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On The Road Article #1
|HIGH VOLTAGE INTERFACE (HVI)
WHY? To protect both power and telephone company (1) personnel, (2) equipment and (3) plant from the exposure of a remote ground due to a Ground Potential Rise (GPR). May also be used to protect anyone from the exposure of a remote ground due to a GPR from a power line fault or a lightning strike.
WHEN? When the GPR is capable of exceeding 1000V peak-asymmetrical an HVI must be used in-place of standard gas tube protection on communications pairs. The failure to use an HVI will result in equipment and cable damage, and represents a significant safety risk to personnel. An HVI must be used when there is any requirement for circuit reliability associated with the circuits that are connected to remote ground.
WHERE? An HVI is required at every high voltage location where there are circuits connected to a remote ground. A remote ground may be another high voltage location. An HVI should be placed on the ground grid and sheltered from the elements in a building.
HOW? This is a very good question! There are many basic rules and a wealth of information on the subject. The best way to learn how is to take a training seminar from Bill Petersen and find an old electrical protection expert to discuss things with. A copy of IEEE Std. #487 IEEE Home will be very helpful as well as US WEST Reference Publication "Special High Voltage Protection"- #77321/June 1988.
Visualize the HVI as having a face (front-side) and a rear (back-side). The face is the high voltage side and isolates circuits from a remote ground. The rear is the low voltage or station side in which circuits do not leave the grid and are bonded to grid ground. Do not forget that PVC conduit is required for your cable entrance and steel conduit is recommended for the cables on the station. Also remember to protect the face and rear of your HVI from lightning surges and switching transients.
For more information you can always invite me to give you an overview of the big picture, but beware of the dangers of doing nothing! Gas tubes are not an HVI! The use of an HVI is a recognized necessity both nationally and internationally and is very common throughout the United States and Canada.
On The Road Article #2
PROTECTION OF THE HIGH VOLTAGE INTERFACE (HVI)
The HVI is designed to separate voltage differences on a cable pair between the station and remote ground, resulting from a ground potential rise (GPR). If you are using Teleline Isolator for your HVI, you have the greatest separation in the industry; i.e., tested at 50kV rms continuous for one minute and guaranteed to survive a 70kV Peak stress voltage. However, the HVI was not designed to withstand large surges induced on the cable pairs from either side! If you do not properly protect your HVI from these surges, you will damage or destroy your interface.
On the station side of the HVI, protect with gas tube or mutual drainage reactor on each cable pair. Also place a #4/0 with the cable as a close proximity conductor and bond both cable shield and conductor to grid ground. This should be done where the cable leaves the HVI location and again where the cable terminates at the end location.
On the remote (CO) side of the HVI, protect with the use of a lightning (surge) arrestor to reduce shield voltages. Also consider the use of gas tube or mutual drainage reactor on the cable pairs at the 300V point.
Protecting your HVI from large transient surges from either the station or remote side will be worth your money and effort. Eighty percent of all returned Teleline Isolator cards are the result of surge damage from the station side. Damage from the remote side is very small, from a number standpoint, but very large from a destruction standpoint! You can be sure this damage will result in your total replacement of the HVI!
On The Road Article #3
CABLE ENTRANCE FACILITY INTO A HIGH VOLTAGE ENVIRONMENT
The engineering design of the cable entrance facility into a high voltage environment is critical to the proper functioning of the High Voltage Interface (HVI). If care is not taken in your design to cover all entrance requirements, the HVI may not function properly and in fact may be damaged or totally destroyed during a ground potential rise (GPR).
Some of the most common problems noted after visiting over 200 HVI locations include the following: (1)grounding the entrance cable shield to station ground, (2)placing the entrance cable in a steel conduit, (3)failure to use a lightning (surge) arrestor in front of the HVI, (4)placing loop back equipment on the remote (CO) side of the HVI, (5)not considering (full count) pair protection at the 300V point (remote drainage location).
The entrance cable must be in a PVC conduit or inter-duct and the shield must float from station ground. This is the only exception in the telephone industry where bonding the cable shield to ground is the first law of telephony! It is very good practice to use a lightning arrestor to protect your HVI from direct lightning surges. Failure to consider this as part of your design may find you rebuilding a totally destroyed interface.
