Voice-over-IP (VoIP), T1, E1, ADSL, HDSL, VDSL, SDSL, video, wireless local loop - all have one thing in common. Each application involves telephone line interfaces that need protection from lightning and power-cross faults, both for the safety of customers and maintenance personnel and to meet circuit protection requirements.
Equipment designers can follow 10 steps to determine which of the many safety requirements a telecom design should meet and to find an appropriate circuit protection design.
- Decide where your design will be sold
Deciding which agency requirements to meet depends on where your design will be installed, as circuit-protection agency requirements are not global. In North America, the dominant agencies for secondary protection of the PSTN interfaces are Underwriters Laboratories (UL), the Federal Communications Commission (FCC), and Telcordia Technologies (formerly Bellcore). In Europe and most of the world, the International Telecommunications Union's (ITU) K-series recommendations are the most widely referenced specifications.
Designers who plan to meet only North American requirements will follow the circuit protection guidelines of UL, FCC, and Telcordia. These include power-cross and lightning fault requirements that are higher than those found in the ITU requirements. If the design is to be sold only outside North America, lower surge rated components can be used to meet the ITU requirements. If the design is to be sold anywhere in the world, the designer can follow the North American circuit protection requirements and meet the ITU circuit protection requirements in the process.
In Canada, the Canadian Standards Association (CSA) requirements apply, rather than the FCC requirements. However, the FCC Part 68 requirements were recently harmonized with the CSA requirements.
- Decide who your customers are.
Knowing your customers is critical to knowing which specific agency requirements should be met. For example, if your design is for customer premise equipment (CPE) in North America, you'll want to meet UL1950 and FCC Part 68 protection requirements. (UL60950 will soon be published and will also cover CPE.) Network equipment falls under the Telcordia GR-1089 requirements.
In the rest of the world, ITU K.21 dictates requirements for CPE, and ITU K.20 dictates requirements for network equipment. A new ITU standard, K.45, will apply to access network equipment installed between a central office and customer premise building.
- Decide which agency requirements to meet.
For CPE, a designer should meet the UL1950 (or soon, the UL60950) and FCC Part 68 requirements. The FCC Part 68 requirements come from the US government and carry the force of law. Any CPE connected to the PSTN must meet these requirements. As discussed above, a designer of network equipment to be sold in North America will typically follow the Telcordia GR-1089 requirements.
In many cases, CPE is designed to meet the Telcordia GR-1089 requirements. If a piece of equipment could be used as CPE or network equipment, the designer can cover all possibilities by meeting the Telcordia GR-1089 network equipment protection requirements. Digital loop carriers, for example, may need to meet both UL/FCC and Telcordia requirements.
- Choose protection against real-world faults.
Meeting agency requirements in a test lab is one thing; surviving in the real world is another. While very few designs can adequately protect against a direct lightning strike, agency requirements cover the more common instance of lightning surges induced by nearby strikes. They also cover the probable worst case power-cross faults that may occur if a power line comes into direct contact with a phone line, or, more likely, induces a fault current in a nearby phone line.
The Telcordia GR-1089 requirements protect against the most severe faults of the agency standards discussed here. And what if the equipment will not connect to the PSTN, but instead to internal telephone wiring, such as the inside lines on a PBX or wireless local loop system? The Telcordia GR-1089 intrabuilding requirements are becoming more common for such situations. These requirements are less stringent than the Telcordia GR-1089 outside line (PSTN) requirements. In most cases, primary protection at the building entrance guards against faults entering the building. Nevertheless, real-world faults such as power-cross can occur on inside lines, and protecting against them is good engineering practice.
- Choose your protection components.
Protection components for telecom interfaces can be divided into two categories: overvoltage protection and overcurrent protection. Overvoltage protection components go in parallel - between the tip and ring, tip and ground, and/or ring and ground. They normally feature a high impedance, switching to a low impedance in the presence of voltage above their breakdown rating. Typical overvoltage components include thyristors, TVS diodes, MOVs, and gas tubes.
Overcurrent protection components are placed in series, in the tip or ring line, or in both. They normally feature a low impedance and switch to a high impedance in the presence of a current that is above their trip current rating.
- Consider resettable vs. non-resettable protection.
The question of resettable versus non-resettable applies to overcurrent protection. Resettable overcurrent devices include polymer PTCs and ceramic PTCs. The term "PTC" refers to a device with a positive temperature coefficient of resistance. Under excess current conditions, PTC devices heat up and undergo a phase transition, which causes their resistance to increase by several orders of magnitude. This greatly reduces the current flowing in the circuit and protects sensitive downstream components. Non-resettable devices include one-shot fuses.
The advantage of resettable devices is obvious; once the overcurrent fault is gone, the device cools down and returns to a low resistance state. No user intervention or technician service call is required. Non-resettable devices must be manually replaced by a technician or other qualified person, creating unnecessary warranty costs or high operating costs when equipment is located at remote sites. Fortunately, most of the overvoltage protection components used today for secondary protection are resettable.
- Prepare your design for agency testing.
Once the protection components are chosen and installed on the prototype board, the unit can be tested.
If an onsite test facility is available, test the design before it is submitted to an official agency's testing lab. Lab time can be expensive, and costs will increase if boards must be redesigned and retested because of some mistake or oversight. Minimize the risk of failing agency testing by employing protection interfaces recommended by circuit protection manufacturers in their application notes and reference designs.
- Submit your design for agency testing.
Find a certified UL, ITU, FCC, or Telcordia test lab to work with and submit your boards for testing and approval.
- Sell your design.
An agency-compliant design will make life easier for your customers. Knowing that the design is protected against power-cross and lightning surges will ease their minds.
- Create your next design.
Once the designer has conquered the obstacles to obtaining good protection and settled on a workable design, that design can be used over and over on new projects, often with minimal redesign.
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