Grounding 2” EMT supporting PTP Radio on metal clad trailer.

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powerplay

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An Point to Point Radio mounted on 2” EMT has an #6 insulated copper wire back to the trailer subpanel inside the EMT pipe along with Ethernet cable through an 2” weather head to the radio. The metal EMT Mast is attached to the Trailer standing 6’ above the roof, but isolated from the support and Radio by rubber and vinyl tape where clamped to be able to provide an separate #6 insulated copper wire to it’s own ground plate.

If lightening did hit the metal EMT support, would the isolated path of least resistance actually reduce damage to the trailer, or likely blow the metal EMT support off the metal clad trailer and damage everything anyways ?

Thanks for any feedback !
 
I think in the event of a direct strike chances are you are not going to be able to protect much of anything no matter what you do, short of extremely elaborate measures.

I don't think the tape is going to do much good. There is a good chance of flashover.
 
petersonra said:
I think in the event of a direct strike chances are you are not going to be able to protect much of anything no matter what you do, short of extremely elaborate measures.

I don't think the tape is going to do much good. There is a good chance of flashover.
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I’m my mind I would imagine everything to just be all bonded together rather than relying on a single conductor to carry all the lightning strike energy?
 
Bonding apparently prevents an potential difference from occurring due to wind blowing particles that hit the mast and transfer energy, that could attract lightening. As far as I know it is supposed to be separately grounded independent of the sub service, but perhaps in reference to an larger community antennae , and not necessarily an point to point radio sticking 6’ off an trailer.
 
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Communications Systems Grounding Rules

Communications systems and circuits in buildings must comply with the applicable rules in National Electrical Code (NEC) 2008 Article 800. Even though these systems operate at lower energy levels, improper grounding and bonding can result in severe consequences for equipment, property and...
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Lighting can jump thousands of feet thru the air your tape won't insulate at its high voltage and amperages. If lightning strikes you antenna your trailer will be toast.
Cell towers are designed to withstand a direct lightning strike, and withstand most. For the 40+ small SCADA radio sites and repeaters I installed antennas for, I used a 2" water pipe mast, with the bottom having a 2 AWG cad welded and run directly to a ground rod. The IS was mounted on an isolated plate and the plate had a ground wire outside to the ground rod, which was connected at one point the building GES. Would it withstand a direct lightning strike, probably not but certainly a nearby one.
Read Art 810 and 820.
The gold standard for radio grounding and bonding is Motorola R56
 
powerplay said:
An Point to Point Radio mounted on 2” EMT has an #6 insulated copper wire back to the trailer subpanel inside the EMT pipe along with Ethernet cable through an 2” weather head to the radio. The metal EMT Mast is attached to the Trailer standing 6’ above the roof, but isolated from the support and Radio by rubber and vinyl tape where clamped to be able to provide an separate #6 insulated copper wire to it’s own ground plate.

If lightening did hit the metal EMT support, would the isolated path of least resistance actually reduce damage to the trailer, or likely blow the metal EMT support off the metal clad trailer and damage everything anyways ?

Thanks for any feedback !
Click to expand...
During my working life I installed a fair number of point to point antennas. These were the older technology commonly called microwave with the antenna and the radio separated from each other by at least several feet and sometimes by a hundred or more feet of tower. The critical fact that needs to be considered is whether the antenna is of a grounded design or not. If it is the grounded type then making any attempt to isolate it from it's support mast will cause damage rather than prevent it. The installation of an antenna on it's support is often enough to bond it to that mast and thus to whatever the mast is mounted on but you have taken measures to prevent that. I cannot see how that could be a good idea. If the radios you are using are the new self contained type, with the antenna mounted on the radio, then the only conductors besides that of the support mast itself is the Power Over Ethernet cable that comes down the mast to connect the point to point radio to the equipment is serves.

If the trailer were mine I would bond it's entire frame to ground using the equivalent of multiple Surface Wire Grounding System kits
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or with multiple short driven rods spaced twice there length apart. 2 to 3 feet is an appropriate length for this purpose. Either of those are easy to take up, if you have the right tool, if the trailer is to be frequently moved. If the trailer is permanently sighted, like the larger ones frequently called Mobile Homes usually are, I would drive in 8 foot rods at both ends of the frame. If that places the rods more than twice their own length apart then I would add additional rods to make them about 16 feet from each other . I would bond the mast to the frame of the trailer and it would extend to within a foot of the ground or at least to the bottom edge of the trailers frame. With a conduit bonding bushing installed at the bottom of the conduit Then bond the mast to the Grounding Electrode system from the grounding bushing. Use a #2 AWG bare copper conductor to make that connection. The US National Electric Code allows a conductor as small as #10 AWG for communications protector grounding conductor but I can assure you that is entirely inadequate. I would bring the POE Ethernet cable from the point to point radio to an Ethernet cable protector mounted at the top of the mast, through the service head you mentioned, then all the way down to an Ethernet surge protector mounted on the Grounding Electrode system and bonded to it. From that Ethernet cable protector I would run the Ethernet cable back up to another arrester bonded to the frame were the Ethernet cable enters the trailer. That protector would also get bonding to the Grounding Electrode System with the largest wire which will fit in it All of that will provide protection from the currents induced into the Ethernet cable by a nearby lightning strike. A direct strike is going to cause some damage but if you install a 4th protector adjacent to the connected equipment and also bond it to the Grounding Electrode System The connected equipment itself is likely to escape damage. This is how it was done on multiplex telephone signal Ground Radio Assemblies that were truck or trailer mounted when I was in the service some 50 years ago. Those mobile radio sets had mast assemblies which varied in length between 35 and 100 feet. The tubing sections which made up the mast were between 4 and 6 inches in diameter. Those units survived several direct strikes and remained operational. Sometimes the mast sections would end up fused to each other but the radio assembly remained operational.

--
Tom Horne
 
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