[TenTec] Lighting and ground loops caused by tying negative lead to chassis.

Stuart Rohre rohre at arlut.utexas.edu
Wed Nov 30 19:27:49 EST 2005


Let's see if I can make it plain.   One of the club folks grounded 
everything in the shack with both the coax shields, and the chassis running 
to ground braids to a Bus.

He then grounded the negative post of the Astron as well, which then put it 
at chassis AC ground. (Grounded it to its case and by virtue of the braid, 
its case back to station ground bus).

The lightning surge came in on the SHIELD of the highest (2m stacked beams), 
in SPITE of plumbers delight, (grounded beam elements), construction.   (And 
a tower grounded to the earth with all coax and control cables inside the 
tower strapped to one leg.)

AND, the cables make a bend to induce high impedance to surges, before 
entering a roof penetration/ weather head.

 It went into the 2m rig on the coax shield, and exited to earth via the 
negative lead (by the burn marks), as well as the braids, but spread enough 
fault current back into the Astron on the way to its AC ground, to blow the 
regulator.  The user turning on the supply, found it now had gone to 
unregulated something-- well in excess of 13.8 VDC.

The burn marks were evident- that the current came on the power supply 
negative lead and back out the AC chassis ground/ braid or vice versa, 
(depending on which way you think the current flowed).  There was clearly a 
ground loop created by both the grounded negative lead, and a ground braid 
to chassis of the Astron; (the Astron negative was tied to chassis).

Now it is possible that the arc to Astron AC ground and chassis would not 
have occurred from the negative if it had gone (ungrounded) straight to the 
rig.  The rig got it in either case, (damage).   The supply might have 
escaped losing its regulator if there had been only the chassis braid 
connection to case to station ground.   BTW, station ground is a copper 
bronze flat strip run around the base board to  wall conduits where the 
electrical and telephone feeds come in, and from the outside box, the strip 
runs to a cold water faucet one foot from the wall conduit path.

The earth conductivity in that area should be pretty good as it is 'black 
land prairie", rather than the rock of the west side of town.  However, we 
have seen problems in AC line grounding nearby; leading to strikes across 
the parking lot behind the station destroying 2/3 of a pole that had a 
faulty ground wire; while that same time, the beams and tower drained 
charges and the station escaped.

An ungrounded telephone PBX at the end of the same station room had blown 
cards.  (The PBX repair folks then grounded it to our copper bronze strip 
when they installed new cards).

Lightning can be very variable in the paths it follows.
What I am saying is if some parts of the station have multiple lower 
resistance ground paths, you may make them desirable paths to a surge, at 
the cost of that supply, while other parts of the station escape. 
Incidentally, while we lost a rotor fuse, the rotor and the HF beam under 
the VHF 2m stack escaped damage, as did HF antenna switch and HF rigs.  On 
lower VHF/UHF antennas, going to packet gear on other side of station, we 
did get some damaging surges that played havoc among the rigs' packet 
computers/ TNCs.  All that gear has a "halo" of copper braid along the wood 
shelves, surrounding the equipment with short paths to ground.

YMMV.

Stuart
K5KVH 





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