UL 60950-21-2016 pdf download
UL 60950-21-2016 pdf download.lnformation Technology Equipment – Safety – Part 21: Remote Power Feeding.
A.1 Overview
The two alternative types of RFT CIRCUIT, RFT-C CIRCUIT and RFT-V CIRCUIT, reflect satisfactory experience in different parts of the world. IW-C CIRCUITS are typically floating and used in Europe, while RFT-V CIRCUITS are typically balanced and used in North America. Some RFT-V CIRCUITS are not balanced but utilize an earthed conductor. The conductors of a floating AFT CIRCUIT are isolated from earth by a large resistance, while the conductors of a balanced AFT CIRCUIT are isolated from earth only by the voltage source.
Remote power feeding at 50 V d.c. to 60 V d.c. is well known on the analogue telephone system.
It has also been used at higher voltages for many years for long distance lines in many
countries. The safety requirements have been specified in national standards, for example, in
Germany in DIN VDE 0800-3.
Such higher voltage remote feeding is planned to be expanded to subscriber lines. One example is HDSL (high bit rate digital subscriber line).
AFT-C CIRCUITS with d.c. currents up to 60 mA and d.c. voltages up to several hundreds of volts in floating circuits have been used without creating hazards. AFT-V CIRCUITS operating at up to 140 V d.c. per conductor to earth, or 200 V d.c. per conductor to earth with monitoring and control devices, have also been used without creating hazards,
Requirements are specified in this standard for a newly defined concept, namely the AFT CIRCUIT (REMOTE FEEDING TELECOMMUNICATION CIRCUIT) for use in such applications.
A.2 Operational considerations
Remote feeding with voltages exceeding the voltage limits of TNV CIRCUITS and currents exceeding the current limits of LIMITED CURRENT CIRCUITS is needed to supply a useful amount of power via a TELECOMMUNICATION NETWORK.
However, it is not practical, in all cases, to prevent access by SERVICE PERSONS to an AFT CIRCUIT.
SERVICE PERSONS should be permitted to work under live conditions (without switching off the
power) on the TELECOMMUNICATION NETWORK, as well as on an RFT CIRCUIT supply equipment and on
AFT CIRCUIT load equipment.
A.3 Safety consideratIons
It is not intended for the USER to have access to an RFT cIRcun. The USER should be protected against electric shock as specified in IEC 60950-1 by limiting access to an RFT ciRcuiT as if it were a SECONDARY CIRCUIT at HAzARDous voLTAGE; however, higher spacings and insulation requirements may be necessary, as an RFT cicun is considered to be subject to overvoitages.
SERViCE PERSONS are protected against contact with RFT CIRCUITS, and large area contact with bare parts of AFT-V CIRCUITS, by insulating these circuits as SECONDARY CIRCU(TS at HAZARDOUS VOLTAGE.
If SERVICE PERSONS touch both conductors of an RFT-C CIRCUIT, most of the current available to provide power to the AFT-C CIRCUIT load equipment or equipments (IRFT) could pass through the body. Therefore. to protect under these conditions. ‘RFI should not exceed the limits that cause no harmful physiological effect, taking into account the hand-to-hand current path through the body (60 mA).
To protect SERViCE PERSONS who touch one conductor of an AFT-C CIRCUIT and are earthed through the feet, the maximum permitted body current is less, due to passing close to the heart. The current from one conductor of the RET-C CIRCUIT to earth under normal operating conditions can easily be limited and should not exceed the current that usually causes no reaction (2 mA). If one conductor of the AFT-C CIRCUIT is accidentally earthed and the person touches the other conductor, the ‘RFT should not exceed the limits that cause no harmful physiological effect (25 mA), taking into account the hand-to-foot current path through the body. It RFT exceeds this value, there should be a mechanism, triggered by the unbalance created by the fault, to reduce it to 25 mA or lower.
