Lightning Strikes. Presented to the Greater Norwalk Amateur Radio Corporation Inc. February 8, 2017 Steven M. Simons W1SMS

Transcription

1 Lightning Strikes Presented to the Greater Norwalk Amateur Radio Corporation Inc. February 8, 2017 Steven M. Simons W1SMS ARRL CT State Technical Coordinator

2 The Power of Lightning What is a Ground? Design of a Lightning Protection System Your Station Protection What we will cover today

3 What is a lightning strike? The potential difference between the clouds and a conductor on the earth. How does lightning start? Polarization of charges in the cloud to positive & negative Rising moisture collides with water droplets electrons are ripped off causing separation of + & - Electrons (-) are repelled by bottom of cloud. Buildings, trees, people are typically positively charged (attracting electrons) The Power of Lightning

4 The mechanics of a lightning strike Static charge in cloud builds up Air is normally a good insulator but, strong electric fields around the cloud ionize the air and make it more conductive Ionized air then turns into a low impedance plasma discharge acting as a conductor from the cloud to earth Initial speeds are 60 miles/second leading to ultimate speeds of 50,000 miles/second Strike current of 30,000 to 250,000 amps! The Power of Lightning

5 How is Lightning Attracted to your Station & Antenna? Lightning is not really attracted but just happens. Height above earth Impedance of the conductor Proximity to other low impedance objects The Power of Lightning

6 How many of you have had lightning damage? To your well? To appliances? To electronics? WHY? Did you have a proper grounding system installed? How many rods? Where? What type of conductor? wire, braid, solid strap What is a Ground?

7 Resistance Rod length Ground rod length vs. resistance Longer is Better What is a Ground?

8 1 Rod = 33 ohms Compliance to electrical code 2 Rods = 15 ohms 3 Rods = 12 ohms 4 Rods = 7 ohms 15 Rods = 3 ohms Resistance # of Rods What is a Ground?

9 The Difference between AC Power and RF Power / Lightning Grounds AC Power ground - typically located at the service entrance / meter provides a redundant return path for the neutral wire and a safety ground for the house electrical system. Usually 2 ground rods connected with a piece of solid copper wire. RF Power / Lightning ground - very low impedance ground consisting of an array of ground rods that are bonded together. The array of rods may circle the tower or tower + structure. The connection may be large diameter solid wire and/or copper strap. Sometimes clamped to the rod but better to be soldered or CAD welded. Typical service entrance grounding What is a Ground?

10 Ground rods and clamps Don t use this type on tower legs - too weak! ½ > 5/8 typical diameter 8>10 long Clamps used to fasten the ground conductor to the rod CAD weld is a commercial method Soldering is not recommended Grounding Equipment

11 1/4wl DC short type Replaceable element type for Ethernet Commercial types: Coax lines - Telephone - CATV - Data/Ethernet Coaxial lightning arrestors & surge protectors

12 Protect connection against corrosion Ground the shields of hard line by exposing the solid copper shield Components available from local electrical supply, ham radio dealers, McMaster-Carr & Grainger Grounding System

13 All equipment and devices MUST be bonded together. Both indoor and outside. Outdoor connections With isolated connection bus Use Star Point connection technique Grounding System

14 Typical commercial grounding system Use Star Point connection technique Grounding System

15 Typical guidelines for connection include: Only one grounding system. All ground rods should be bonded together to form a single electrode system. All grounding in or on a structure shall be interconnected to provide a common ground potential - AC power system - tower/antenna - lightning protection - telephone system - exposed structural building steel - underground metallic piping. Coaxial cables should have their shields grounded at the base of the tower and at the entrance to the shack. Clamps used to secure ground rods/wires should be weatherproofed and sealed to protect against moisture. Use multiple ground rods for lowest impedance. Use lightning arrestors at cable entry points to the shack. The take away here is that during a lightning strike, the grounding conductor system will rise above ground based on massive current flow - up to 200,000 amps - if all devices are at the same potential, no damage occurs. COMMON GROUNDING (EARTHING)

16 Do not forget grounding of tower & shack Lightning protection Additional Safety Preparations

17 How complex is your station? What is your budget? Create a BOM (Bill of Materials) Consult the literature and ARRL Handbook for detailed information Remember, some grounding is better than no grounding! Your Station

18 Ground rods Typically 5/8 diameter x 8 long - steel with copper plating At least 2 in parallel outside of the house by the coax entrance Spacing between rods 8 Arranged in a ring around protected structure Use #6 copper wire or flat strip to connect Use proper clamps & protect against weather Your Station

19 Tower Grounding Multiple ground points from tower base Coax cable shields grounded as they leave tower Your Station Image by NW3Y

20 Station Grounding Bus Copper pipe or flat aluminum strip - drilled for screws Conductors to radios and devices can be copper braid taken from old RG8 cable Crimp ring lugs or just flatten braid, flood with solder and punch a hole. Use braid indoors only Limit soldering as it can explode during a lightning event Your Station

21 Shack Grounding Inside Ground Panel with lightning arrestors ¼ wave shorted stubs made with coax and tee adapters Grounded panel Commercial coaxial arrestors Copper bus bar ground Your Station

22 ¼ wave shorted stubs Stubs are your friends Fabricate from coax and tee adapters 8.3 MHz Your Station 1:1 SWR 50 Ohm impedance Provides a DC short between center conductor & shield

23 ¼ wave shorted stubs Fabricate from coax and tee adapters Shorted end Tee connector to place this stub in parallel with the antenna or equipment Your Station

24 ARRL Handbook Tessco MFJ McMaster-Carr Supply DX Engineering Grainger Rohn Towers Polyphasor Resources