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REO Load Banks Covers Testing House Needs

KTL, based in Hull, is a world leader in testing, certification and approvals, offering an extensive range of test and conformance services for voluntary and regulatory markets worldwide. One of its most frequent tasks is to test equipment for manufacturers of telecommunications network equipment that wish to gain access to the U.S. market. In such instances, it may be necessary to comply with “Nebs” requirements for electromagnetic compatibility, product safety, and environmental performance. This is in addition to Federal Communications Commission requirements. The term “Nebs” is derived from Network Equipment Building Systems and is required by manufacturers which sell their equipment to the US Regional Bell Operating Companies (RBOCS), which includes companies such as Bellsouth, SBC and Verizion, or Competitive Local Exchange Carriers (CLEC). In addition, suppliers to other US markets may also be asked if their equipment is Nebs compliant as many distributors use these requirements as part of their procurement specifications.

Many standards fall within the scope of Nebs. The standards most commonly used are GR-1089-CORE, which tests for electromagnetic compatibility and electrical safety, and GR-63-CORE which tests physical protection. KTL provides testing services that include all aspects of GR-1089-CORE which covers the following requirements: system level electrostatic discharge (up to 8kV contact discharge and 15kV air discharge) electromagnetic interference, lightning and AC power fault tests, steady state power induction tests, electrical safety, bonding and grounding arrangements.

To simulate lightning and power faults that could occur on the American mains distribution system, various surge tests are applied to the telecommunications ports. Maximum voltages applied are 2500V for first level tests and 5000V for second level tests. Surge tests are also applied to the AC power ports. The maximum levels are 2000V for first level tests and 600V for second level tests. Numerous other AC power fault tests are applied to the telecommunications ports, with the potential maximum power occurring during a dual circuit 600V rms 60 Amp test. Untested telecommunication ports must remain operational before, during and after the test is applied.

In order to provide the required voltages and limited currents for the various tests, KTL uses a custom built REO load bank with fixed and variable resistors fed from up to two variable transformers. The combination of transformers and resistors gives a range of dual outputs variable from zero to 1000 Volts, with a maximum current limited to a total of 120 Amp. Some of the high current tests have a five second duration which has allowed de-rating of some of the designed resistors power specification. The maximum power dissipation during short circuit conditions could be as high as 80kW.

The custom built REO load is of a modular construction and comprises three pairs of resistor banks, each with tens of Kilowatts power ratings and overall fan cooling. The same apparatus is also used in other KTL testing scenarios for loading telecommunications equipment, for loading inverters, auxiliary supplies and mains operated equipment.

Access to each resistor load is by pairs of 32 Amp laboratory terminals on a patch panel at the input. The output is fed via relays to a wall panel, where connection is made to the apparatus under test. Screw terminals behind a safety-interlocked cover provide a simple way of making this link. A single-phase integral fan is the preferred cooling option and is protected with an over-current isolator in the access door panel. The resistor banks are contained within an internal force-cooled enclosure with ventilation from the top and bottom of the surrounding cabinet.

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