Originally posted by: johnnmann
It has been suggested to me that the screening is not good enough on our D connectors. The reason being that the connector sits in grooves in the two halves of the backshell but because it is not clamped tight and is able to "float", EMC continuity between the connector and backshell cannot be guaranteed. Even when the jackscrews are tightened, they only hold the connector to its mating socket and don't act on the backshell.
I've searched extensively, but it seems that all D connector backshells work like this, even the super-duper EMC ones, so - is the suggestion valid and if so what can I do about it? If anyone can point me at some suitable products or information it would be appreciated.
Assume you are talking here about "manually-wired" connectors: some of the moulded-types can address this issue rather better (but to guarantee you'd need to audit the manufacturing process, of course
Many of the "EMC hoods" have a metal cable-clamp or spring-clip for the drainwire/screen to be wrapped around for connection to the hood, and the connection to the "shell" is fortuitous at best. I've seen an odd one or two types with an arrangement having springs on the jackscrews to ensure the hood can take the screen from the shell of the connector itself.
For applications that require "good" screening practice, I do often see this:
Metal hood with screw-type cable clamp, under which the braid or drainwire of the overscreen is terminated: if drawinwire, then this is wrapped around the conductive side of the foil a few times to get the recommended 360-degree connection. "pigtail" is then taken from the clamp and soldered onto the shell (if extended drainwire, then this is insulated from the clamp to the shell to prevent inadvertent electrical connection, otherwise something like 24AWG tri-rated is used, soldered to braid before clamping).
However, there may be alternative approaches: can you provide more information on what interface standards you are using, and whether you must take the screening from the shell, or whether the interface pinout arrangement you're employing has a "screen" or "ground" connection intended for the screen. As an example, the original intention of RS232, for example, where the DB25 had a "SCN" pin, which with many manufacturers' applications you could select whether this was connected to the outer shell only, or both the shell and ground?
I think the upshot of this, whatever method we use, is that most of the "manually-wired" D-type connectors is not really suited for the absolute very best EMC practices as stated in BS IEC61000-5-2, particularly if the disturbances we are trying to protect against are very short wavelength signals: there are always little gaps between the hood and shell for the signals to leak through.
If this abolsute very best practice is required, and you are forced to use manually-wired connectors because of the application, then the solution would be to enclose the equipment at each end, in earthed, or functionally-bonded, metallic enclosures, and provide conductive containment (having 360-degree bond to the enclosures) - of course, this provides mechanical protection too.
EUR ING Graham Kenyon CEng MIET TechIOSH
Principal and Proprietor,
G Kenyon Technology
16 February 2012 at
09:32 AM by