THE PROTECTION OF WIRELINE COMMUNICATIONS ENTERING HIGH VOLTAGE ENVIRONMENTS

FOR IMMEDIATE RELEASE
(Free-Press-Release.com) December 29, 2008 -- 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.

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THE PROTECTION OF WIRELINE COMMUNICATIONS ENTERING HIGH VOLTAGE ENVIRONMENTS

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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 (IEEE members Search Database) 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.

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!

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.

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.

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 for tower ground, to divide the lightning strike energy. (3)Always ground the coax shield at the top, every 75 feet 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 isolation equipment to prevent surges from attempting to head back to the Central Office (remote ground).

More information can be found online at http://www.gpr-expert.com

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