Whether the telephone company or the power company owns the HVI, the loop back equipment must be on the station side of this interface. This is both for the protection of the equipment as well as the safety of personnel working at the HVI. The customer network interface (NI) is on the station side of the network channel terminating equipment (NCTE). Thus, if the power company owns the HVI, they own 'interposed' equipment on the communications network.
It is always a good practice to consider the use of pair protection at the 300V point. This will protect the HVI from large surges on the cable pairs and will also protect the communications network, should the HVI fail to function properly. Heed these five common errors in special high voltage protection design and you will be well on your way to a maintenance free HVI.
On The Road Article #4
EQUIPMENT DAMAGE FROM LIGHTNING INDUCED GPR
In five years on the road and in the air with over 350 visitations to all types of sites in nine countries, I can place nearly all electrical equipment damage into two categories; improper local grounding and ground potential rise (GPR). Improper local grounding will result in the equipment being stressed (potential difference) from nearby equipment, metal objects, etc. A GPR will result in the equipment being stressed from its attachment to a remote ground at some other location through a pair of communication wires.
A wealth of recommendations by grounding consultants cover virtually everything from single point grounding, to multi-point grounding. These recommendations include everything from shielding lightning's E and H fields to diverting the strike energy away from the system ground. These recommendations are all very good, however, rarely is GPR mentioned. When GPR is alluded to, gas tubes, MOV's, SCR's, etc. are usually recommended as possible solutions.
These devices will not protect electronic equipment from a GPR, whether induced from lightning or from a faulted power line. They merely offer an additional path off the site to remote ground and guarantee a connection to the communication path in the reverse direction from which they were intended to operate!
Since the elimination of the remote ground path (communications wires) may not be a reasonable solution, why not the next best thing? Isolate the conducting path with six inches of fiber optic cable. Yes, use Teleline Isolator! QED.
On The Road Article #5
DANGER AT 911 PSAP LOCATIONS
The typical 911 center (PSAP) is a small underground building beneath a very large radio tower. This tall tower is for the dispatch of emergency services and is also a very likely target for lightning. Personnel taking emergency calls coming into the PSAP must be at the phones at all times and do not have the luxury of remaining off during lightning storms, as recommended in virtually every telephone book in the country!
This places PSAP equipment and personnel at a significantly higher risk if additional precautions are not taken. My experience shows that there is substantial damage to 911 PSAP locations throughout the country, over a single lightning season! Those of you that do not think your PSAPs are experiencing lightning strikes may want to make a visit to some of your sites and listen to the personnel tell their 'war stories'!
In addition to finding that the tower and coax are usually not grounded in accordance to recommended practices and procedures, the radio and 911 equipment within the PSAP has not been properly grounded either. This allows lightning to spark all over the consoles, turning the room into a blue glow, (which is mentioned quite often) and damages much in its path. Personnel have reported acoustical shock also.
Damage to 911 equipment can be shown to be the result of ground potential rise (GPR) at the PSAP. A typical location with a well grounded radio tower experiences approximately 8kV in GPR. The use of Teleline Isolator on the remote ground communication pairs has virtually eliminated further damage from GPR.
Why accept 911 outages? Be safe and follow these steps: (1)Review the building and tower grounding of your PSAPs and determine the damage that is being experienced from GPR. (2)Use radials (copper strap and ground rods) for tower ground, to divide the lightning strike energy. (3)Always ground the coax shield at the 150 foot point and at the base of the tower and never enter the coax into a PSAP without going through a protected bulkhead panel! (4)Bond all equipment to a common single point ground. (5)Use Teleline Isolator to isolate 911 equipment (connected to communications pairs) from remote ground.
On The Road Article #6
REMOTE DRAINAGE LOCATION (300V-POINT)
How do I find the 300V-point from the edge of a ground grid? This question is asked quite frequently. The answer requires that the following facts be known: (1)peak-asymmetrical ground potential rise (GPR), (2)grid area in square feet, (3)soil type and resistivity, and (4)graph depicting the fraction of total GPR in relation to the distance from the edge of the grid.
