Mystery of Lightning Lightning is mysterious because it occurs in a time frame that is so different from our normal experiences. We are familiar with 60Hz AC distribution and DC battery systems. This is our normal frame of reference. AC is like driving a car down a highway and DC is like walking along the same highway but lightning is like a supersonic jet flying at car level shooting off super- supersonic rockets. So Lightning doesn’t seem to react the same way, it doesn’t need a conductor to move, it makes its own paths usually invisible paths through the air.
Mystery of Lightning Lightning’s time horizons are microseconds, millionths of a second. Lightning can be understood if we think in the timeframe of the lightning event and analyze the paths based on high-speed charge movement not the time frame of familiar electrical activity.
Surge Damage Investigation: Radio/Fiber Bungalow 150’ Tower 150’ Rolling Sphere of Lightning Protection Zone Maple Creek Saskatchewan Communication Tower Site View from the East
CPR 10 by12 Communication Site 2by6 wood/drywall construction with metal skin Antenna Coax & Lighting Cables Ground Wire
Site Physical Geometry: Remote Connections East North
Electrical Connections Telephone Grounds Fiber with Shield Ground AC Power Radio Racks Fibre Racks Coax AC SPD Skid Frame
Telephone Grounds Fiber with Shield Ground AC Power Radio Racks Fibre Racks Coax AC SPD Skid Frame Coax Arrestors- No DamageDamage Locations
Damage Summary: 1) All three telephone line connections (at both end of wires) 2) Radio Equipment In-Line AC protector and internal power supplies 3) Fibre AC connections to Power Supply 4) No Damage on Coax Antenna connections 5) No Damage to upstream AC power 6) No significant damage to Telephone equipment in Dispatcher Office where 25 pair cable originated from 7) No Damage to the Fibre Equipment- still functioned from Battery
How Did the Damage Occur? What was the Damage Mechanism? Challenge: What current flow path can explain: - the observed damage - the undamaged equipment - the undamaged remote connections.
Radio2 Radio1 RTI Fibre 2nd Row Charger Radio Telephone Interface Lightning Leader Strike Equipment Ground Plate Strike Raises Potential of Ground Plate Forcing Current into Site Telco Remote Cable Current Flow Through Equipment Vaporizes Conductors Fibre Charger Internal wiring Inductance Voltage moves faster than external Grid Inductance Voltage driving Current through AC Chargers No Coax Damage In Line 120V SPD
Multiple Equipment Damage Caused by: Ground Plate Potential Rise driving current into site Current Flow through equipment out to remote telco connections vaporizing small wires Potential Difference between Ground Plate and far side Power Ground driving current through Power Supplies vaporizing conductors
Surge Mitigation Strategies: 1.Reference all external connections to a Low Impedance Reference Plane (LIRP) with Surge Protection Devices (SPD’s). 2.Incorporate this Low Impedance Reference Plane (LIRP) into a conductive enclosure following the Faraday Cage shielding principles. (Charge applied to a conductive enclosure is forced to the outside of the enclosure).
CPR’s Implementation of Faraday Cage and Low Impedance Reference Plane (LIRP) Wiring
4.Reduce Z*di/dt voltage rise across equipment by avoiding surge current paths through equipment. Designating one location ie top or bottom as the common path for external connections and routing all external connections via this path best accomplishes this. Avoid hidden paths such as connection of rack mounting bolts into frame of building beneath floor. 5.Use current division to control the dissipation of lightning strike energy on an antenna tower grounding system through multiple paths 6.Treat Surge Energy Conduction as high frequency current (~100Khz). The more surface area the surge path has the less Voltage Rise. For the same amount of metal a flat bar or hollow tube has less IZ (Impedance) voltage rise.
7.Maximize bend radius in surge current wire paths. Surge currents are a high force event like water surging into a fire hose. Bends greater than 12” do not increase IZ voltage rise. 8.Minimize coupling between surge current carrying wires and signal wires. Insulated wires run close to each other couple 70% of surge energy between wires Insulated wires run with a 3” separation couple less than 40%, 6” less than 25% Wire run against a conductive plane to radiate energy into plane and less into free space. Wires separated by a conductive plane have minimal coupling. Wires crossing at right angles have minimal coupling
CPR Lightning Damage Investigation and Mitigation Strategies Peter Brackett Canadian Pacific Railway Peter_Brackett@cpr.ca 403-319-7781