Terminating Shielded Cables

[richwaskowitz]

Consider an application where shielded 480 volt power cables for inverted controlled motors are brought into a control panel and are terminated at the prescribed terminals on the variable frequency drives. In order utilize the full benefits of the shielding designed into the cable, is it necessary to terminate not only the drain wire but also to bring the braided shield to that same "shield" terminal? Some here are of the opinion that the braid should simply be cut off and insulated (shrink tube or taped) where the outer jacket ends and only the drain wire needs to be landed. Please cite some authoritative document if possible, but saged experience would also be appreciated. I am pretty sure that a lot of people will say that it may be better to terminate both, but I need to know if one MUST terminate the braid.

 

[jcormack]

All of the terminating kits (3M typically) we use on higher voltage cables have a pigtail to connect to the braid to allow connection to ground, although we don't typically use shielded on 480 I would think it would be prudent to connect to the braid as well as the drainwires, even though they "should" be connected within the cable structure - grounding the braid also provides a ground connection "you can see & verify". Generally you only need to connect the shield to ground in the VFD cabinet - not at the motor, but read your drive requirements.

 

[dbuckley]

Generally you only need to connect the shield to ground in the VFD cabinet - not at the motor, but read your drive requirements.

In the audio world we used to think that too, until some work done about a dozen years ago categorically proved this to be the wrong approach. The issue of keeping noise out of screened audio cables is identical the the issue of keeping noise within screened VFD cables. Basically, the screen needs to surround the cable from the boundary of the metal box at both ends, the so-called 360 degree protection. Otherwise the screen acts as an antenna, and makes the noise radiation problem worse.

It's very audio but have a read of a paper entitled Bonding Cable Shields at Both Ends to Reduce Noise, noting particularly how bad pigtails are at making the problems worse.

Possible more relevantly, this paper entitled The factors to be considered in the correct cable for VFD applications on cranes(PDF), particularly page 2259 (it starts on page 2257, its not a 2000+ page document!) regarding Faraday shields.

Heres a link to UK advice on EMC compliance by radiation mitigation, for cables acting as unintentional antennas. Again, this is mostly about signal cables, but the principles remain. It also includes the statement "never use pigtails".

 

[richwaskowitz]

According to Belden...

According to Belden...

Belden tech support assured me that the braid does not need to be terminated, only the drain wire. They recommend cutting the braid off where the jacket is removed adn taping or heat shinking the exposed ends.

 

[Ranch]

Here is another spin - what it takes to comply with a VFD on a submarine in terms of allowable radiated emmissions levels from the VFD output to motor conductors.

You need a complete enclosed metallic entry around your VFD output terminals where you connect this EMI fexible conduit with the appropriate fitting, and it runs all the way to the motor conduit box where you do the same.

There is really no drain or rule here, simply a completely metallically housed circuit from start to finish. Works great, costs a fortune.

This technique and the audio circuit analogy appear to support one another

If the picture I am attemplting to attach doesn't show, I guess this reply is pointless. Well - here goes