atcomsystems.ca/forum Forums General General Telephone & Systems Information Correct Install of Ferrite Beads on 25pr into PBX or KSU

Generally speaking, does anyone know the correct method of installing ferrite beads onto the 25 pair 'tail' into the pbx or ksu?

Should the 25pr be 'wrapped' around the bead or should it be straight through the center?

Should the bead be at the system or at the block?

linkage would be helpful, I've googled till I'm blind and haven't found anything usefull.

Thanks

Usually the ferrite is at the device in order to reduce noise entering the cable along it's journey. I'm sure there is a more thorough explanation for why it's at the device then this as opposed to being at the source (block). This is merely more of an observation then hard facts. I've always seen ferrites used in front of the signalled device, not the signalling one.

Each time the cable passes through the ferrite it increases the impedence (or attenuation? both?) of the cable. So if the idea is to add more impedence to the cable to remove interference picked up from AC sources/EMI/RFI/etc, then I guess you would want to loop it as many times as it takes up to the point where you dont have enough signal strength to do what you need to do.

I'm sure someone else on here has a more in depth explanation (or correct one if I'm mis-remembering it) but I believe that is the general gist of it.

Dunno, I was lead to believe it was to reduce the RF traveling OUT of the ksu. And the ferrites provided with most equipment only allows 1 pass-thru of the cable. My 2 cents worth. John C. (Not Garand)

The number of turns matches the frequency of the interference that you're trying to tune out. i.e., one turn might be 800 - 1000 (AM range), two turns might be 1000 - 1250, etc.

Someone used to have a definitive guide... I thought it was Sandman, but last time I looked, he didn't have it.

Maybe I should mention I was thinking about Mitel SX50 and Panasonic TDA specifically when I posted my previous babble. I've seen the ferrites with 1-2 turns for line cords, etc., but never really investigated them. Used 'em if they came with the equipment, and that's about it. John C. (Not Garand)

The ferrite turns into a choke if it's all "tuned" right. Has to do with wire diameter, wrap rate, core size, etc etc...

Sounds like the question is wether you should use a ferrite core on noisy lines. All I can think of is call the manufacturer? They may have a spec all laid out already. Certain Size ferrite core with a certain rating with a min and max # of wraps/etc.

First, how about you tell us WHY you think you need to do this. We'll take it from there.

-Hal

My original question stems from the fact that some manufacturers include the ferrite beads with the system cabinet, along with instructions to "install on the tails" but no details on how specifically.

My boss thinks that the tails should be wrapped around at least one turn, I think that straight through is better, I used to turn them but was told not to by a former boss though I cannot remember his reasoning.

I always thought the beads were to prevent any 60hz hum from getting into the station ports.

I also understood that 'closer to the source' is better because letting the emi run the length of the cable before choking it out could degrade the signal more than choking it as close to the source as possible.

Since we are talking about choking 60hz noise that means as close to the switch as possible on the tails.

I always like a lively discussion...

Ferrite cores are used in this instance to keep RF noise from radiating FROM the system through the cables. The usually get used because somebody in the original engineering dropped the ball and the system didn't meet FCC part 68 requirements for RF emissions when it was tested. So somebody sticks a ferrite in the cables and it passes. I wouldn't worry much about using them, it will make no difference to the system and might prevent AM radio interference if one is used nearby.

If you do use them they should be placed close to where the cables exit the system. If it looks like you would need two turns (doughnut) then that is what you need to do. But again, I wouldn't worry about them.

No way would they be useful at 60Hz, you would need lots of iron for that.

-Hal

Ferrite beads will attenuate in both directions; whether you need them to reduce noise coming out of the system or going into it is a different matter of course, but simple inductance is not selective in attenuation with regard to direction.

As Hal said, you'd need much larger values of inductance to even start affecting 60Hz, since reactance is a function of both inductance and frequency: X = 2 x pi x f x L.