Русская версия: Молнии больше не страшны
One of the most actual question among wireless connection operators is the safety of active equipment while thunderstorm. Today the telecommunication equipment stagnation costs the internet service providers enormous losses. And the problem is not about what falls out: switch or access point (though it also has to be taken into account), the problem is that client would not pay money for missing service. Second no less important factor is the time of standstill, because damaged equipment is not always stored at operators or suppliers warehouses. In addition, warranty obligations do not spread at such equipment faults.
It is important to say that any active transmitting equipment has an operating license, and in case of death equipment, operator ought to provide examination works and acceptance trials for new equipment. It also takes additional money and time.
As the result operator can lose a credit of client trust and get negative reviews. So how much actually costs equipment standstill?
The cause of equipment faults may be a factory defect, unskilled installation or natural phenomena. In first case it is possible to send a product on warranty repair, the second case could be avoided, and only the natural phenomenon usually becomes a problem. But not such big and unsolvable, as before…
Each the internet service provider knows that storm with lightnings is the most damages natural phenomena. The reason is that lightnings are the powerful EMIS source and they cause faults of base stations and nodes. Therefore while designing wire Ethernet networks to reduce risks and increase reliability it is necessary to take into account EMIS influence on the communication links and active network equipment and use lightning guard.
A company ASP24 produces a variety of lightning protection equipment for networks with bandwidth of 1000 Mbps without POE, and 100 Mbps with support of POE (feed on the uninvolved pairs of line of connection). Today ASP24 is presented at the market by the next models of lightning guards:
- Грозозащита FTP mcWit 100
- Грозозащита UTP mcWit 100
- Грозозащита UTP mcWit 100 light Box
- Грозозащита для NanoBridge и AirGrid AN-100-Poe
- Грозозащита для NanoStation NS-100-Poe
- Грозозащита для NanoStation M5 и M2 NSM5-100-POE
- Грозозащита mcWit 1000
Protection is executed on a classic, proved pattern, without galvanic isolation. A big part of element base was transferred to surface mounting, which reduced dimensions and weight, and made setting of lightning guard directly into the protected device (in a corps) possible. The model NS-100-Poe, designed for wireless access points Ubiquti Nanostation, is outstanding example of this.
How does it work?
The main task of lightning guard is to limit induced interference when it starts to exceed a dangerous for equipment level. It is necessary to organize the static charge removing from a wire line connecting, for example, wireless access point and switchboard. A static charge on a line can be accumulated even in clear sunny weather, when small particles (dust, shallow garbage and other) brought by wind influence on the outdoor part of line. When the air current comes across a line, it causes a friction that electrifies the dielectric shell of cable. Accumulated static electricity can top a few kV, and without protecting devices, active network equipment could be damaged with electric hasp.
In a period before the storm wind intensity increases, and electric field generated by lightning, starts to affect the communication links. These factors create hindrances in a communication link. Sometimes hindrances amplitude can override the useful signal, and even incapacitate equipment, attaining a critical value.
Lightning guards presented by company ASP24 provide protection limits for amplitude emissions between pairs at the level of 8 volts. When static charge storage tops 90V, discharger snaps into action and all static electricity “flows” down in the earth and a charge is taken off from a communication link.
Installation does not require any additional effort!
Setting the lightning guard inside the equipment is simple enough and requires only standard tools set usage. For example, to install the lightning guard NS – 100 – Poe or NSM5 – 100 – Poe you need a screwdriver and an insulation removal instrument. For the models mcWit-100 or AN-100–Poe, when crimped cable is already brought to the point, you need only wire stripper, which takes off insulation. Anyway all set of the above-stated instrument could always be found. To make things obvious, let’s consider the lightning guard NS-100-Poe connecting example: the algorithm is represented on pic.1. First of all we are getting rid of protective insulation from cable and twisted-pair wires (pic. 1a), after that we are weakening cogs in terminal block (pic. 1b), connecting wires in accordance with the color marking (pic. 1c) and tightening cogs back (pic. 1d).
After connecting the cable we set a lightning guard into a corps, as shown on pic.2. We insert it into RJ-45 port till the click and put on a protective lid, preliminary passing through the slot a ground wire and cable. Done! As you can see, nothing complicated.
Do not forget to properly ground!
The special attention must be turned on electrical earthing (grounding). Without it equipment remains unprotected, even if a lightning guard is set! It is categorically forbidden to use for grounding a “zero wire” of 220V network, as you expose equipment to the greater risk. At breakage of a “zero wire” in the distribution board high voltage affects the equipment and can damage it. In addition, sometimes the length of a “zero wire” is very big, and as a result the resistance of such “pseudo-grounding” exceeds the legitimate value.
