Article we talked about the new Zyxel switches designed to build video surveillance systems and other segments of the IT infrastructure using power over PoE.
However, just buying a good switch and connecting the appropriate devices is far from everything. The most interesting may appear a little later, when this farm will have to be serviced. Sometimes there are peculiar pitfalls, the existence of which should be known.
Copper-plated twisted pair
In various sources of information on the use of PoE, you can find a phrase like "Use only copper cables." Or "Do not use for CCA twisted pair". What do these warnings mean?
There is a well-established misconception that twisted wire is always made of copper wire. It turns out not always. In some cases, the manufacturer, in order to save money, uses the so-called copper-bonded cable.
In fact, this is an aluminum cable, in which the conductors are covered with a thin layer of copper. Full name "copper-clad aluminum twisted pair"
A twisted pair of solid copper conductors is marked as "Cu" (from the Latin "cuprum"
Copper-plated aluminum is referred to as "CCA" (Copper Coated Aluminum - copper-coated aluminum).
Manufacturers of CCA may or may not label at all. Sometimes completely unscrupulous manufacturers draw the “Cu” parameter on a copper-plated aluminum twisted pair.
Note. According to GOST, it is not necessary to put such a marking.
The only indisputable argument in defense of a copper-bonded cable is its low price.
Another much less significant argument is less weight. It is believed that coils with aluminum cable are easier to drag during installation, because the specific gravity of aluminum is less than that of copper.
Note. In practice, not everything is so clear. The weight of the package, the weight of the insulation, the availability of improvised means of mechanization, and the like play their role. It takes about the same amount of time and effort to bring on a trolley and lift on an elevator 5-6 boxes with coils of CCA cable, as the same number of boxes with coils of "full copper".
How to accurately recognize an aluminum cable
Copper-plated aluminum is not always easy to recognize. Tips like “Scratch the surface of the wire or estimate the weight of the cable coil by lifting it in your hand” work very relatively.
The most accessible and fastest test: set fire to the stripped end of the wire, for example, with a lighter. Aluminum quickly begins to burn and crumble, while the end of a conductor made of pure copper can become red hot, but retains its shape and, when cooled, returns physical properties, such as elasticity.
The dust left from the ignition of copper-plated aluminum is, in principle, what such an “economical” cable turns into over time. All the scary sysadmin stories about "sleeping cables" are just about the "copper".
Note. You can strip the wire from insulation and weigh it, calculating the specific gravity. But in practice, this method is rarely used. You need accurate scales installed on a strictly horizontal flat surface, and free time to do this.
Table 1. Comparison of the specific gravity of copper and aluminum.
Our friends from NeoNate, which, by the way, makes a very good cable itself, made this help you.
Loss of transmission power
Let's compare resistivity:
-
resistivity of copper - 0 ohm*mm0175/m;
-
specific resistance of aluminum - 0 ohm*mm0294/m/
The total resistance of such a cable is calculated by the formula:
Given that the thickness of the copper coating on a cheap copper-plated cable "tends to zero", we get more resistance due to aluminum.
But what about the skin effect?
The skin effect is named from the English word skin - eng. "leather".
When transmitting a high-frequency signal, an effect is observed in which the electrical signal is transmitted primarily along the surface of the cable. This phenomenon serves as an argument by which manufacturers of cheap twisted pair are trying to justify the savings in the form of copper-plated aluminum, they say, "the current will still go through the surface."
In fact, the skin effect is a rather complex physical process. To say that in any copper-plated twisted pair the signal transmission will always go strictly along the copper surface, without “capturing” the aluminum layer, is not a completely fair statement.
Simply put, without having a laboratory study on this particular brand of wire, it is impossible to reliably state that this CCA cable, due to the skin effect, transmits no worse than a high-quality copper cable in terms of characteristics.
Less strength
Aluminum wire breaks much easier and faster than copper wire of the same diameter. However, "take and break" is not the biggest problem. A much bigger nuisance is microcracks on the cable, which increase the resistance and can lead to a floating signal attenuation effect. For example, when the cable is subjected to bending or temperature effects from time to time. Aluminum is more critical to such influences.
Criticality to temperature difference
All physical bodies have the ability to change volume under the influence of
temperature. With a different coefficient of expansion, these metals will change in different ways.
This can affect both the integrity of the copper coating and
quality of contacts at the junctions of aluminum conductors and devices
mounts. The ability of aluminum to expand more with increasing temperature
contributes to the appearance of microcracks that worsen electrical
characteristics and reduce the strength of the cable.
The ability of aluminum to oxidize faster
In addition to thermal expansion, one must take into account the property of aluminum to oxidize quickly, as evidenced by the lighter test.
But even if the aluminum wire is not exposed to open flames and external high-temperature heaters, over time, when the temperature drops or when heated by the transfer of electric current to power devices (PoE), more metal atoms come into contact with oxygen. This does not improve the electrical properties of the cable.
Contact of aluminum with other non-ferrous metals
Aluminum is not recommended to be connected with conductors made of other non-ferrous metals, primarily with copper and copper-containing alloys. The reason is an increase in the oxidizability of aluminum at the junctions.
Over time, you will have to replace the connectors, and redo the conductors in the patch panel. It's frustrating that floating bugs can be associated with this.