Frequently, this information is not readily available and an approximation would be good enough to keep the job on schedule. Thus, I have developed a chart making two assumptions. Assume a GPR = 6000V-peak asymmetrical and a two layer soil 100/20 meter-ohms.
This is a fairly typical GPR from either an earth return fault or a lightning strike, and the assumed soil type is the most common of the three basic earth types.
GPR=6000V-peak asy. / two layer soil 100/20 meter-ohms
(40)2 sq. foot grid 300V-point = 100 feet from edge of grid
(187)2 sq. foot grid 300V-point = 700 feet from edge of grid
(539)2 sq. foot grid 300V-point = 3000 feet from edge of grid
(967)2 sq. foot grid 300V-point = 5000 feet from edge of grid
This chart will provide a rough approximation for distances to the 300V-point or remote drainage location. This is the location where the high dielectric dedicated cable for the high voltage environment should merge into the general use cable.
This location provides protection to both the High Voltage Interface (HVI) from incoming surges (lightning) as well as the communications network from outgoing surges (an HVI failure). This protection is accomplished with the use of gas tube or mutual drainage reactor on each cable pair.
The graphs used to develop the above chart can be found in US WEST Reference Publication, "SPECIAL HIGH VOLTAGE PROTECTION" - #77321, June 1988. For copies call Faison Office Products in Littleton, Colorado (303) 340-3672.
On The Road Article #7
GROUND POTENTIAL RISE (GPR) FROM LIGHTNING STRIKE ENERGY
There is a 50% probability that a first stroke from lightning will not exceed 30kA. This is from Anderson-Eriksson, "Lightning Parameters for Engineering Applications". If the self-inductance of a ground grid is estimated very conservatively at .5x10-6H and lightning takes the form of a pulse which has a typical rise time of 2x10-6S, then from V=Ldi/dt; the estimated GPR will be 7.5kV. Values of GPR could easily double or triple for poorer grounding systems!
A GPR of 7.5kV on a ground grid of a high voltage environment or on the ground system of any structure will result in damage to equipment that is tied to a remote ground. Thus, any equipment that is connected to communication pairs is in jeopardy!
If we are considering a very large structure with many (1000+) communication pairs, such as a central office, the effect will be greatly reduced with the many multiple paths to remote ground. However, if we are considering a small structure with relatively few communication pairs to remote ground then we must consider isolating the conducting path.
Gas tubes, MOV's, SCR's, SAS's, etc. will not protect equipment from a GPR by isolating the conducting path. These devices merely offer an additional path to remote ground through the communication pairs. In fact, they guarantee a connection to the communication path in the reverse direction from which they were intended to operate!
Teleline Isolator is the best method for isolating equipment from remote ground. It offers over 50kV rms continuous and over 70kV Peak separation. The use of Teleline Isolator will also put you money and time ahead with less equipment damage and maintenance costs for continuous repair calls.
On The Road Article #8
PERSONAL COMMUNICATION SERVICES (PCS) PROTECTION IN POWER CORRIDORS
PCS providers are joining with power companies to acquire space for their antenna sites within power company high voltage corridors. These site locations require T1 Carrier/telephone communications service and must be protected with a High Voltage Interface (HVI), similar to the requirements outlined in IEEE Std. 487-1992. IEEE Home
A particular location along a power line corridor will generally experience more faults in a year than a substation location, because of the fault current distribution. Most of these faults are initiated from lightning. Although a corridor may experience more faults than any one substation, the fault current at any tower location is expected to be generally less in magnitude, because of this current distribution.
This lower fault current at any particular tower location is countered with a generally higher resistance platform ground built at the base of these towers. Thus, the expected Ground Potential Rise (GPR) at these antenna sites is comparable to the GPR expected at most power plants and substations. Recently calculated GPRs have been between 7kV Peak and 40kV peak, well within the capability of our Teleline Isolator Product Line.
The HVIs at these site locations are exposed to the elements and must be placed in environmentally controlled cabinets. These HVI Cabinet Assemblies are being manufactured by our EF&I Division and shipped ready for splicing immediately into the communications service.
