Local Area Network (LAN) Protection problems. Discussion and suggestions.

The IS-50BB/18 is a coaxial cable protector designed to conduct energy between center conductor and the protector’ s metal housing when a voltage differential of greater than +/- 18 Volts occurs. Lightning energy is directed through the protector to earth, rather than going through the connected circuitry on its way to earth. This "shunting" action saves the equipment from damage.

The IGA 90V (in this application) is attached to the metal housing of the IS-50BB/18 and isolates the protector’ s metal housing (to a 90 Volt differential) from the conductive surface to which it is mounted (earth ground). The purpose of this configuration is to isolate earth ground from the coaxial cable shield to eliminate ground loops during normal operation.

When a lightning event occurs, The voltage on the coax center conductor and shield quickly elevate in potential creating a voltage drop across the equipment with current flow through the equipment causing damage. When protected by the above products, the center conductor to shield differential will not exceed 18 Volts peak and the shield to local ground attachment will not exceed 90 Volts peak.

The only true test would be with a low current high potential tester to determine protector turn on. The resistance measured above is typical for the IS-50BB/IGA90V combination, before gas tube turn on.

"What are your recommendations on the grounding of protectors? Our problem was to decide which end to be grounded (as one end is to be isolated) or to ground both?"  Due to the long distance of the coax, does it make sense to isolate one end?

How the protectors are grounded is the most critical aspect of a lightning protection system. The protector assembly (IS-50BB/18 & IGA90V) must be connected to a large surface area, low inductance conductor, connected below earth, to a fast transient response, low impedance (at lightning frequencies) ground system. Shield continuity can be continued through to the equipment port since the IGA-90V will turn on and direct shield energy to earth before large currents can reach the equipment. Isolating (parting) the shield at one or both ends could create a problem with signal returns unless some way were provide to bridge the signal across the open shield connection.

During a lightning event, large fast rise time currents are directed toward earth. Any conductor in series with this path will develop an E= Ldi/dt voltage drop across its length. From the formula, the actual voltage drop across the conductor is a function of the conductor’ s inductance, the stroke peak current, and the stroke current rise time to 90% of peak value. (See PTD1011.)

"Each of the buildings have Air Terminations. The grounding panel of the IS-50BB is bonded to the down conductor of the Air Terminal. So does the electrical earth." What are your recommendations on the grounding of units to lightning rod ground conductors?

It is OK to connect to the building’ s structural lightning protection system only if all protected equipment is located on the roof. If the equipment is on any lower floor, do not bond the protector grounds to the Air Terminal down conductors! Route separate low inductance conductors from the protectors directly to earth ground.

The Air Terminals send up an upward going streamer in the presence of a downward moving step leader. When they connect, a path for the main lightning stroke is established. The Air Terminals are designed to attract a lightning stroke to themselves rather than the building structure. This reduces damage to the building structure, but concentrates the stroke current on the Air Terminal’ s down conductor. The fast rise time, high current pulse creates a distributed voltage drop across the length of the down conductor. This instantaneous peak voltage can be hundreds of KV!

If any other "ground" conductors were connected to the air terminal’ s down conductor somewhere along its length, a high voltage differential between the local "floor potential" and air terminal down conductor distributed potential could occur at that connection. This high peak voltage could exceed the "turn on" threshold of both protectors and switch energy back through the connected critical circuitry towards "floor potential" chassis ground. This potential difference could cause current flow through the critical circuitry towards any other building ground connection. The circuitry would probably be destroyed in the process. The condition would be even worse if there were coax cables from roof mounted antennas coming down to the lower floor. If there were separate protector ground conductors connected, through the building, to the earth ground system, damage could be avoided.

Twisted pair protector grounding methods are the same as coax protectors – with the same precautions. My personal preference would be to connect a jumper with a cut back, isolated shield on the protector side, from the protector to equipment, at both ends of the run. Connect the jumper shield to the low impedance ground system if possible, not to the protector housing. Do this only if it is practical and doesn’ t create ground loop problems.