Problems with PoE for copper-clad twisted pair
In the case of PoE, the electric current to power devices is transmitted partly through the copper coating, but mainly through the aluminum filling, that is, with high resistance, and, accordingly, with large power losses.
In addition, other problems arise: due to the heating of the wires during the transmission of the power supply current, for which this twisted pair was not calculated; due to microcracks, wire oxidation and so on.
What to do if the SCS with copper-plated aluminum cable was "inherited"?
It must be borne in mind that some of the segments will eventually have to be replaced (for one reason or another). It is better to immediately reserve money in the budget for this case. (I know it sounds like fantasy, but what else can you do?)
Monitor the status of the SCS. Monitor temperature, humidity and other physical indicators in rooms and other places where twisted pair cable passes. If it is hotter, colder, more humid there, or there is a suspicion of mechanical effects, such as vibration, it is worth considering preventive measures. In principle, in a situation with a traditional copper twisted pair cable, such control also does not hurt, but aluminum wires are more capricious to these phenomena.
There is an opinion that it no longer makes much sense to purchase some particularly good patch panels, network sockets, patch cords for connecting users and other passive equipment. Since the wired part, let’s say, is “not a fountain”, then it may not be worth spending on a cool “body kit”.
On the other hand, if over time you still want to replace such a wonderful “basically no different” twisted pair CCA with time-tested “copper” - is it worth it to act on the principle of “one step forward, two steps back”, buying now patch panels and sockets at a bargain price?
You also need to be very careful about sudden loss of communication. When for some time there was not even a ping, but for now they were looking for - “everything miraculously” was restored. The quality of the cable and connection can play a significant role in such incidents.
If you plan to use PoE, for example, for CCTV cameras, for this section it is better to immediately replace the twisted pair with copper. Otherwise, you may encounter a situation where you first hung up a camera with low power consumption, then changed it to another and you have to puzzle over why it doesn’t work.
5E is good, but category 6 is better!
Category 6 is more resistant to interference and temperature effects, the twisting of conductors in such cables is carried out with a smaller pitch, which improves electrical characteristics. In some cases, in cat. 6 separators are installed to separate the pairs (remote from each other in order to prevent mutual influence). All this increases operational reliability.
For connecting devices with PoE, such changes will come in handy, for example, to ensure stable operation of the network during temperature fluctuations.
SCS cables are sometimes run in rooms with weak climate control, for example, through the space above the ceiling, in the basement, technical or basement floor, where the temperature difference during the day reaches 25°C. Such temperature jumps affect the characteristics of the cable.
Laying a more expensive, but more reliable category 6 cable with better characteristics instead of category 5E is not an increase in “overhead”, but an investment in better and more reliable communications.
The Russian representative office of Zyxel conducted their own study of the dependence of the allowable distance for PoE power transmission on the type of cable used. Switches were used to test
Figure 1. External view of the GS1350-6HP switch.
Figure 2. External view of the GS1350-18HP switch.
For convenience, the results are summarized in a table divided by video camera manufacturers (see tables 2-8 below).
Table 2. Test procedure
Test Procedure
Step
Description
1
Enable extended range in port 1,2
-GS1300: DIP switch to ON and press reset&apply button on front panel
-GS1350: Login Web GUI > Go to "Port Setup" > enable extended range and apply.
2
Connect PC or Laptop on the switch for camera access
3
Connect Cat-5e 250m cable on Port 1 and connect camera to power up.
4
Use PC/Laptop to PING the camera IP, should not see ping loss.
5
Access camera and check if video quality is good and smooth.
6
If step#4 or 5 failed, swap the cable to Cat-6 250m and re-test from step#3
7
If step#4 or 5 failed, swap the cable to Cat-5e 200m and re-test from step#3
Table 3. Comparative characteristics of cables for connecting LTV cameras
Table 4. Comparative characteristics of cables for connecting LTV cameras (continued)
Table 5. Comparative characteristics of cables for connecting LTV cameras (continued 2).
Table 6. Comparative characteristics of cables for connecting UNIVIEW cameras.
Table 7. Comparative characteristics of cables for connecting UNIVIEW cameras (continued).
Table 8. Comparative characteristics of cables for connecting Vivotek cameras.
Conclusion
The problems described in the article are not mandatory for purchase. Perhaps there is a person who will say: “In my projects, I always use exclusively copper-plated twisted pair of category 5E and I don’t know any problems.” Of course, the quality of workmanship, operating conditions, periodic monitoring and timely maintenance play an important role. However, there is still a need to use PoE, and for such a situation, the use of Category 6 twisted-pair copper is a more promising solution.
Possible savings when using cheap copper-plated twisted pair is quite specific. If we are talking about large-scale projects of the Enterprise level for an IT-critical business, it is wiser to use a high-quality copper pair from proven, well-established manufacturers. If we are talking about small networks, then saving on a twisted pair cable, especially in the context of a “coming admin”, looks rather doubtful. Sometimes it's better to pay extra for a quality cable to eliminate potential problems, increase reliability, extend the range of capability (PoE), and reduce the cost of service.
We thank our colleagues from the company for help in creating the material.
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Sources of
Source: habr.com