The dedicated cable serving the antenna site should be high dielectric cable of approximately 150 feet in length and perpendicular to the high voltage corridor. This cable should be in PVC conduit for at least the last 50 feet coming into the corridor. This cable run should be kept away from the tower legs to prevent lightning damage from arcing. Lightning will arc 8 feet in 100 meter-ohm soil and will arc 25 feet in 1000 meter-ohm soil.
To insure the complete safety of the general purpose telephone cable plant (serving other customers), the dedicated cable can be spliced into gas tube protectors where it meets the general purpose cable. Thus, if the HVI fails, the general purpose cable plant would not be exposed to the fault current.
Many HVI Assemblies have been shipped to and installed by PCS providers over the last two years. These working installations have been functioning properly and without a single outage.
It is important to remember that a HVI is absolutely required to protect the safety of personnel, equipment and cable plant in High Voltage Locations from the dangers of a remote ground. This remote ground comes from wire line communications serving these locations.
It is equally important to remember that these HVI locations should not be worked on during lightning storms. In addition, working at a HVI requires proper protection of personnel by utilizing special rubber gloves and rubber mat.
On The Road Article #9
DESIGN CONSIDERATIONS FOR THE 300V-POINT LOCATION
The 300V-Point, also known as the remote drainage location, is defined as the cable plant location where the Dedicated Cable (substation cable entrance facility) and the general-use cable are spliced together. The 300V-Point, originally defined in Bell System Practices, was the remote ground from the substation where the cable shield of the Dedicated Cable is first grounded.
It is officially defined as a location point away from the substation ground grid where the Ground Potential Rise (GPR) profile has decreased to 300V-Peak or below. This point was also chosen to minimize protector block operation, which is, of course, approximately 300V-Peak! This definition may be found in IEEE Std. 487.
The best engineering design is to directly splice the Dedicated Cable into the general use cable, using individual splice connectors. This direct splicing of all pairs into the general-use cable assumes, of course, that the substation has a High Voltage Interface (HVI) which is isolating all pairs from remote ground, i.e., the Central Office (CO) or another substation, etc.
The less preferred engineering design is the use of shunting devices at this splice location. The use of shunting devices such as gas tubes or mutual drainage reactors at the 300V-Point reduces reliability of the communication services, with one more layer of protection.
These devices may also fire from induced voltage on the communication pairs, and if three element gas tubes are not used, may damage the HVI from transverse currents between tip and ring. Transverse currents may be very significant if two element gas tubes are used on each wire in the dedicated cable, an may contribute to significant damage of the HVI. Two element gas tubes are NOT RECOMMENDED.
On The Road Article #10
WINDFARMS A GROUNDING NIGHTMARE
Windfarms represent a grounding nightmare and GE grounding specifications requiring less than a two ohm system ground to remote earth are not being followed and are actually being ignored. Why? Because windfarms are being placed on mountain ridges where there are good winds, and where there are good winds there is almost always very poor soil conditions and a very lot of lightning.
Poor soil conditions result in grounding design problems and lots of equipment damage from lightning induced Ground Potential Rise (GPR). What makes the whole situation even worse is that there are generators spread out all over the place instead of all together. This places stress on the entire power generation plant. Also the switchyard is at a considerable distance from the various generators resulting in significant transients from GPR. All of these issues make the problems at a power plant look nonexistent by comparison.
Most windfarms today do not come close to achieving a two ohm system ground to remote earth. What is actually worse is that the grounding design is not capable of dissipating lightning strike energy or protect the generators spread out all over the country side from the GPR caused by the lightning strike. Why? Because those responsible for the grounding design do not understand how to dissipate lightning strike energy.
Ground rods driven in rock are lousy grounds and if they are absolutely perfectly bonded to the rock they are only going to protect against a small part of the lightning strike energy at the lower frequencies. Most of a lightning strike is higher frequency than the old 60 HZ. In fact much of it is up at 1.5 MHZ to 2.0 MHZ.
Capacitive couple grounding using radials is the answer to the grounding problem and may require the use of a low-ohm cement to make the radials look like large capacitive plates. The rest of the problem requires electrical protection engineering to evaluate what requires isolation and what can be protected by the less expensive shunting devices.
Think green but don’t forget to think proper lightning protection grounding and electrical protection also. If you don't you will be spending greenBACKS until you do!
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