Telecommunications

Happy families are segregated families in cabling

The world of electronics can be a cruel one for cables – particularly the sensitive ones that carry sensor readings. They can be badly disturbed by the big power cables or relay switch arcs.

The world of electronics can be a cruel one for cables – particularly the sensitive ones that carry sensor readings. They can be badly disturbed by the big power cables or relay switch arcs.

The answer is to keep them apart, or segregated. To that end I like to group them into four main families according to the kind of signals they carry. Let’s call the families the Ones, the Twos, the Threes, and the Fours.

In the cable families who are the disturbers and the disturbed?
The Ones are power electronics lines. They carry heavy currents and aren’t sensitive to disturbances from other cables. They are big disturbers at frequencies from 50Hz up to 100kHz.  The Twos are the relay switch family.Like the Ones, they’re not very sensitive, and they are “disturbers”. When they open, they cause mega-disturbing arcs to form.

The Threes are communication links, like buses. They’re very sensitive to disturbances from the Ones and the Twos. And they’re disturbers, too, in their own right. The Fours are “victims”. They’re analogue input/output circuits and are highly sensitive to disturbance from all the other families.  But it’s not always easy to spot the difference at glance. A practical tip: colour-code them.

How does disturbance happen?
Signals don’t just flow down wires regardless. Capacitive or inductive coupling creates crosstalk, and worse, between parallel wires. Unless, that is, they’re at the right distance.

Happy Families are segregated families. Ones and Twos must be kept a safe distance from the sensitive Threes and Fours. A good spacing is 30cm, but you may not have enough room. In that event, mount all your cables on metal surfaces in your installation enclosure. They’re a free gift. You can reduce the spacing between cables and minimise the coupling that causes unwanted electromagnetic effects. Spacing can be as little as 10cm.

And if you separate cables carrying different signals with metal partitions, distance is not an issue at all. Partitions should be higher than the cables are thick and the exposed conductive parts should be connected by equipotential bonding.
Sometimes, though, you have to cross cables. You can avoid crosstalk by crossing the cables at right angles. The thing is, cables often come in bundles and you can’t always be sure what’s inside. The segregation principles applies again: separate the twisted pairs carrying different signals from each other.

Does segregation apply to cable management systems?
Cable management systems (CMSs) are increasingly used to shield against electromagnetic emissions in power and information cables. Segregation minimises disturbance in CMSs. The sketch below shows how cables should and should not be arranged.

Power cables, big disturbers

In the tray on the left, cables are packed together regardless of their families – Ones and Twos pressed against Fours. There is no segregation. And Ones and Twos’ diameters exceed the height of the tray. You can be sure you’ll have electromagnetic issues.
On the right the different families have been segregated and as much distance as possible has been put between the sensitive Fours and the powerfully disturbing Ones. And the sides of the tray are higher than the cables.

Unfortunately, the sketch on the left is how cables are arranged in the vast majority of installations worldwide. I’ve lost count of the number of times I’ve been to the sites of customer irate at their dirty signals. Yet as soon as I spaced out the cables a little, the problem abated. Just check out the rules and procedures set out in the manufacturer’s documentation.

2 Responses to “Happy families are segregated families in cabling”

  1. John Kruebbe

    One time we were connecting up a Remote Transmitter Controller (RTC) and it was not transmitting. After checking with a Data General Sniffer, we saw the correctly formatted message going to it, but what gives with it not keying the transmitter? We called Scientific Atlanta, had them self test it and no issues there. We separated the wires as suggested in keeping our Ones, Twos, Threes and Fours straight, things got better, but still it was not working reliably… We even shielded the cables especially the data bus. Ok, so we saw an improvement, but still only 90% communication with the device. What was wrong…? After checking the schematics, I noticed that there was a frame ground available, but not connected. Nothing in the docs about it. I asked them how to properly connect this… We connected the Frame Ground and presto, problem gone. Don’t forget about proper grounding as well. This was probably the most challenging problem I have still faced to this day.

    Reply
  2. Bernard Jover Bernard Jover

    Dear John,
    You are right, for communication links such as Modbus, CanOpen, DeviceNet, grounding is very important.
    Normally the Common (Ground) is mandatory to improve the immunity of signals (stable signal reference).
    In that case the use of a screened/shielded 3 twisted wires cable is mandatory. Unfortunately such cables are not so common.
    Anyway cables segregation and separation are very important and the use of screened/shielded twisted pairs with the correct impedance is also important. Best regards

    Reply

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