ligthing strike protection on MR-74

Comes here often

ligthing strike protection on MR-74

Hi folks,


we are implementing a wireless project with 35x MR74 with MA-ANT-20 antennas.


Regarding the best practices guide (


there is the recommendation for grounding the antenna connectors or using an passive RJ45 lighning arrestor.


The customer don't want to grounding each antenna connector because it looks very ugly. Acutally I'm not sure if RJ45 lighting arrestor (I bought some from ubiquity) is enough to protect the LAN infrastructure in case of an lighnting strike.


Do you have any additional recommandations or best practices how I can get a good optical AND a safe electrical solution.


Thanks in advance!


Kind of a big deal
Kind of a big deal

This is my personal opinion.  Others will have different ideas.


First, nothing is going to protect the kit against a direct strike.  It will tend to explode. RF or RF45 filters will melt (if they don't explode).  If you have the kit grounded then the grounding wire will probably melt (if it doesn't vapourise in an explosive reaction).


So what you are now trying to protect against is an indirect strike within several hundred meters.  For me, nothing beats (at least) several meters of optical fibre for isolation - and that is to protect the switch (and the devices plugged into that switch).  Think about how far the lightening strike has travelled from the sky to get to your device - and how much you are extending that journey to get to earth via your optical fibre.  You really want the distance to ground via your optical fibre to be longer than any other path to ground.  Lightening likes taking the shortest path.

Having the kit grounded will help bleed off excess charge - hopefully enough to stop the kit from dying.


Also note that lightning tends to go along the surface of objects.  So the outside edges of a building are the riskiest places to put APs (including indoor APs).  Of course, the outside edges (aka the outside walls) also tend to be the best locations for outdoor APs).



You need to weight up the cost against the weather in your environment.  Putting in fibre, media converters, or switches with SFP ports, grounding points, etc - all add up.

In my area - I have personally never had an AP killed by lightening.  The sunshine and corrosion are usually bigger problems over a long time than lightening.  So for me, I can't justify the cost of using lightening arrestors for my clients based on the small probability of any damage happening as a result.

It would be cheaper to accept that you need to replace the odd AP.  If you get a major amount of kit damaged (such as a direct strike) lodge an insurance claim.



Another option you could consider if you are really worried is to erect a lightning pole up high near the area to draw the lightning to it.  The ones that have counters on them are pretty cool.  With this approach you accept you will get strikes, and are just controlling where the strike will be.


If you must use lightning arrestors, you may use the Cisco AIR-ACC245LA-R kit with Meraki APs.  I say if you "must" because depending on the installation, like if antenna cable(s) or Ethernet cable(s) are entering a building, lightning arrestors may be mandatory due to either insurance requirements or local fire regulations. 


As @PhilipDAth no arrestor will protect the AP from a direct strike, but it will tend to protect the more valuable equipment like upstream switches and whatever else is in an IDF closet.  Like Philip mentioned, if the AP is on a pole out in a field and there's a length of optical fiber (no metallic wiring connections back to IT equipment) then the arrestor would be less critical.  


So I'd say the simple rule of thumb is if metallic cables enter a building (antenna cabling or Ethernet cabling, but not necessarily optical) than arrestors should be used.  If it's standalone APs on a pole (like mesh/repeater APs or fiber connected) then likely not much advantage and the added cost/labor of adding lots of arrestors will be more expensive than maybe needing to replace a fried AP, in the unlikely event it gets directly hit.


The correct part number for Meraki Outdoor AP's would be AIR-ACC245LA-N= which provides the N type connector. 


The AIR-ACC245LA-R= is for RP-TNC connectors.


We use the TerraWave LAN/PoE surge suppression.  Models is TW-SP-1GBPS-10-1T.  As they have mentioned direct strikes kill everything, but if you need to protect your IDF/MDF equipment you can use something like this. These allow the PoE to go through but will kill static surges or indirect strikes. We have these in front of several MR74s.

Here to help

Lightning arrestors more specifically Surge Protection Devices (SPD) should only be used as the last step in your full defense against lightning, as these only protect against the surge effect of lightning notably the Galvanic (direct Electrical Contact), Electrostatic coupling, Electromagnetic Induction and some also protected against Radio Frequency interference (RFI).


A well-designed lightning protection system, which if designed well can mitigate 20,000°C temperatures and currents of 250,000 amperes or more that can result from a direct lightning strike. Also, even if you avoid a direct lightning strike remember indirect lightning strikes from under 1KM radius can result in the increase in the ground voltage which in turn could damage your gear again.


Having said that I have witnessed firsthand in one project where the location was assessed to be in a 'lightning belt' (extremely prone to lightning) where a ground conductivity assessment, addition of grounding enhancers, copper earthing conductors linked to air terminals (lightning rods) have successfully protected sensitive a Port and container logistics equipment against direct lightning.


When designing it is best method is to conduct an assessment of grounding properties of the site, first obtain the

Isoceraunic maps and check the frequency and strength of lightning in your client’s location. Next is to get a specialized company to test of the soil substrates to ascertain grounding conductivity as this identifies that conductivity of the soil or whether additives are required as some soil types are poor conductors. Next, I would strongly consider the use of air terminals is the region is lightning prone, ascetics aside.


Finally, any lightning protection system implemented requires maintenance so all of the above recommendations comes down to risk management and the total cost of your ICT investment against the cost of implementing a lightning protection system. However, I believe starting with Isoceraunic maps will aid you better make those decisions. Hope this helps.


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