I wonder if I can solder another wire to white #7 (ground) and crimp the cable again
Unless you have a great deal of knowledge about the process, never solder network cables. By the nature of this question, you do not have anywhere close to the required level of knowledge.
I found several PoE cable DIY videos and they just solder + and - wires from a power adapter to white-blue/blue (+) and white-brown/brown (-) wires of an ethernet cable. To me, that looks similar to what I want to do.
People making online videos may have the knowledge required to do so properly. Unfortunately many such videos are done by people like you who "think" they found a great solution and want to share it with the world without knowing why what they are proposing is a bad idea.
Not sure which videos you refer to or if you are referring to the properly (maybe you misunderstood what they were doing or I misunderstood what you are trying to pass along), but the description you provide would indicate that the videos in question are in the second video category.
I often think of these videos as the "blind leading the blind" and following their example is simply bad. Ask an expert/knowledgeable professional in the field and respect their advice.
Will such scheme damage my video recorder or camera? Will it work or it's a way to make a short circuit?
Since the device you list in question is standards compliant, it expects to be connected to a standard connection. Your fix is not standards compliant so how a device will respond is a significant question mark.
Could it be damaged? Sure. Could it short circuit? Possibly, if not likely. In any case, simply don't do it.
I crimped RJ-45 connectors on both ends so that dead wire became #8 (ground). I expected that 1 ground wire could be enough to make PoE work. My naive approach didn't work.
There is no "ground" wire in a network cable. That isn't how things work. The pairs are used as pairs for a purpose. Feel free to look into balanced-pair/differential signalling for more detail if you want more background.
Looks like swapping 6 and 8 wires works fine and there is no need to swap pairs.
Looks like you took good advice and promptly ignored it. Sure it works, but now you have split pairs and introduce a significant amount of crosstalk between the circuits (i.e. "pairs"). There are sound electrical reasons for the use of pairs and now you have violated these.
So let me say it again. Swap your damaged 3-6 pair for either the 4-5 pair or the 7-8 pair. Do not simply swap a single wire. Personally, I would then omit the old 3-6 pair and the unused 4-5/7-8 pair and only use two pair in your termination (so you have one pair terminated to 1-2 and one pair terminated to 3-6). Mark both ends of the cable as non-standard to help avoid misunderstandings/problems in the future.
Video recorder. POE: 8 ports (IEEE802.3at/af). Power consumption: PoE: Max 25.5w for single port, 80w in total
IP camera. Ethernet RJ-45 (10/100Base-T)
Assuming your Video recorder is driving the data/PoE to the camera and it is listed as IEEE802.3at/af, then I would assume your camera is as well. Of course, my assumptions could be wrong in which case you should get standards compliant equipment.
The 802.3af amendment defined two modes of operation, mode A (end span power source) and mode B (mid span power source). The two modes of operation primarily differ in the pairs that were used to provide power with mode A using the same pairs as the data and mode B using the unused pairs on a 10/100 Mbps connection. Compliant devices to be powered by PoE must be able to accept either mode.
Since the description you provide indicates your video recorder is acting as a switch, which would be a end span power source and should be using mode A. As such you should now be fine using the two pair cable as described above.
Note: you will not be able to get 1000 Mbps or 802.3bt to work on such a cable and it will need to be replaced if you need to implement either.
1"So funny to read theories of coach experts with their "35 years" of experience advising me to rebuild the concrete walls in my house" Home networking is explicitly off-topic here, and it is actually a simple matter to drill concrete for cabling. – Ron Maupin – 2019-09-24T04:05:09.743
What you are suggesting and ponder to do is most definitely not recommended. I would refrain from doing it and look to replacing the cable instead. Maybe not in the same way as the original, but install a new cable in cable trays on the wall or something. – None – 2019-09-23T08:15:04.660
1@user56700 that is not possible, unfortunately. If there is a chance to fix the cable I'd like to use it. – Nik – 2019-09-23T08:17:26.120
1The cabling standard requires four pairs for UTP, and it will fail the test suite for the cable category, meaning that the cable must be replaced. There should be a pull string to run a new cable, or you can use the existing bad cable as a pull string for the new cable (be sure to also run a new pull string). In any case, The cable is probably more damaged than you think, and the wires must be used in the pairs in which they are twisted. You cannot use two wires in different pairs as a pair. – Ron Maupin – 2019-09-23T13:29:10.607
3@Nik Re Update: you should be aware that a throughput test is no connection quality test. For the latter, you should examine the error counters on both ends, most prominently for failed FCS. – Zac67 – 2019-09-23T14:39:13.897
1@Zac67 already using the camera and video quality is perfect. I may run some tests in the future but at the moment the problem is solved – Nik – 2019-09-23T15:28:33.783
1@Nik This site is about professional networking that you can rely on for your business - just wanted to make sure that came across. – Zac67 – 2019-09-23T15:38:54.877
Yes, NE is about professionally managed networks. Sometimes, we all need to make a temporary compromise in such a situation, but a network professional would never allow such a situation to be a permanent solution. If it works for you now, you should also be scrambling to correct the situation. The way you have it is certainly not something on which you bet your business, and you would never brag about it when seeking a new job. – Ron Maupin – 2019-09-23T15:51:45.077
3Why can't you pull a new cable? Or ideally several cables? The old scrap broken non-spec existing cable can be used as a draw wire. If someone has installed the old cable poorly, now's your excuse to cut open the wall, or run capping/trunking to fix it properly, permanently, and pretty. – Criggie – 2019-09-23T22:23:01.623
1@RonMaupin It would be nice if IT got all the money it wanted, but sometimes, laid wiring infrastructure is prohibitive to repair/replace, e.g. Would require unjustifiable workplace disruption or blow the annual telecom budget. We're dealing with a hum dinger right now because of a cut of a 1500' direct burial cable... there are places not to wing-ding (mains) but a "hold it together til the next budget cycle" deal seems reasonable in many cases. – Harper - Reinstate Monica – 2019-09-23T23:16:53.273
@Harper, "'hold it together til the next budget cycle' deal seems reasonable in many cases." You seem to have misinterpreted what I wrote, which is that it should not be a permanent solution, and you should be scrambling to get it repaired, and that may mean pushing numbers up the management chain, but it should never be considered a permanent fix. We bought companies where is was always a permanent fix, and, in the long run, they generally spent a lot more in workarounds than we did to scrap and replace the entire cable plant. – Ron Maupin – 2019-09-23T23:23:05.047
@Harper, by the way, fiber is meant to be cut and spliced, but that simply isn't allowed in UTP. It introduces impedance mismatches and other problems, and may seem to work correctly, at first, but cause giant headaches in the future when it is forgotten. – Ron Maupin – 2019-09-23T23:25:18.600