No less risky is grounding on the heating circuit. Firstly, it is dangerous, as so you endanger habitants of house, and secondly – foolishly, because this heating circuit may be badly or in general not earthed. Also, it happens that house habitants use the heating circuit as ground, and the electric potential between the real ground and this improvised “grounding” reaches tens or more volts.
Qualitative grounding should be organized along the shortest path with heavy gauge wire. If a lightning rod is set on an object, you can connect a lightning guard to the existing grounding tire. Make sure, that lightning rod grounding is performed safely and efficiently, from a low resistant material with a large cross-section. In addition to an electrical contact the reliable mechanical fastening of wire to the earthing strip must be provided through bolted connection.
Sometimes it is necessary to use the “soft grounding”. What is it, and what cases is it used in? Imagine a local network between two houses. Both of them have electrical earthing, but the potential difference between them is not equal to zero. So if we conduct the screened communication link between houses and earth it on either side, then shielding braid of this cable will be under tension equal to the potential difference. When this tension is small it is possible to use the “hard grounding”, but in case it exceeds a few ten volts it is necessary to use the “soft grounding”. In “soft grounding” connection is performed not directly, but through a capacitor for galvanic isolation. The capacitor does not pass constant voltage but transfers induced interference on the grounding.
Also, the “soft grounding” must be used in those cases, when points connected to the link have their individual grounding.
Let’s see how to organize a soft grounding in the lightning guard mcWit 100. On a card there is a place under three jumpers: JP1, JP2 and JP3. In case of the “soft grounding” the jumper JP1 is not set (pic. 3а).
Jumpers JP2 and JP3 are not set, if POE is used. If the power is supplied to the device directly, not using POE, then it is necessary to set jumpers JP2 and JP3 into their places.
In the lightning guard mcWit 100 UTP jumper JP1 is absent, because this model is intended for work in apartments with the use of cable without shielding braid. In the model mcWit 1000 (“Gigabit brother” of mcWit 100), conversely, there is only a jumper JP1, and all pairs of wires are involved in data transmission.
Does always a lightning guard save the equipment?
A lot of people ask themselves this question thinking about buying this device.
Testing results showed protection at the level of 80%, which means that approximately in 80% cases the residual voltage spike on the exit of protective device remains within the limits of possible and does not destroy entrance chains of the protected port.
It is important to understand that energy carried by the lightning is hugely great, and lightning guard cannot always manage with it. In practice lightning guards decrease a possible risk to the minimum, but they cannot guarantee 100% defense. The very adverse conditions can appear during a heavy gale, especially when few lightning discharges happen near a communication link with a small interval. Under such conditions a crystal in protective diodes doesn’t have time to get cold between periodic lightning discharges and finally gets overheat.
Also, when discharge hits directly at communication link the enormous amount of heat can be distinguished on the elements of lightning guard. Sometimes that burns down a lightning guard, because of what equipment is deprived defense and at next discharges can be put out of action. But despite all this the protected equipment falls out much rarer, and a risk goes down to a few percent.
How we tested lightning guards
Lightning guard were tested according to the instructions presented in the document ITU 1Vc – Per IEC 61000-4-2 Level 4, where are marked the form and methods of influence on the scheme of standard storm impulse imitation.
On the graphic (pic. 4) is displayed the form of the test impulse named “impulse 8/20 uS”. In nature duration of real lightning can reach a higher value, but the most destructive power has a part of the impulse limited by growth time of 8 uS and falling time of 20 uS.
The test head consisted of a chain of RC elements with nominal 150 pF and 150 Om accordingly and charged from a low-power voltage source 8 kV is used to make an impulse of necessary form. A discharge on the elements of testable device takes place at a complete electric contact. Wiring diagram for test setup is represented on pic. 5.
Every channel of lightning guard is exposed to influence no less than 20 discharges, wherein a form and amplitude of output signal are controlled by a storage oscilloscope.
Lightning guards are required for networks of both small and large internet service providers. Unfortunately it often happens that providers try to save money for the construction of their first links and eliminate lightning guards from the list of obligatory equipment. Such a small saving at the beginning can become a trouble in the future. Do not disregard the fact that in the future will keep your calmness and guarantee qualitative reliable connection to users.
First published in “Wireless Ukraine”. No 13-14 (1-2). 2013: pp. 54-57
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