it all inside one RF tight enclosure (aka Faraday cage). It doesn't have to be earthed at all, fortunately for aircraft.
In practice this means multiple boxes, interconnected by coaxial cables, which connect shield to box at each end

Yes, but in this specific case we are not talking about a closed, totally enclosed system - we are talking about a system with a long, unprotected antenna (the mains wiring) with a short final piece being screened...

 

[ntom]

Shielding a RF SYSTEM means logically putting it all inside one RF tight enclosure (aka Faraday cage). It doesn't have to be earthed at all, fortunately for aircraft.
In practice this means multiple boxes, interconnected by coaxial cables, which connect shield to box at each end

The majority of hi-fi components these are double insulated. So there will be no connection of case to mains earth. Therefore for most modern kit this isn't relevant surely? It's the draining of RF that the screen is intended for is it not?