Description of the problem

The user network is organized as follows, the network has been running stably since its completion. A year later, the user declared that the Industrial and Commercial Bank of China often has the phenomenon of instantaneous disconnection, the duration of 1 to 2 seconds, and later, the phenomenon of instantaneous disconnection is getting more and more serious, and it occurs several times a day until it is completely blocked in the end; at this time, the SDH transmission Metro1000 equipment 155/622H reported that the P-LOS and the red light of the router RX-Alrame is on (the reception signal is not normal).

Alarm information

P-LOS

Processing

Inspection found that the grounding situation at the site is shown in the above figure, the router of company C uses 220V AC power supply, and its protective ground is naturally connected to the 220V AC protective ground, so there is no potential difference; however, the Metro 1000 equipment uses -48V DC power supply, and its protective ground is not ultimately connected to the 220V AC protective ground, and the test found that there is a status difference of about 1.7V between the DC protective ground of the Metro 1000 and the 220V AC protective ground of the server room (0.7V of the 1.7V can be regarded as inductance, which has no energy, and can be eliminated after the coaxial cables of both sides are connected to each other, whereas the other 1V voltage can only flow through the coaxial cables from the Metro 1000 to the Cisco router, and ultimately flow to the 220V (AC ground, causing interference).

It can be seen that the cause of the problem is the 1V voltage difference between protective ground 1 and protective ground 2, and the maximum amplitude of the 34M signal is just 1V, which leads to service interruption. Since the protective ground could not be rectified, the protective ground of the Metro 1000 was connected to the 220V AC protective ground (red wire), after which the transient interruption no longer occurred.

Root cause

SDH transmission Metro1000 equipment and router's protective ground is not common ground

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