Lightning surge protection for 3G/4G/5G Radio Cellular Masts

Do you actually mean the antennas, or the power supply to the base station electronics. 

BS7671 does not have a lot to say about the former, and as far as  the latter is concerned may be TT or TNC, and then as far as the mains supplies are concerned it is SPDs and filters.  

How is the back-haul link being provided ?

The antennas, lightning,  and the earthing system will need to be considered as one, depending on the location it may well be the largest lightning conductor for quite an area unless you try and design it so something else is instead.

Mike.

It doesn't really make sense. Presumably your trailer has a generator, and if this is off some batteries. Your only significant risk is a lightning strike to the mast (probably not the antennas) and you will not be able to prevent damage to something. The BS7671 “surge protection” says nothing about any of this, but it is an operational risk of mobile equipment in high places. What is done about normal masts, because something similar might work, but don't try to reinvent the wheel? You may well have big problems fitting any kind of LPS Earth on a mobile site, and it is probably not worth the effort or cost.

Your RF coverage will be very little affected whatever you do with the mast Earthing. I suggest this is not a suitable project for you to consider in any way! Earth resistance is not the biggest factor in LPS.

I have done some work in respect of earthing for a well known Mobile Phone company. I can confirm that they have very comprehensive earthing systems, LPS and surge protection. As David above says this is not a job for someone without a comprehensive engineering knowledge of the subjects.

but please can you tell me what TT and TNC mean

It's a shorthand description for the Earthing arrangement for the (LV) supply - T = Earth (Terra), N=Neutral, C=combined, S=Separate, I=Isolated (or Impedance Earthed)

So TT - means the supply star point (and hence supply N) is earthed by its own Earth electrode and the consumer's equipment is also Earthed by it own electrode. So you end up with two (or more) separate electrodes. Traditionally seen on overhead rural supplies, but also seen where the supplier's earth facility is considered unsuitable for some reason.

TN - means again the supply star point (and hence supply N) is earthed by its own Earth electrode, but this time the consumer earth is connected to the same point (i.e. the supply N). There are a few different sub-options:

• TN-S - where the consumer-side earth is connected to the star point (N) of the supply by a separate PE conductor throughout
• TN-C - where the consumer-side earth uses the supply N coductor to connect to the star point (N) - i.e. a single conductor serves as a combined N and PE.
• TN-C-S - where the earth is partly combined and partly spearate in the same system - often TN-C in the DNO's cables and separate within the consumer's installation.

Where you have a local generator, it's usual to have a TN-S arrangement where it's Earthed (or a separated or IT system where the system isn't Earthed).

Each system has its pros and cons. TN-C and TN-C-S systems have a particular issue where a single break in the supply PEN conductor can result in exposed metalwork in the consumer's installation being made hazardous live, so it's avoided in certain situations such as the supply to caravans or boats. TT systems are typically totally reliant on RCDs for shock protections (rather MCBs or fuses) and RCDs have a notorious failure rate. TN-S systems can have a faulty PE connection without anyone noticing until it's too late.

TN and TT systems also have somewhat different requirements when it comes to SPDs.

   - Andy.

Ah mobile temporary radio. Nothing like fixed base stations at all really.

The microwave backhaul and gensets makes it simpler, as there are now no external cables coming on to site at all, so there is little scope for network induced transients.

You will not really ever mitigate a direct strike, just make sure the equipment is insured against it, and if there is anything critical that there are spares in stores ready to roll.

You may need policies in place that say that the gensets may not be refuelled  while there is a lightning state, but in reality it is no big deal for containerised units, as you can have extra fuel tanks on ‘tiger loop’, and the run time is so long you can always pick the moment.

Given your antenna array beam angles, I presume these are sector panels, and then the mast is ‘behind’ them and has only a modest effect on rad pattern. The usual is to extend the tower, or at least a spike off the top of it, some way above the antennas, then bond the coax outers to the tower at the bottom  and have electrode(s) there, and include gas discharge devices in the coax, if such things are not already part of the diplexing. Surge protection on the mains side is largely irrelevant as you have total control of that - it is your own genset !!!  And as has been explained above, it will be TNS, so no worries there.

But given the costs and risks, I suggest you ask the equipment providers to review your plans and  advise.

And as an aside be aware that a genset for XX watts cannot take a shock load from nothing to XX without stalling, more like maximum load step of perhaps XX/3 so you may need a larger genset than you realised if a bang-on start is possible.

Mike.